The Facts On File Dictionary of Botany

The Facts On FileDICTIONARYofBOTANYEdited byJill Bailey

The Facts On File Dictionary of BotanyCopyright © 2003 by Market House Books LtdAll rights reserved. No part of this book may be reproduced or utilized in anyform or by any means, electronic or mechanical, including photocopying,recording, or by any information storage or retrieval systems, withoutpermission in writing from the publisher. For information contact:Checkmark BooksAn imprint of Facts On File, Inc.132 West 31st StreetNew York NY 10001Library of Congress Cataloging-in-Publication DataThe Facts on File dictionary of botany / edited by Jill Bailey.p. cm.Includes bibliographical references (p. ).ISBN 0-8160-4910-6 (hc)—ISBN 0-8160-4911-4 (pbk.)1. Botany—Dictionaries. I. Title: Dictionary of botany. II. Bailey, Jill.III. Facts on File, Inc.QK9.F33 2002580'.3—dc2l 2002035202Checkmark Books are available at special discounts when purchased in bulkquantities for businesses, associations, institutions, or sales promotions. Please callour Special Sales Department in New York at (212) 967-8800 or (800) 322-8755.You can find Facts On File on the World Wide Web athttp://www.factsonfile.comCompiled and typeset by Market House Books Ltd, Aylesbury, UKPrinted in the United States of AmericaM P 1 09 8 7 6 5 4 3 2 1This book is printed on acid-free paper

AABASee abscisic acid.abaxial In structures such as a leavesand petals, the side facing away from themain axis, i.e. the lower surface. In lateralorgans such as leaves, abaxial is synonymouswith the underside. Compare adaxial.abiotic environment The nonlivingfactors of the environment that influenceecological systems. Abiotic factors includeclimate, chemical pollution, geographicalfeatures, etc.Abscisic acidabscisic acid (ABA) A plant hormone,that functions chiefly as an inhibitor ofgrowth and cell elongation. Abscisic acidhas a variety of effects related to seed dormancyand stress responses: it regulatesprotein expression in seed developmentleading to dormancy and is one of the hormonesinvolved in bud dormancy; it regulatesstress responses by, for example,closing stomata in times of water shortageand increasing the ability of roots to carrywater. It can also promote root growth andinhibit shoot growth. Despite its name itdoes not directly promote abscission, butonly indirectly through increasing ethyleneproduction. Formerly it was known as abscisinII or dormin.abscission The organized loss of part ofa plant, usually a leaf, fruit, or unfertilizedflower. An abscission zone occurs at thebase of the organ. Here a separation layer(abscission layer) is formed by breakdownor separation of cells and final severanceoccurs when the vascular bundles are brokenmechanically, e.g. by wind or rain. Theabscission layer is activated by increasinglevels of ethylene and a decreasing concentrationof auxinabsolute humiditySee humidity.absolute pollen frequency (APF)pollen analysis.Seeabsorption 1. The uptake of liquid bycells and organs. In plants, water and mineralsalts are absorbed mainly by the roothairs, just behind the root tips.2. The capture of radiant energy by plantpigments. About 80% of the visible lightfalling on a leaf is absorbed, and about10% of the infrared radiation.absorption spectrum A plot of the absorbanceby a substance of radiation at differentwavelengths, usually of ultraviolet,visible, or infrared radiation. It can give informationabout the identity or quantity ofa substance. Chlorophylls, for example,have absorption peaks in the red and blue(and therefore reflect green light). Compareaction spectrum.accessory cellSee B chromo-accessory chromosomesome.accessory pigmentpigments.See subsidiary cell.See photosynthetic1

acellularacellular Denoting relatively large tissuesor organisms that are not composed ofdiscrete cells and are, in effect, unicellular.In flowering plants, for instance, the earlystages of endosperm development are oftenacellular. Other examples include aseptatefungal hyphae and certain green algae,such as Acetabularia. The term is used inpreference to unicellular to distinguishsuch structures (which are often multinucleate)from conventional cells and showtheir equivalence to multicellular structures.See also coenocyte.Aceraceae A family of temperate andtropical trees and shrubs that includes themaples and sycamores.acetaldehyde (ethanal) An aldehyde,CH 3 CHO, that is an intermediate in theconversion of pyruvic acid to ethanol duringthe final stage of glycolysis duringanaerobic respiration in plants. It is involvedin the synthesis and breakdown ofthe amino acid threonine.acetic acid (ethanoic acid) A carboxylicacid, CH 3 COOH, obtained by the oxidationof ethyl alcohol. Acetic acid is a componentof vinegar (which is obtained bybacterial oxidation of wine waste). It isused as an alternative carbon source by certaingreen algae. When combined withCOENZYME A to form ACETYL COA, it plays akey role in AEROBIC RESPIRATION.acetocarmine A stain used to colorchromosomes deep reddish-black for viewingwith a light microscope. Tissues arefixed in acetic acid before applying acetocarmine.See fixation; staining.acetyl CoA (acetyl coenzyme A) A compoundmade up of acetyl and coenzyme Alinked by a sulfur bridge. Acetyl CoA playsa key role in metabolism, being a precursorof the KREBS CYCLE and GLYOXYLATE CYCLE,and the starting point for synthesis of fattyacids, terpenes, and some amino acids. Thesynthesis of acetyl CoA is a high-energyprocess, requiring energy from ATP, whichis converted to AMP.achene A dry indehiscent fruit formedfrom an ovary with a single carpel containinga single seed, e.g. oak (Quercus). Differenttypes of achenes include theCARYOPSIS, CYPSELA, NUT, and SAMARA.acicularNeedle-shaped.acid A substance that gives rise to hydrogenions (or H 3 O + ) when dissolved inwater. An acid in aqueous solution willhave a pH below 7. Lowry–Brønstedtheory defines an acid as a substance thatexhibits a tendency to release a proton, anda base as a substance that tends to accept aproton. Strong acids (e.g. HNO 3 ) reactcompletely with water to give H 3 O +. Weakacids (e.g. CH 3 COOH) are only partly dissociatedbecause H 3 O + is a stronger acidthan the free acids.acidic stainSee staining.acid rain The deposition of acids by naturalprecipitation, mainly by rain but alsoby snow and fog. Acids are formed by reactionof gaseous waste products, particularlysulfur dioxide, and also nitrogenoxides, with moisture in the air to form sulfuricand nitric acids. The subsequent precipitationhas led to raised acidity in forestsand lakes, especially in Scandinavia andsome other parts of northern Europe, damagingthe environment. Acid rain is oftenused more loosely for any atmospheric pollutantthat dissolves in precipitation causingenvironmental damage, such as carbonmonoxide or ozone, and these may interactwith the acids.The most serious pollutant is sulfurdioxide, which comes mainly from burningcoal and is, consequently, less serious nowthan in the twentieth century. Sulfuric acidin soils may lead to the formation of ammoniumsulfate, which causes the releaseof toxic aluminum and heavy metal ionsthat inhibit metabolic activity. In waterwaysthese can damage the gills of fish. Unpollutedrain is normally slightly acidicwith a pH of 5.0–5.6.acid soil A soil with a pH less than 6.0.Such soils usually form in areas of heavy2

activation energyrainfall, which causes leaching of limefrom the surface layers, or over acid substratasuch as granite or sand. In the acidicconditions, decomposition of organic materialin the soil is slow. Acid soils oftencontain substantial concentrations of ironand aluminum hydroxides. See brownearth; podsol.Acrasiomycota (cellular slime molds) Aclass of protoctists in the phylum Rhizopoda.Formerly considered to be fungi,they are made up of independently livingamebas that feed on bacteria, which theyingest by phagocytosis. Under conditionsof food shortage or other stresses, theycome together to form a slug-like aggregationof amebas called a pseudoplasmodium,which acts like an individualorganism.acrocarpous Describing mosses inwhich the reproductive organs are borne atthe top of the main axis, which is usuallyerect, so that subsequent growth is sympodial.acrocentricSee centromere.acropetal Describing a process (such asgrowth or development) that progressesfrom the base or point of attachment, sothat the oldest parts are at the base and theyoungest are at the tip. Compare basipetal.actin A globular contractile protein (Gactin)that makes up 10–15% of the totalcell protein in eukaryotic cells. It is a majorcomponent of the CYTOSKELETON. It alsocontrols motility, cellular movements,and cyclosis. G-actin can polymerize intohelical strands that coil together to formmicrofilaments called fibrous actin (Factin).Actinobacteria (ray fungi; actinomycetes)A phylum of Eubacteria that contains thetrue actinobacteria and the coryneformbacteria. The true actinobacteria formbranching filaments that resemble smallfungal mycelia, with hyphae usually lessthan 1.5 m m in diameter; the coryneformbacteria are rodlike structures. TheActinobacteria are distinguished by producingactinospores: entire cells encystedin thick walls to form resistant spores.actinodromous (palmate; digitate) Describinga form of leaf VENATION in whichthree or more primary veins radiate fromthe base of the lamina toward the margin,as in sycamore (Acer pseudoplatanus), resultingin a leaf that has several large lobesor several leaflets all originating at thesame point.actinomorphyactinomycetesSee radial symmetry.See Actinobacteria.actinostele A type of protostele inwhich the xylem is star shaped and thephloem lies between the points of thestar. Protosteles are found in the primaryroots of higher plants, and in some speciesof Lycopodium and Psilotum. Seestele.action spectrum A graph showing theeffect of different wavelengths of radiation,usually light, on a given process. It is oftensimilar to the absorption spectrum of thesubstance that absorbs the radiation andcan therefore be helpful in identifying thatsubstance. For example, the action spectrumof photosynthesis is similar to the absorptionspectrum of chlorophyll. It showsthe net assimilation plotted against wavelength.See absorption spectrum. See illustrationoverleaf.activation energy The minimum extraenergy that must be put into a system to enablea reaction to occur. It increases the energylevels of participating molecules, andhence raises their reactivity, often by raisingelectrons to an excited state. ENZYMESenhance reaction rates by lowering the activationenergy. This is often achieved bythe attraction of the reactants to the enzyme’sactive site, thus coming into closerproximity, or by the enzyme causing conformationalor electrostatic changes in oneor more of the reactants.3

adventitiousAerobic respirationADP (adenosine diphosphate) A nucleotideconsisting of adenine and ribosewith two phosphate groups attached. Thesecond phosphate is attached to the first bya high-energy bond.adventitious Describing plant organsthat arise in unexpected places, for examplethe development of adventitious rootsfrom stems, and adventitious buds fromleaves.aerenchyma A plant tissue containinglarge intercellular air spaces, usuallyformed as a consequence of the death ofparenchyma cells. Aerenchyma is typical ofthe stems of many aquatic plants whoseleaves float at the surface of ponds andlakes, giving added buoyancy to the tissues.aerial root A root that arises above soillevel. Examples include the roots of epiphytesand climbers, which hang down inthe air or stick to a trunk or branch. Theroots of many orchids and other epiphytesdeveloped a sheath of dead cells, the velamen,which helps to absorb water from theatmosphere.aerobe An organism that can live andgrow only in the presence of free oxygen,i.e. it respires aerobically (see aerobic respiration).All plants and most bacteria andfungi are aerobes. Compare anaerobe.aerobic respiration Respiration inwhich free oxygen is used to oxidize organicsubstrates to carbon dioxide andwater, with a high yield of energy. The reactionoverall is:C 6 H 12 O 6 + 6O 2 = 6CO 2 + 6H 2 O +energyIt occurs in a number of stages, the first ofwhich (GLYCOLYSIS) also occurs in anaerobicrespiration in the cell cytoplasm. Withglucose as the substrate, a sequence of reactionsresults in the formation of pyruvate.The remaining stages, which do notoccur in anaerobic respiration, take placein the mitochondria. Pyruvate is convertedto ACETYL COA, which enters a cyclic seriesof reactions, the KREBS CYCLE, with the productionof carbon dioxide and hydrogenatoms. These and other hydrogen atomsproduced at earlier stages are now passedto the ELECTRON-TRANSPORT CHAIN (involvingCYTOCHROMESand FLAVOPROTEINS).Here they combine with atoms of free oxygento form water. Energy released at each6

air bladderstage of the chain is used to form ATP duringa coupling process (see oxidative phosphorylation).There is a net production of38 ATPs (2930 kJ) per molecule of glucoseduring aerobic respiration, a yield of about19 times that of anaerobic respiration (300kJ), and the mechanism of the majority oforganisms. Compare anaerobic respiration.aerotaxis (aerotactic movement) ATAXIS in response to an oxygen concentrationgradient. For instance, motile aerobicbacteria are positively aerotactic, whereasmotile obligate anaerobic bacteria are negativelyaerotactic. Some photosynthetic eukaryoticunicells are also aerotactic.aerotropism A TROPISM in which theorientating stimulus is oxygen.afforestation The establishment of foreston land not previously forested, eitherby natural succession or by planting.aflatoxin One of a group of carcinogenictoxins produced by fungi of the genusAspergillus, especially A. flavus. Thisspecies is a common contaminant of crops,such as peanuts. Contaminated feed causesserious outbreaks of disease among livestock.after-ripening The collective name forprocesses that are necessary before germinationcan take place in certain seeds, eventhough external conditions may be suitable.For example, a period of dormancymay be imposed on the seed, preventingpremature germination before an unfavorableseason such as winter. After-ripeningis common in areas with marked seasonality.agamospermySee apomixis.agar A gelling agent prepared from seaweed(normally a red alga, Gelidium), usedto set liquid nutrients. It can withstandsterilization at high temperatures and is resistantto attack by most bacteria. Agargels are extensively used for growingmicroorganisms and tissue cultures.Agaricales (agarics) An order of basidiomycetesthat contains the mushroomsand toadstools. The cap of the fleshy fruitingbody – the mushroom or toadstool –bears a series of parallel gills on its lowersurface that greatly increase its surfacearea. These are covered in the sporebearinglayer (hymenium). Agarics arecommon in leaf litter and rotting wood,where they live as saprobes.aggregate fruit (compound fruit) A fruitlikestructure that is made up of several individualfruits derived from the carpels of asingle flower, e.g. strawberry (Fragaria),blackberry (Rubus fruticosus). These maybe achenes, follicles, berries, or drupelets.An aggregate fruit is sometimes called anetaerio, but this term may also be restrictedto an aggregate of drupelets. A fruit thatdevelops from a group of flowers is termeda multiple fruit, e.g. pineapple (Ananas comosus)and hop (Humulus lupulus).Agrobacterium A genus of soil bacteria,the species A. tumefaciens being thecausative agent of CROWN GALL, a type oftumor in plants. A segment of DNA (transferredDNA, T-DNA) from a PLASMID inthe bacterium is transferred into the hostDNA and induces tumor formation. Sincethe plasmid is capable of independentreplication in host cells of many dicotyledonousplants, it has been used as a cloningvector in GENETIC ENGINEERING.agroforestry A system of cultivationcommon in many parts of the tropics, especiallyin rainforest regions, in whichforestry and arable farming are mixed.This reduces soil degradation, as freshlytilled soils are shaded and protected fromheavy rains and run-off by the trees. Thetrees help to draw nutrients up from thedeeper soil layers into the zone occupied bythe roots of the crop plants.agronomyA branch of agriculture deal-7

air planting with soil management and crop production.air bladder 1. A structure on the pollengrains of some conifers, such as Pinus, thataids the wind dispersal of the pollen. Itconsists of an air-filled protuberance oneach side of the pollen grain developed bythe separation of the exine layers, givingthe pollen a characteristic winged appearance.2. An air-filled chamber in the thallus ofmany Phaeophyta (brown algae) that increasesthe buoyancy of the thallus, enablingit to float toward the surface whenunderwater to increase light absorptionand gaseous exchange for photosynthesis.Air bladders are highly developed in certainwracks, such as the bladder wrack(Fucus vesiculosus).air plantSee epiphyte.alanine A simple AMINO ACID, one of theearly products of photosynthesis. Alanineis formed by transamination when anamino group is donated by gutamine topyruvic acid. It may be deaminated back topyruvate for use in the KREBS CYCLE.alcohol dehydrogenase An enzymethat converts ethanol into acetaldehyde(ethanal). It is important in a plant’s abilityto withstand waterlogging. Ethanol is aproduct of anaerobic respiration in plants,and waterlogged roots suffer from a shortageof oxygen. Accumulation of ethanol isharmful to a plant.alcoholic fermentation A form ofANAEROBIC RESPIRATION in which glucose isbroken down to form ethanol and carbondioxide. It is carried out by yeasts andsome other fungi and certain bacteria.Alcoholic fermentation is catalyzed byenzymes of the zymase complex, which aresecreted by the cells or released after thecells die. The process is self-limiting, sincethe organisms usually die once the alcoholconcentration in the medium exceeds 15%.Alcoholic fermentation is the basis of thepreparation of alcoholic beverages, and ofbread making (in which the carbon dioxidealbumin One of a group of simple, lowmolecular-weightproteins found in plants,for example in the endosperm of barleyand wheat seeds. Albumins are watersolubleand coagulate when heated.albuminous cell 1. A vertically elongatedparenchyma cell, found in groups inthe rays of the secondary phloem in gymnosperms,where they are associated withsieve cells. Unlike the companion cells ofangiosperms, they are not derived from thesame mother cell as the sieve cell.2. An albumin-containing cell found in certainseeds.alcohol A type of organic compound ofthe general formula ROH, where R is a hydrocarbongroup. Examples of simple alcoholsare methanol (CH 3 OH) and ethanol(C 2 H 5 OH). Ethanol is a product of anaerobicmetabolism.Alcoholic fermentation8

alkaloidreleased causes the dough to rise). Duringalcoholic fermentation acetaldehyde actsas a hydrogen acceptor instead of oxygen.The pyruvic acid formed by GLYCOLYSIS isbroken down into acetaldehyde and carbondioxide. The acetaldehyde is furtherreduced by NADH to form ethanal, releasingonly about a tenth of the energy thatwould be released by aerobic respiration.aldehyde A type of organic compoundwith the general formula RCHO, wherethe –CHO group (the aldehyde group) consistsof a carbonyl group attached to a hydrogenatom. Simple examples ofAldosealdehydes are methanal (formaldehyde,HCHO) and ethanal (acetaldehyde,CH 3 CHO).aldose A SUGAR containing an aldehyde(CHO) or potential aldehyde group. Examplesinclude the sugars ribose (with fivecarbon atoms) and glucose (with six carbonatoms).aleurone grain (aleurone body) A modifiedvacuole found in the embryo and endospermof seeds and containing mostlyreserve proteins, but also phytic acid andvarious enzymes associated with mobilization(digestion) of these reserves. The proteinsand phytic acid are present incrystalline form in the dormant seed.aleurone layer The outermost proteinrichlayer of the endosperm of grass fruits(e.g. cereal grains). At germination, the embryoproduces gibberellin, which stimulatesthe aleurone layer to synthesizeenzymes, especially amylase. The lattercauses hydrolysis of the starch in the endosperm.The enzymes are synthesizedfrom the amino acids supplied by breakdownof ALEURONE GRAINs.algae (sing. alga) A large mixed group ofphotosynthesizing eukaryotic organisms,now usually placed in the kingdom Protoctista.They often resemble plants and arefound mainly in marine or fresh-waterhabitats, although some algae are terrestrial.Algae differ from plants in lackingany real differentiation of leaves, stems,and roots, in having no layer of sterile cellsaround the reproductive organs,and in nothaving an embryo stage in their life cycle.Algae can be unicellular (e.g. Chlamydomonas),colonial (e.g. Volvox), filamentous(e.g. Spirogyra), or thalloid (e.g.Fucus). All algae contain chlorophyll butthis may be masked by various accessorypigments, these being one of the majorcharacteristics used to divide the algaeinto their various phyla. Other charactersused to classify the algae are the natureof storage products, the type of cell wall,the form and number of undulipodia(flagella), ultrastructural cell details, typeof food reserves, and reproductive processes.alkaline soilalkaloidSee calcareous soil.One of a group of organic com-9

allelepounds found in plants, which are poisonousinsoluble crystalline compounds.They contain nitrogen and usually occur assalts of acids such as citric, malic, and succinicacids. Their function in plants remainsobscure, but it is suggested that theymay be nitrogenous end-products of metabolism,or they may have a protectivefunction against herbivores, since theytaste bitter. Important examples in humanuse are quinine, nicotine, atropine, opium,morphine, codeine, and strychnine. Theyoccur mainly in the poppy family, the buttercupfamily, and the nightshade family ofplants.allele (allelomorph) One of the possibleforms of a given gene. The alleles of a particulargene occupy the same positions(loci) on homologous chromosomes. Agene is said to be homozygous if the twoloci have identical alleles and heterozygouswhen the alleles are different. When twodifferent alleles are present, one (the dominantallele) usually masks the effect of theother (the recessive allele). The allele determiningthe normal form of the gene is usuallydominant while mutant alleles areusually recessive. Thus most mutationsshow in the phenotype only when they arehomozygous. In some cases one allele is notcompletely dominant or recessive to anotherallele (INCOMPLETE DOMINANCE).Thus an intermediate phenotype will beproduced in the heterozygote. See also incompletedominance; multiple allelism.allelomorphSee allele.allelopathy Inhibition of the germination,growth, or reproduction of an organismeffected by a chemical substancereleased from another organism. It is probablyan anticompetition mechanism inplants; for example, barley secretes an alkaloidsubstance from its roots to inhibitcompeting weeds, but it is not known howimportant it is generally.allogamy Cross-fertilization in plants.This promotes genetic variation in thepopulation especially in plants that areDIOECIOUS and MONOECIOUS. Most plantsare hermaphrodite, but have mechanismssuch as self-INCOMPATIBILITY thatpromote allogamy. Compare autogamy.See also heterostyly; chasmogamy; dichogamy.allopatric speciesSee species.allopolyploidy A type of polyploidy involvingthe combination of chromosomesfrom two or more different species. Allopolyploidsusually arise from the doublingof chromosomes of a hybrid betweenspecies, the doubling often making the hybridfertile. The properties of the hybrid,such as greater vigor and adaptability, areretained in the allopolyploid in subsequentgenerations and such organisms are oftenhighly successful. Many plant species havebeen derived originally from allopolyploidy,e.g. cultivated wheat. Compareautopolyploidy.allosteric enzyme An enzyme whosecatalytic activity can be modified by thenoncovalent binding of a particularmetabolite (a modulator) at a site (the AL-LOSTERIC SITE) other than the active site. Itmay either inhibit or enhance the enzymeactivity. Allosteric enzymes may have morethan one modulator.allosteric site A part of an enzyme towhich a specific effector or modulator canbe attached. This attachment is reversible.Allosteric enzymes possess an allosteric sitein addition to their ACTIVE SITE.allotetraploid (amphidiploid) An allopolyploidwhose chromosomes are derivedfrom two different species and whichtherefore has four times the haploid numberof chromosomes, e.g. Spartina anglica,derived from the diploids S. alternifloraand S. maritima. See allopolyploidy.alluvial soil A type of soil formed onriver floodplains and deltas, where newsediment is deposited on the land duringfloods.10

amino acidsalpha helix A highly stable structure inwhich peptide chains are coiled to form aspiral. Each turn of the spiral contains approximately3.6 amino acid residues. TheR group of these amino acids extends outwardfrom the helix. Hydrogen bondingbetween successive coils holds the helix together.If the alpha helix is stretched thehydrogen bonds are broken but reform onrelaxation. The alpha helix is found inmuscle protein and keratin.alpha-naphthol test (Molisch’s test) Atest for detecting the presence of carbohydratesin solution. The test solution isplaced in a test tube, and a small amount ofalpha-naphthol added; concentrated sulfuricacid is then trickled slowly down theside of the tube. If carbohydrate is present,a violet ring will form at the junction of theliquids.alpine Describing a BIOME (regionalcommunity) of plants above the treelineand below the snowline on high mountains.The lower limit of the alpine zonevaries in different mountain regions, accordingto the rainfall and other climaticand topographic factors, from 100 metersabove sea level in parts of Scotland to 3700meters in the western Himalayas. There areoften considerable differences between thenature and distribution of vegetation onnorth- and south-facing slopes and betweenwindward and leeward slopes.Alpine vegetation is often similar to TUN-DRA vegetation, being adapted to harsh climateconditions, including high windspeeds.alternate Describing a leaf arrangementin which there is only one leaf at each node,as in hazel. This is the commonest form ofleaf arrangement. See phyllotaxis.alternate host Any host other thanthe main (most common) one. For example,many rusts overwinter on alternatehosts. Some parasites and pests are able tolive on more than one host, and someneed a second host to complete their lifecycle.alternation of generations The occurrenceof two, or occasionally more, generationsduring the life cycle of an organism.In all plants and some algae there is an alternationbetween sexual haploid andasexual diploid stages. They usually differmarkedly in morphology. The haploidplant produces gametes mitotically and isthus termed the gametophyte while thediploid plant produces spores meioticallyand is called the sporophyte. The gametesfuse to form a zygote, which develops intothe sporophyte, and the spores germinateand produce the gametophyte, so forminga cycle. In bryophytes (mosses, liverwortsand hornworts) the haploid gametophyte isthe dominant phase of the life cycle and thesporophyte is represented only by the capsule,seta, and foot. In vascular plants thediploid sporophyte is the dominant phaseand in the ferns, for example, the gametophyteis a small prothallus. The concept ofan alternation of generations can be extendedto the flowering plants, in which theembryo sac and pollen represent the muchreduced female and male gametophyte generationsrespectively.ameboid Describing an organism thatresembles an ameba in shape and movement.amensalism An association betweentwo different species, at either the level ofthe individual organism or the populationlevel, in which one is harmed and the otheris unaffected. Compare commensalism;mutualism.amino acids Compounds containingboth carboxylic acid and amino groups intheir molecules. The amino acids have thegeneral formula RCHNH 2 –COOH. Here,the group R ranges from a simple hydrogenatom to complex ring structures. All aminoacids are white, crystalline, soluble inwater, and with the sole exception of thesimplest member, glycine, all are opticallyactive.Amino acids are the basic componentsof proteins, which consist of chains ofamino acids. Plants also contain some 20011

amino acid sequencingAmino acidsamino acids that do not occur in proteins,some of which have protective or storageroles, while others are intermediates in thesynthesis of commoner amino acids.Amino acids contain both acids (carboxylicacids) and bases (amino group), sothey react with both acids and bases. Thusthe charge on the amino acid depends on12

amitosisAmino acidsthe pH. This feature is used to separateamino acids for analysis by chromatographyand other methods. Each amino acidhas its own characteristic isoelectric point,where the net charge on the molecule iszero. See illustration overleaf.13

AMPamino acid sequencing The determinationof the amino acid sequence of a proteinor peptide. This sequence is the primarystructure of a protein; it influences secondary,tertiary, and quaternary structure.amino sugar A monosaccharaide sugarin which a hydroxyl group (OH) has beenreplaced by an amino group (NH 2 ). Glucosamine(from glucose) is one of thecommonest amino sugars. Galactosamine(from galactose) is a major component ofglycolipids. Amino sugars are importantcomponents of bacterial cell walls.amitosis Nuclear division characterizedby the absence of a nuclear spindle andleading to the production of daughter nucleiwith unequal sets of chromosomes.The ordered process of division, duplicationof chromosomes, dissolution of nuclearmembrane, and production of aspindle as in mitosis is apparently absent.Cells produced amitotically inherit variablenumbers of chromosomes. Thechances of a daughter cell lacking essentialgenes are less than may be expected sincemany cells that characteristically divideamitotically are polyploid, e.g. the endospermnucleus in angiosperms. Compareendomitosis; mitosis.AMP (adenosine monophosphate) Anucleotide consisting of adenine, ribose,and phosphate. AMP has an importantrole in the regulation of glycolysis, promotingthe production of fructose bisphosphatefrom fructose 6-phosphate. See ATP;cyclic AMP.amphidiploidSee allotetraploid.amphimixis True sexual reproductionby fusion of gametes. Compare apomixis.amylase An enzyme, found widely inplants, animals, and microorganisms, thathydrolyzes starch or glycogen to the sugarsmaltose, glucose, or dextrin. Amylase hydrolyzesthe a(1-4)glycosidic bonds instarch. It occurs in two forms, a and b. b-amylase attacks the nonreducing ends ofthe starch molecule, while a-amylase canattack bonds within the starch molecule.Both a- and b-amylases occur in plants, thelatter particularly in malt (being used in thebrewing industry), but only b-amylase isfound in animals, having an important rolein digestion. During seed germination a-amylase breaks down the starch of theendosperm, the transcription of the a-amylase gene being greatly enhanced bygibberellic acid produced by the embryo.a-amylase is also involved in the release ofstored starch from plastids.amylopectin The water-insoluble fractionof STARCH. It is a branching polymer ofglucose units.amyloplast A plastid that synthesizesand stores starch grains. Amyloplasts arecommon in storage organs, e.g. the potatotuber. They have a physiological role in theroot cap and elsewhere, where the starchgrains act as statoliths.amylose The water-soluble fraction ofSTARCH. It is an unbranched polymer ofglucose units.anabolism Metabolic reactions inwhich molecules are linked together toform more complex compounds. Thus, anabolicreactions are concerned with buildingup structures, storage compounds, andcomplex metabolites in the cell. Starch,fats, and proteins are all products of anabolicpathways. Anabolic reactions generallyrequire an input of energy, usuallyprovided by ATP produced by catabolism.An example is the Calvin cycle. See alsometabolism. Compare catabolism.anaerobe An organism that can live andgrow in the absence of free oxygen, i.e. itrespires anaerobically (see anaerobic respiration).Anaerobes can be facultative, inthat they usually respire aerobically butcan switch to anaerobic respiration whenfree oxygen is in short supply, as are mostyeasts; or obligate, in that they neverrespire aerobically and may even be poisonedby free oxygen, as are denitrifying14

aniline stainsbacteria and lactic acid bacteria. Compareaerobe.anaerobic respiration Respiration inwhich oxygen is not involved, found in almostall organisms. The organic substrateis not completely oxidized and the energyyield is low. The glycolytic pathway,whereby glucose is degraded to pyruvate,with the production of a small amount ofenergy, is the same as in aerobic respiration,but after that, in plants and fungi,ethanol and carbon dioxide are producedas the end products of the process, normallyknown as FERMENTATION. In animalslactic acid is produced and normally remetabolizedwith oxygen at a later stage. Onlytwo molecules of ATP are produced by thisprocess. See anaerobe; glycolysis. Compareaerobic respiration.analogous Describing structures thatare apparently similar (structurally orfunctionally) but have a different evolutionaryorigin, and thus a different embryologicalorigin and structure. Phyllodes areanalogous to leaf blades, but are derivedfrom petioles (leaf stalks). See also homologous.anaphase The stage in mitosis or meiosiswhen chromatids are pulled toward oppositepoles of the nuclear spindle. Inmitosis each of the chromatids moving towardthe poles represents a single completechromosome. During anaphase I of meiosispairs of chromatids still connected at theircentromere move to the spindle poles. Duringanaphase II the centromeres divide andsingle chromatids are drawn toward thepoles, thus sister chromatids are separated.anatomy The organization of the partsof the body and the structural relationshipsbetween them, including the arrangementof tissues and their component cells.anatropousSee ovule.androdioecious Describing species inwhich there are two forms, one bearingonly male flowers, the other bearing onlyhermaphrodite flowers (or occasionallybearing male and female flowers, i.e. MO-NOECIOUS). Compare gynodioecious.androecium The collective name inhigher plants for the male parts of a plant,i.e. the STAMENS. It is denoted in the floralformula by a letter A.andromonoecious Describing speciesin which both male and hermaphroditeflowers are borne on the same plant. Comparegynomonoecious.anemophily Pollination by wind. Plantspollinated in this manner (e.g. grasses) usuallyhave insignificant unscented flowerswith large, often feathery stigmas.aneuploidy The condition, resultingfrom nondisjunction of homologous chromosomesat meiosis, in which one or morechromosomes are missing from or added tothe normal somatic chromosome number.If both of a pair of homologous chromosomesare missing, nullisomy results. Monosomyand trisomy are the conditions inwhich one or three homologs occur respectively,instead of the normal two. Polysomy,which includes trisomy, is thecondition in which one or more chromosomesare represented more than twice inthe cell. See nondisjunction.aneuspory The production of an unusualnumber of spores during meiosis of aspore mother cell. In dandelion (Taraxacum),for example, the chromosomes ofthe megaspore mother cell remain in onecell after the first meiotic division, forminga restitution nucleus. The second meioticdivision thus gives rise to two cells with thediploid number of chromosomes, one ofwhich divides parthogenetically to form anembryo. This is a form of APOMIXIS. Sincecrossing over can occur during the firstmeiotic division, it does allow for some geneticvariation in the offspring. See parthenogenesis.angiosperms The flowering plants constitutingby far the largest phylum (Antho-15

anionphyta or Angiospermophyta) of vascularseed plants. They differ from conifers andother gymnosperms by having the ovuleenclosed within an ovary, which after fertilizationdevelops into a fruit. The femalegametophyte is represented by the embryosac, the archegonia being absent. The contentsof the pollen grain represent the malegametophyte. The pollen grain germinateson a special extension of the carpel, thestigma, and double fertilization results in adiploid zygote and triploid (usually) endospermnucleus. Angiosperms are dividedinto two major classes, the monocotyledons(MONOCOTYLEDONAE) and dicotyledons(DICOTYLEDONAE), with a smallbasal group known as primitive dicotyledons.angstrom Symbol: Å A former unit oflength equal to 10 –10 meter (one thousandthof a micrometer, one tenth of ananometer). It is still used occasionally formeasurements of wavelength or interatomicdistance.aniline stains A group of stains withvarious properties that are derived fromaniline. For example, aniline hydrocholorideand aniline sulfate will stainlignin yellow. See staining.anion A negatively charged ion, formedby addition of electrons to atoms or molecules.In electrolysis anions are attractedto the positive electrode (the anode). Comparecation.anisogamy (heterogamy) The sexual fusionof nonidentical gametes. Anisogamygrades from situations in which the gametesdiffer only in size to the extreme ofoogamy, in which one gamete is a largenonmotile ovum and the other a smallmotile sperm. Compare isogamy. Seeoogamy.annual A plant that completes its lifecycle within a year. Examples are the commonfield poppy (Papaver rhoeas) and thesunflower (Helianthus). Compare biennial;ephemeral; perennial.annual ring (growth ring) The annualincrease in girth of the stems or roots ofwoody plants, as a result of cambial activity.The annual rings of plants growing intemperate climates can be seen in crosssectionas two consecutive rings of lightanddark-colored xylem tissue. These areformed from a zone containing larger vesselelements produced by the cambium inthe spring (lighter layer), followed by azone containing smaller vessel elements(darker layer) produced during the latesummer. This process is repeated annuallyso that the number of light or dark rings indicatethe age of that part of the plant.annular thickening Rings of thickeninglaid down on the inner wall of protoxylemvessels and tracheids. Such thickening allowsextension of the XYLEM between therings so that it is not ruptured as the surroundingtissues grow.annulus 1. The ring of tissue surroundingthe stalk of the mature fruiting body(mushroom or toadstool) of the basidiomycetefungi. The annulus is all that remainsof the veil, which joined the rim ofthe pileus (cap) to the stalk in the immaturemushroom or toadstool. It is sometimestermed the velum.2. A special arc or ring of cells in the sporangiaof ferns that constitutes the mechanismfor spore dispersal. The cells of theannulus are thickened except on the outerwall, so that they contract on drying out.This causes the stomium to rupture, exposingthe spores within the capsule. The capsulewall gradually bends back as the waterin the annulus cells continues to evaporateuntil a point when the remaining water inthe cells suddenly turns to vapor. Thisresults in the wall springing back to itsoriginal position, the sudden movementdispersing any remaining spores.3. A ring of large cells separating the epidermisfrom the operculum in certain bryophytes(e.g. Funaria).anterior Designating the part of aflower or axillary bud facing away fromthe inflorescence axis or stem, respectively.16

antiportanther The part of the stamen that producesthe pollen. The anther is usuallyjoined to the tip of the filament (stalk) andis made up of two lobes. Each lobe containstwo pollen sacs that produce verylarge quantities of small pollen grains. Thepollen is released when the lobes split openlongitudinally.The anther is made up of an outer epidermis,a middle fibrous layer, and aninner nutritive layer, the tapetum. The haploidpollen cells develop in the tapetal zonefrom spore mother cells.anther culture (pollen culture) The generationof haploid plants from immaturepollen grains or intact excised anthers. Theresultant plants are generally smaller thantheir diploid counterparts.antheridial cell The cell from which theantheridium develops. In seed plants it isthe generative cell in the pollen grain,which divides in the pollen tube to producetwo sperm cells.antheridium The male sex organ of thealgae, mosses, liverworts, hornworts,ferns, horsetails, clubmosses, and fungi. Itmay be made up of one cell, or one ormany layers of cells. It produces gametesthat are usually motile. Compare archegonium.antherozoid (spermatozoid) The malegamete of algae, some gymnosperms, andthe non-seed-bearing plants. It is motileand is produced in an antheridium,except in certain gymnosperms (e.g. Cycas,Ginkgo) in which antherozoids developfrom the generative cells of the pollentube.Anthocerophyta (hornworts) The smallphylum of nonvascular plants. Like thalloseliverworts, the hornworts have analternation of generations in which thegametophyte is a dorsiventral flattenedgreen thallus and the sporophyte is an uprightstructure that has a foot embedded inthe gametophyte, from which it draws nutrition.anthocyanin One of a group of watersolublepigments found dissolved in the sapof higher plant cell vacuoles. Anthocyaninsare red, purple, and blue and are widelydistributed, particularly in flowers andfruits where they are important in attractinginsects, birds, etc. They also occur inbuds and sometimes contribute to theautumn colors of leaves. They are naturalpH indicators, often changing from red toblue as pH increases, i.e. acidity decreases.Color may also be modified by traces ofiron and other metal salts and organic substances,for example cyanin is red in rosesbut blue in the cornflower. See flavonoid.AnthophytaSee angiosperms.antibiotic One of a group of organiccompounds, varying in structure, that areproduced by microorganisms and can killor inhibit the activities of other microorganisms.One of the best-known examplesis penicillin, which is produced by themold Penicillium notatum. Another exampleis streptomycin, from the actinobacteriumStreptomyces griseus. Antibioticsare also commonly used in medicine tocombat bacterial infections. Antibiotics arewidely used in research to inhibit proteinsynthesis. Some antibiotics selectively inhibitprotein synthesis by the ribosomes ofprokaryotes, but not by those of eukaryotes(excepting the ribosomes of chloroplastsand mitochondria), lending supportto the theory that these organelles evolvedfrom symbiotic microorganisms. Antibiotic-resistantgenes are used as markers ingenetic-engineering experiments.anticlinal Describing something that isat right angles to a surface. An anticlinaldivision results in the formation of anticlinalwalls between daughter cells, allowinga tissue to increase its circumference. Theanticlinal wall of a cell is perpendicular tothe surface of the plant body. In cylindricalorgans, such as stems and roots, the termradial is often used instead of anticlinal.Compare periclinal.anticodonA nucleotide triplet on trans-17

antisense DNAfer RNA that is complementary to andbonds with the corresponding codon(triplet) of messenger RNA in the ribosomesduring protein synthesis, thus ensuringthe correct sequence of amino acids isbonded together in the final peptide. Seetransfer RNA.antioxidant A substance that slows orinhibits oxidation reactions, especially inbiological materials or within cells, therebyreducing spoilage or preventing damage.Natural antioxidants include vitamin Eand b-carotene.antipodal cells The three haploid cellsfound in the embryo sac of seed-bearingplants that migrate to the chalazal end ofthe sac farthest from the micropyle. Thesenuclei arise as a result of the three meioticdivisions that produce the egg cell, synergidcells, and polar nuclei, but they donot themselves take part in the fertilizationprocess. They are eventually absorbed bythe developing embryo, and their functionis uncertain. They do not occur in all floweringplants.antiport A membrane transport proteinin which the energy liberated by the passivemovement of H + ions across the membraneis coupled to the active transport of anothersolute, e.g. Na + , in the opposite directionbut against its own electrochemicalpotential gradient.antisense DNA The DNA strand that isnot transcribed. In transcription the DNAdouble helix unwinds and only one of thestrands acts as the template for messengerRNA synthesis.antitranspirant A chemical that reducestranspiration. Antitranspirants maywork by depositing a waterproof film overthe stomata or by inducing stomatal closure.ant plant A plant that is adapted to providea home for ants. Such plants usuallyhave one or more domatia, cavities instems, leaves or roots that house ants.These may be elaborate systems of cavitieswith many chambers, e.g. Myrmecodia, orsimple hollows in internodes, leaf veins(Coffea arabica), petioles (Tococa guyanensis)or stipular spines, e.g. whistlingthorns (Acacia sphaerocephala).APFSee pollen analysis.aphid (greenfly; blackfly; plant louse) Abug of the family Aphididae of the insectorder Hemiptera. Aphids feed by suckingplant juices. Many occur in such numbersthat they are serious pests of crops andgreenhouse plants. Others carry plantviruses.aphotic zone The lower part of a bodyof water where light intensity is insufficientto support photosynthesis. Comparephotic zone.AphyllophoralesaphyllousSee bracket fungi.Without leaves.Apiaceae (umbellifers; carrot family) Afamily of dicotyledons distinguished by itsINFLORESCENCES, which are umbels. Mostspecies are herbaceous, but a few areshrubs and trees. The plants have a characteristicodor and some species, such asparsley, fennel, and dill, are used as herbs;the seeds of many others are popularspices, e.g. aniseed, caraway, coriander,cumin. Many species develop swollen taproots,and the family includes several importantroot vegetables, e.g. carrot andparsnip; the leaf stalks of celery are alsoeaten.apical dominance The phenomenon inwhich the presence of a growing apical budon a plant inhibits the growth of lateralbuds. It is controlled by the interactions ofplant hormones, particularly AUXIN (producedby the shoot tip) and ABSCISIC ACID.apical meristem The actively dividingcells constituting the growing point at thetip of the root or stem in vascular plants.New cells are cut off on the lower side toform new stem tissue at the stem apex, andon both sides in the root apex to form root18

Archaeatissue and a protective root cap. The apicalmeristems in the lower plants consist ofone cell only, as in the ferns, but becomemore complex and consist of groups ofcells in the higher plants. See histogentheory; tunica–corpus theory; meristem.aplanospore A nonmotile spore, characteristicof the pin molds and certaingreen algae of the phyla Clorophyta andChrysomonada. It is an asexual sporeformed in a sporangium and is usuallythick walled.apocarpousSee carpel.apocarpy An ovary made up of unfusedcarpels, as in the buttercup (Ranunuculus).Compare syncarpy.apoenzyme An ENZYME whose cofactoror prosthetic group has been removed (e.g.via dialysis) rendering it catalytically inactive.It is the protein part of a conjugate enzyme.When combined with its prostheticgroup (coenzyme) it forms a complete enzyme(holoenzyme). Examples include thecytochromes, which require ferrous or ferricions for activity.apogamy In certain ferns, algae, andfungi, the development of the sporophytedirectly from a cell of the gametophyte, sofusion of gametes is bypassed. It frequentlyoccurs in gametophytes that have beenproduced aposporously and are thusdiploid. The term also describes the developmentof an unfertilized female gameteinto the sporophyte, a phenomenon describedas PARTHENOGENESIS. See apospory;apomixis.apomixis Any form of asexual reproduction.The term is sometimes used in arestricted sense to describe a modified formof reproduction by plants in which seedsare formed without fusion of gametes, aprocess also termed agamospermy. A plantthat reproduces in this way is called anapomict, e.g. many species of Rubus.Apomicts are often polyploids that wouldhave difficulty reproducing sexually.Apomixis is comparable to the conditionsof apogamy and apospory, which are seenin many ferns. It includes the processwhereby a diploid cell of the nucellus developsinto an embryo giving a diploid seedwith a genetic constitution identical to theparent. Another form of apomixis in whichseeds develop from unfertilized gametescan also be termed parthenogenesis. Seedsproduced in this way may be either haploidor diploid depending on whether or not themegaspore mother cell undergoes meiosis.Often, in the process termed pseudogamy,entry of the male gamete is required tostimulate the development of the femalegamete, even though nuclear fusion doesnot occur. Such cases of apomixis are difficultto distinguish from true sexual reproduction.Compare amphimixis. See alsoapogamy; apospory; parthenogenesis; sexualreproduction.apoplast The system of cell walls and intercellularmaterial extending through aplant body and along which water containingmineral salts, etc. can move passively. Itis an important pathway for movement ofthese substances outside the xylem, for exampleacross the root cortex. Comparesymplast.apospory The development of the gametophytedirectly from the cell of a sporophyte,thus bypassing meiosis and sporeproduction. Gametophytes produced inthis manner are thus diploid instead ofhaploid. If such gametophytes produce fertilegametes, the resulting sporophyte isthen tetraploid, and large polyploid seriesmay subsequently be developed. Aposporyis found in some mosses and ferns. In angiosperms,a diploid embryo may developfrom a cell of the nucellus or chalaza; thisis also apospory. See also apogamy;apomixis.apotheciumSee ascoma.apposition The deposition of successivelayers of cellulose on the inner surface of aplant cell wall after elongation is completed,resulting in an increase in thickness19

archaebacteriaof the wall. These extra layers are calledthe secondary cell wall. Compare intussusception.appressorium (pl. appressoria) In manyparasitic fungi, a flattened hyphal structurethat presses closely against the host epidermis.From its undersurface a narrow infectionhypha or penetration tube pushesthrough the cell wall or intercellular spacesand develops into a series of hyphae or aHAUSTORIUM.aquaporin (water pore) A protein channelin the plasma membrane that facilitateswater movement across the membrane.Aquaporins can open or close to increaseor decrease water flow.Archaea (archaebacteria) In the FiveKingdoms classification scheme, a subkingdomof the BACTERIA, but in most modernclassifications, a DOMAIN in its ownright. It contains bacteria-like organismswith distinctive ribosomal RNA whose nucleotidesequences differ significantly fromthose of other prokaryotes. Because RNAis considered to evolve very slowly, thesedifferences are considered important in assigningthe Archaea the status of domain.The shape and structure of their ribosomesare more similar to those of eukaryotesthan to bacterial ribosomes. Whether theyare more closely related to the eukaryotesthan to the bacteria is hotly debated. Theyappear to have a mixture of prokaryoticand eukaryotic genes. Like eukaryotes, butunlike bacteria, they have introns in theirtransfer RNA, but like bacteria they havepolycistronic operons (gene regulators).Archaean cell walls lack murein, a commoncomponent of bacterial cell walls, andtheir lipids have ether links rather thanester links.The Archaea contain many organismsof extreme environments, such as hydrothermalvents, hot springs, saline sediments,volcanic craters and boiling muds.They include methane-generating organisms(methanogens), sulfate reducers, andextremophiles.archaebacteriaArchean (Archeozoic)See Archaea.See Precambrian.archegonium The female sex organ ofthe mosses, ferns, clubmosses, horsetails,most gymnosperms, and some red andbrown algae. It is a multicellular flaskshapedstructure made up of a narrow neckand a swollen base (venter) that containsthe female gamete. Compare antheridium.archesporium The single cell or groupof plant cells in the sporophyte from whichspore mother cells may eventually developin a sporangium.Arecaceae (Palmae; palms) A family ofmonocotyledonous plants found mainly intropical regions. The typical growth formis an evergreen tree with unbranched trunkand a crown of large spirally arrangedleaves at the apex. The trunk is oftensheathed in old leaf bases. Some palms,e.g. the doum palms (Hyphaene), branchdichotomously. Many palms grow in theunderstorey of rainforests in Asia and theAmericas.arginine An AMINO ACID found especiallyin the histone proteins that surroundDNA in chromosomes.aril A brightly colored fleshy or hairyoutgrowth from the funicle at the base ofthe ovule that may partly or completelycover the seed. The mace-yielding outgrowtharound the fruit of the nutmeg is anexample, as is the red fleshy cup surroundinga yew seed (Taxus baccata). See alsocaruncle.artificial parthenogenesis See parthenogenesis.ascocarpSee ascoma.ascogonium (pl. ascogonia) The femalegametangium of certain fungi of the phylumAscomycota (e.g. Erysiphe, Eurotium).20

ATPascoma (ascocarp) The fruiting body ofmost fungi of the phylum Ascomycota, inwhich the asci are borne.ascomyceteSee Ascomycota.Ascomycota (sac fungi; ascomycetes) Alarge phylum of fungi characterized bytheir distinctive reproductive structure, theASCUS. In some species the asci are cylindrical,and the ascospores are discharged violentlyas a result of hydrostatic pressurebuilding up inside the ascus. In others, theasci are globular, and the ascospores are releasedpassively. The tip of the ascus mayopen through a simple pore, or it may havea cap. Most ascomycotes form branchingseptate hyphae, but some are yeastlike.Ascomycotes are found in a wide rangeof habitats. Some are saprobes, such asblue and green molds (e.g. Neurospora),morels, and truffles, which are importantdecomposers. There are also parasites,such as ergot of rye (Claviceps purpurea),peach leaf curl (Taphrina), the powderymildews (Erysiphales), and Fusarium wilt.A few are symbionts in lichens and mycorrhizas.The yeasts are economically importantin fermentation processes such asbrewing (Saccharomyces cerevisiae), winemaking, and bread making. Some yeastsare parasites of humans, e.g. Candida albicans;others, such as Penicillium, aresources of antibiotics.ascorbic acidSee vitamins.ascospore One of (usually) eight haploidspores produced inside an ASCUS bymembers of the phylum Ascomycota.ascus (pl. asci) The spore-producing cellof fungi of the phylum ASCOMYCOTA. It is asaclike structure that is formed eithersingly or in large numbers in ascomata.After meiosis the ascus contains four oreight haploid ASCOSPORES that are liberatedthrough a pore at the end of the sac.asexual reproduction The formationof new individuals from a single parentwithout the production of gametes or specialreproductive structures. It occurs inmany plants, usually by vegetative propagationor spore formation; in unicellularorganisms usually by fission or budding;and in some algae by fragmentation. Such‘offspring’ are genetically identical clonesof their parents. See apomixis.asparagine An AMINO ACID formed bythe addition of ammonia to aspartic acid.This is a way of removing ammonia, whichis highly toxic, from living cells. Asparaginecan later be broken down and thenitrogen used in amino acid synthesis.aspartic acid An AMINO ACID formed bytransamination, in which the amino groupof glutamic acid is transferred to oxaloaceticacid. Aspartic acid can be brokendown in the KREBS CYCLE. It is an importantprecursor of several amino acids, and alsoacts as a source of the amino group in variousmetabolic reactions.association A climax plant communitynamed according to the dominant type ofspecies. Examples are heath associationsand coniferous forest associations. See alsoconsociation.assortative mating Sexual reproductionin which the pairing of male andfemale is not random. If similar phenotypes(e.g. plants with similar flowershapes and color that attract the same insects)breed together, this a positive assortativemating; it may lead to INBREEDING.This is likely in insect-pollinated plants,where the insect looks for similar colorsand guide markings. Negative assortativemating occurs when dissimilar phenotypesbreed together, leading to OUTBREEDING.asterSee centriole.Asteraceae (Compositae) A large familyof flowering plants, with some 21 000species found worldwide and including almostevery kind of growth form, butmainly herbs. The leaves, which may besimple or compound, are usually alternate,occasionally opposite or whorled, and21

ATPaseoften in basal rosettes. Latex is present insome genera, e.g. lettuce (Lactuca). Taprootsare common. Cultivated compositesinclude sunflower (Helianthus annuus), asource of oil and seeds; Jerusalem artichoke(H. tuberosus), grown for food;globe artichoke (Cynara scolymus), whoseyoung flower heads are eaten; salsify(Tragopogon porrifolius), whose roots areeaten; chicory (Cichorium intybus), used asa substitute for coffee; lettuce (Lactucasativa); and Chrysanthemum cinerariifoliumand C. coccineum, which aresources of the insecticide pyrethrum.astrosclereid An irregularly branchedSCLEREID found in the mesophyll of leavesof certain dicotyledons.ATP (adenosine triphosphate) The universalenergy carrier of living cells. Energyfrom respiration or, in photosynthesis,from sunlight is used to make ATP fromADP. It is then reconverted to ADP in variousparts of the cell, the energy releasedbeing used to drive cell reactions. ATP is anucleotide consisting of adenine and ribosewith three phosphate groups attached. Hydrolysisof the terminal phosphate bond releasesenergy (30.6 kJ mol –1 ) and iscoupled to an energy-requiring process.Further hydrolysis of ADP to AMP sometimesoccurs, releasing more energy. Thepool of ATP is small, but the faster it isused, the faster it is replenished.ATPase (adenosine triphosphatase) Anenzyme that catalyzes the hydrolysis ofATP to ADP and inorganic phosphate.Found in every cell, it is responsible for therelease of energy to power metabolic reactionsand active transport. In oxidativephosphorylation, it forms part of the enzymecomplex ATP synthetase, catalyzingthe synthesis of ATP from ADP and inorganicphosphate using energy from theproton gradient generated by the electrontransportchain.atropous (orthotropous) See ovule.autecologyThe study of the interactionsof an individual organism or a singlespecies with the living and nonliving componentsof its environment. Compare synecology.autodiploid 1. A diploid plant producedby doubling the chromosomes of a haploidplant. Such a plant is homozygous forevery allele.2. A gamete containing a diploid numberof chromosomes, rather than the usualhaploid number. Such a gamete may beproduced by tetraploid tissues, or it may bethe result of faulty meiosis. Two diploidgametes may fuse to give a tetraploid zygote,or gamete may develop directly byparthenogenesis.autoecious Denoting rust fungi that requireonly one host species to complete thevarious stages of their life cycle. Examplesare Puccinium antirrhini found on antirrhinumand the mint rust (Puccinia menthae).Compare heteroecious.autogamy Self-fertilization in plants.This restricts genetic variation but allowsisolated individuals to reproduce. It isfound particularly in pioneer weed speciesand in ecosystems such as the tundra,where insect vectors are rare. Very fewplants rely exclusively on autogamy. Compareallogamy.autolysis The self-destruction of cells bydigestive enzyme activity. It is the finalstage of cell senescence resulting in completedigestion of all cell components. Seelysosome.autonomic movements (autogenic movements;spontaneous movements) Movementsof plants in response to internalrather than external stimuli. Examplesare cytoplasmic streaming, chromosomemovement during nuclear division, andgrowth itself. Compare paratonic movements.autophagy The process by which wornoutcell organelles are enclosed in vacuoles,then broken down by hydrolytic enzymes22

Azotobactersecreted into vacuole, the breakdownproducts being reabsorbed afterward. Seealso lysosome. Compare autolysis.autopolyploidy A type of polyploidyinvolving the multiplication of chromosomesets from only one species. Autopolyploidsmay arise from the fusion of diploidgametes that have resulted from theNONDISJUNCTION of chromosomes at meiosis.Alternatively, like allopolyploids, theymay arise by the nondisjunction of chromatidsduring the mitotic division of a zygote.Autopolyploids often have largercells than the parent plants but are morphologicallyvery similar. They may, however,be at least partially isolated from eachother reproductively and occupy differentecological niches. Compare allopolyploidy.autoradiography A technique wherebya thin slice of tissue containing a radioactiveisotope is placed in contact with a photographicplate. The image obtained ondevelopment shows the distribution of theisotope in the tissue.autosomes Paired somatic chromosomesthat play no part in sex determination.Compare sex chromosomes.autotetraploid An autopolyploid thathas four times the haploid number of chromosomes.See autopolyploidy.autotrophism A type of nutrition inwhich the principal source of carbon isinorganic (carbon dioxide or carbonate).Organic materials are synthesized from inorganicstarting materials. The processmay occur by use of light energy (photoautotrophism)or chemical energy (chemoautotrophism).Autotrophic organisms(autotrophs) are important ecologically asprimary producers, their activities ultimatelysupplying the carbon requirementsof all heterotrophic organisms. Compareheterotrophism. See chemotrophism; phototrophism.auxanometer An instrument designedto measure the increase in length of a plantpart. A growing plant is attached by threadto the end of a lever that magnifies anygrowth movement. The opposite end of thelever is used to record a trace on a slowlyrotating drum.auxin Any of a group of plant hormones,the most common naturally occurringone being indole acetic acid, IAA.Auxins are made continually in growingshoot and root tips. They are activelymoved away from the tip, having variouseffects along their route before being inactivated.They regulate the rate of extensionof cells in the growing region behind theshoot tip and are involved in phototropicand gravitropic curvature responses ofshoot tips (but probably not root tips)moving laterally away from light and towardgravity.Auxins stimulate cell enlargement,probably by stimulating excretion of protonsleading to acid-induced wall looseningand thus wall extension. They help maintainapical dominance by inhibiting lateralbud development. Root initiation may bestimulated by auxin from the shoot, andauxins have been shown to move towardthe root tips. Pollen tube growth is stimulatedby auxin and its production by developingseeds stimulates fruit set andpericarp growth in fleshy fruits. It interactssynergistically with gibberellins and cytokininsin stimulating cell division anddifferentiation in the cambium. A highauxin:cytokinin ratio stimulates rootgrowth but inhibits regeneration of buds intobacco pith callus. It is antagonistic to abscisicacid in abscission.awn In grass flowers (Poaceae), a stiffbristlelike structure formed from a continuationof the central nerves of the LEMMASor GLUMES.axenic culture A culture that consists ofonly one type of organism or cell, i.e. it is23

BBacillariophytaSee Diatoms.bacillus (pl. bacilli) Any rod-shaped bacterium.Bacilli may occur singly (e.g.Pseudomonas), in pairs, or in chains (e.g.Lactobacillus). Some are motile.backcross A cross between an individualand an individual of the original parentgeneration. If a homozygous dominant AAis crossed with a homozygous recessive aa,the F 2 generation obtained by selfing the F 1would be 25% AA, 50% Aa, and 25% aa.To distinguish between the phenotypicallyidentical AA and Aa, these can be backcrossedto the homozygous recessive parentaa (a test cross). The offspring from AA´ aa will all be identical (Aa) whereas in thecross Aa ´ aa, 50% will have the dominantphenotype (Aa) and 50% will show the recessivecharacter (aa).Bacteria A kingdom containing all theprokaryotic organisms, formerly calledMonera or Prokaryotae. This is a large anddiverse group of organisms, which, interms of numbers and variety of habitats,includes the most successful life forms. Innature, bacteria are important in the nitrogenand carbon cycles, and some are usefulto humans in various industrial processes,especially in the food industry, and in techniquesof genetic engineering (see alsobiotechnology). However, there are alsomany harmful parasitic bacteria that causediseases such as botulism and tetanus. Othersare sources of antibiotics such as streptomycinand erythromycin. Some bacteriarequire oxygen for respiration, while otherscan survive without oxygen. There maybe at least ten times more bacteria living incracks in the rocks deep inside the earththan in the surface layers of the planet, includingthe soil. Some derive their carbonfrom inorganic sources, and their hydrogenfrom hydrogen sulfide or other inorganiccompounds, while other bacteria may bephotosynthetic or saprobic.Bacterial cells are simpler than those ofanimals and plants. They lack well-definedmembrane-bound nuclei and histonecoatedchromosomes, and do not containcomplex organelles such as chloroplastsand mitochondria. Their DNA is usuallylocated in a specific part of the cell, calledthe nucleoid. Bacteria may divide every 20minutes and can thus reproduce veryrapidly. They also form resistant spores.The Bacteria are divided into two maingroups or subkingdoms: the ARCHAEA, orarchaebacteria, which often occur in extremeconditions, such as hot springs; andthe EUBACTERIA, which include the vast majorityof bacteria. Some taxonomists considerthese groups to be full kingdoms intheir own right. See Actinobacteria;Cyanobacteria; myxobacteria; sulfur bacteria.See also prokaryote.bacterial chromosome The nakedDNA of bacteria and Archaea. Bacteria donot have true CHROMOSOMES – linear piecesof DNA complexed with histone proteins.Their DNA is circular and is not complexedwith proteins. It lacks the centromere,telomere, and other structures ofthe eukaryote chromosome.See transfor-bacterial transformationmation.bactericidal Used to describe a compoundthat has a lethal effect on bacteria.A bactericidal compound may act by interferingwith a vital biochemical pathway, or24

aseby destroying the molecular structure ofthe cell.bacteriochlorophyll Any of severaltypes of chlorophyll found in photosyntheticbacteria, such as the purple bacteria.There are seven forms, designated bacteriochlorophyllsa–g. All are structurally similarto chlorophyll a of plants. Thebacteriochlorophylls absorb light at longerwavelengths than chlorophyll a enablingfar-red and infrared light to be used inphotosynthesis. See photosynthetic pigments.bacteriophage (phage) A virus that infectsbacteria. Phages usually have complexcapsids (protein coats) composed of apolyhedral head, containing the nucleicacid (DNA or RNA), and a helical tail,through which nucleic acid is injected intothe host. Phages are usually specific to particularspecies or even strains of bacteria.Infection by a phage may or may not leadto the death of the bacterium, dependingon the phage and on the conditions. AllRNA phages cause the death of the hostcell. After reproduction of the viral nucleicacid the host cell usually undergoes lysis(LYSOGENY). Some DNA phages, calledTEMPERATE PHAGES, may become integratedinto the host DNA and are replicated whenit replicates. Some of these phages arecapable of transferring genes between bacteriain a process called TRANSDUCTION. InGENETIC ENGINEERING, nonviral DNA canbe inserted into a phage, which is then usedas a cloning vector.bacteriostatic Used to describe a compoundthat prevents reproduction of bacteria,but does not kill them.bacteroid A modified bacterial cell in aroot nodule, typically a cell of the bacteriumRhizobium in the root nodule of aleguminous plant (family Fabaceae).Banks, Sir Joseph (1743–1820) Britishbotanist. Banks used his considerable fortuneto finance several important expeditions,including his most famous one to theSouth Pacific in 1768 with Captain JamesCook on the Endeavour. This voyagelasted for three years and resulted in thediscovery of the unique flora and fauna ofAustralia. On his return with his enormouscollection of specimens, he received an invitationto meet King George III, who hadexpressed a great interest in the expedition.His continued friendship with the king increasedhis influence in scientific circlesand he became president of the Royal Societyfor life in 1778. Following his appointmentto the post of honorary director ofKew Gardens, he helped to establish amajor collection of living plant species andto make Kew famous as a center of excellencefor plant cultivation. He is also famousfor introducing the tea plant to India(from China) and breadfruit to the WestIndies (from Tahiti). His extensive libraryand herbarium are housed in the BritishNatural History Museum.bark The outermost tissue of the stemand roots in woody plants, on the outsideof the phellogen (cork cambium). If thesame phellogen functions from year toyear, as in beech and oak, then the bark issmooth and consists only of tissue externalto the phloem, mainly cork. If new corkcambia arise every few years, as in mostspecies of woody plants, then the bark includesdead phloem and cortex as well ascork, and is termed rhytidome. As bark increasesin thickness, the outer layers maysplit in distinctive patterns. The textureand patterning of the bark is often characteristicof the species. The bark is punctuatedby lenticels, small patches of loosecells, which allow air to penetrate the livingtissues below.basal bodySee kinetosome.base 1. (Chemistry) A compound thatreacts with an acid to produce water plus asalt. In solution, it forms ions that can reactwith hydrogen ions.2. (Biochemistry) A nitrogenous molecule,either a PYRIMIDINE or a PURINE, that combineswith a pentose sugar and phosphoricacid to form a nucleotide, the fundamental25

Benedict’s test A test for the presence ofa dissolved reducing sugar. Benedict’s solutionis an alkaline solution of copper(II)sulfate, which is reduced to insoluble copbasepairingunit of nucleic acids. The most abundantbases are cytosine, thymine, and uracil(pyrimidines) and adenine and guanine(purines).base pairing The bonding relationshipbetween purine and pyrimidine bases in thenucleotides of DNA and RNA. The pairingof these bases, sustained by hydrogenbonding, is highly specific: adenine (A) onone strand of the nucleic acid pairs onlywith thymine (T) in the complementarystrand of DNA, or with uracil (U) in RNA;guanine (G) pairs only with cytosine (C).The large numbers of hydrogen bonds thatform between the base pairs along a DNAor RNA molecule stabilize the doublehelix. The specificity of the pairing ensuresthat during REPLICATION and TRANSCRIP-TION accurate copies are made of the geneticmaterial. The number of base pairs isa measure of the length of the DNA orRNA molecule, which is often expressed inunits of 100 bases (kilobases), meaning100 base pairs.base ratio The ratio of the molar quantitiesof two bases in DNA, or of one basepair with the other base pair. The baseratio of 1:1 for the bases adenine andthymine and the bases cytosine and guaninewas the first clue to the specificity ofBASE PAIRING in the nucleic acids. The basepair ratio, AT:GC, is constant within aspecies but differs between species.basic stainSee staining.basidioma (pl. basidiomata) The fruitingbody of the Basidiomycota, exceptingthe rusts and smuts, formerly called the basidiocarp.It may be of a fleshy, corky, orspongy nature. The basidioma is composedmainly of sterile tightly packed hyphae; inmost species there is a distinct fertile layer,the hymenium, on the surface of the basidiomathat bears the basidial hyphae which,after meiosis, give rise to the basidiospores(see basidium).basidiomyceteSee Basidiomycota.Basidiomycota (basidiomycetes) A phylumof the Fungi that includes the mushroomsand toadstools. Basidiomycotes arecharacterized by their spore-bearing structures(see basidium), each of which usuallybears four sexually produced spores(basidiospores) on its outer surface. Inmost basidiomycotes the basidia are borneon a special fertile layer, the hymenium, onthe surface of a fruiting body, the BASIDI-OMA.See Basidiomycota; basid-basidiosporeium.basidium (pl. basidia) The cell of the BA-SIDIOMYCOTA that produces the sexualspores (basidiospores). In most basidiomycotesthe basidia develop in the BASIDIOMAfrom hyphal tips in the hymenium and maybe club-shaped or cylindrical.basifixed Stamens in which the antherlobes are attached at their base to the filament,and are not capable of independentmovement. Compare dorsifixed; versatile.basipetal Developing from the apexdownward so that the youngest structuresare furthest from the apex. Basipetal differentiationis seen in the formation of protoandmetaxylem in the stem. The term mayalso be applied to the movement of substancestoward the base, for example themovement of auxin in shoot tissues. Compareacropetal. See also centrifugal; centripetal.B chromosome (accessory chromosome)An extra chromosome found in addition tothe normal number of chromosomes characteristicof a species (the A chromosomes).B chromosomes are known from morethan 20 families of plants and are alsofound in some fungi.beet sugarSee sucrose.26

inomial nomenclatureper(I) oxide by reducing sugars, giving ared precipitate.Bentham, George (1800–84) Britishbotanist. Having developed an interest inplant taxonomy from the age of 17, Benthamabandoned (1833) his career in lawto devote himself entirely to his botanicalcollection and library. In 1854 he presentedhis collection to the Royal BotanicGardens at Kew, where he worked for therest of his life. His famous Handbook ofBritish Flora was published in 1858.Between 1862 and 1883 he collaboratedwith Joseph HOOKER to publish GeneraPlantarum, a world flora describing 7569genera and 97 000 species, which is consideredhis greatest achievement.benthic Of, relating to, or living in thezone at the bottom of a sea, lake, or otherbody of water. See benthos. Comparepelagic.benthos The organisms that live on thebottom sediments of freshwater and marineecosystems. They include plants andalgae (phytobenthos) that grow below lowtide mark.berry A succulent fruit, usually containingmore than one seed, that does not burstopen when ripe. The ovary wall remainsfleshy after fertilization of the ovule exceptfor the development of a thin outer skin. Aberry with a hard rind, such as a cucumber,is called a pepo. A berry with a leatheryrind in which the seeds are separated bysegments, as in the citrus fruits, is called ahesperidium. The epicarp and mesocarpmay be brightly colored thus attractinganimals, which disperse the fruit, as in thetomato. Compare drupe. See also pericarp.beta-oxidationSee oxidation.beta-pleated sheet (b -pleated sheet) Atype of protein secondary structure inwhich polypeptide chains run close to eachother and are held together by hydrogenbonds at right angles to the main chain betweenthe NH and C=O groups on onechain and the C=O and NH groups on theother chain. The structure is folded in regular‘pleats’. Fibers having this type ofstructure are usually composed of aminoacids with short side chains. The chainsmay run in the same direction (parallel) oropposite directions (antiparallel). See conformation;secondary structure.bicollateral bundle The arrangementof tissues in the vascular bundles in whichthe phloem is situated on both sides of thexylem. This is thought to aid transport ofnutrients in plants with long trailing stems,such as the marrow. Compare concentricbundle; collateral bundle.biennial A plant that completes its lifecycle within two years. In the first year itproduces foliage only and photosynthesizes.The food is stored during the winterin a swollen underground root or stem. Inthe second year, the stored food is used toproduce flowers, fruits, and seeds. Manyimportant crops, such as carrot andparsnip, are biennials. Some biennials canbe induced to act as annuals and flower inthe first year by appropriate cold or hormonetreatments. Compare annual;ephemeral; perennial.bilateral symmetry The arrangementof parts in an organism in such a way thatthe structure can only be divided into similarhalves (mirror images) along oneplane. Bilateral symmetry is characteristicof most free-moving animals, where oneend constantly leads during movement. Inplants, bilateral symmetry is seen particularlyin flowers (e.g. snapdragon), the conditionbeing termed zygomorphy. See alsoradial symmetry.binomial distribution (normal distribution;Gaussian distribution) The idealizeddistribution of a population ofstatistical values centered around a mean,when departures from the mean are due therandom occurrence of a large number ofsmall independent effects. It represents theprobability of each value occurring and is27

iogeochemical cycle The movementof chemical elements from the physical environmentinto organisms and back to theenvironment in a more or less circularpathway. If the elements concerned are essentialto life, this cycle is called a nutrientcycle, e.g. the NITROGEN CYCLE. Some ofthese cycles are regulated mainly by physicalfactors. For example, the main constrainton the phosphorus cycle is the slowrelease of phosphates from rocks by weathering.Others, such as the nitrogen cycle,involve biological feedback mechanisms,as nitrates are incorporated into proteinsand released by decomposition. In the CARbioassayrepresented on a graph by a bell-shapedcurve or histogram.binomial nomenclature A system ofclassification introduced by Linnaeus, theSwedish botanist, in which each species isgiven two names. The first is the genericname, written with a capital letter, whichdesignates the genus to which the speciesbelongs. The second is the specific name,indicating the species. The generic and specificnames are in Latin and are printed initalic type. For example, the sunflower belongsto the species Helianthus annuus.Helianthus is the generic name and annuusis the specific name. See International Codeof Botanical Nomenclature.bioassay An experimental technique formeasuring quantitatively the strength of abiologically active chemical by its effect ona living organism. For example, the vitaminactivity of certain substances can bemeasured using bacterial cultures. The increasein bacterial numbers is comparedagainst that achieved with known standardsfor vitamins. Plant growth hormonescan be estimated by their effect in causingcurvature of oat coleoptiles.Biochemical Oxygen Demand (BOD)The standard measurement for determiningthe level of organic pollution in a sampleof water. It is the amount of oxygenused by microorganisms feeding on the organicmaterial over a given period of time.Sewage effluent must be diluted to complywith the statutory BOD before it can bedisposed of into clean rivers.biochronology The measurement of geologicaltime using biological events. Particularstrata of rocks can be approximatelydated according to the presence or absenceof certain fossils. The first and last appearancesof particular fossil species representspecific moments in geological time.biodegradable Describing organic compoundsthat can be decomposed by bacteriaand other microorganisms, such as theconstituents of sewage, as compared withnonbiodegradable compounds, such asmost plastics. See also decomposer; pollution.biodiversity The number and variety oforganisms in a given locality, community,or ecosystem. High biodiversity is typicalof complex and highly productive ecosystems,such as tropical rainforests, where asmall area can contain many differentspecies of animals, plants, and other organisms.Biodiversity is often used as an indicatorof the health of such ecosystems.See also Convention on Biological Diversity.bioenergetics The study of energytransfer in living organisms.biogenesis The theory that living thingsoriginate only from other living things asopposed to nonliving matter. The theorybecame accepted as a result of the work ofRedi and Pasteur, who showed that dirt,for example, does not itself produce bacteriaor maggots, but that bacteria and maggotscome only from spores or eggs alreadyexisting in the dirt. This theory satisfactorilyexplains the occurrence of existing organisms,but not the origins of the firstorganisms.biogenic Describing structures producedas a result of the activity of living organisms,in particular rocks (e.g. chalk,limestone, diatomaceous earth) and fossils.28

iosynthesisBON CYCLE, photosynthetic organisms arethought to be important in balancing thecycle by carbon fixation; carbon compoundsare slowly released as dead organismsdecompose.biogeographical barrier A barrier tothe migration of plants and animals thatprevents different populations mixing.Such barriers may be physical, such asmountain ranges, large rivers, lakes, orseas; or they may be climatic.biogeographical region A subdivisionof the Earth’s surface based on the particularspecies of plants and animals present.There are several different schemes of biogeographicalregions, but the most widelyrecognized are: Boreal (northern Eurasiaand North America), Paleotropical (Africaand southeast Asia as far as New Guinea),Neotropical (South America, except the farsouth), Australasian, Cape (South Africa),and Antarctic (New Zealand and southernSouth America). These biogeographical regionsare subdivided into biogeographicalprovinces.biogeography The study of the distributionof plants and animals, both past andpresent, and its interpretation in terms ofecology, in particular the distribution ofworld vegetation types and the interactionsbetween humans and their environment.bioinformatics The creation and maintenanceof databases of biological information.With the rapid expansion ofbiological knowledge and research, sophisticatedcomputerized information-retrievalsystems are necessary to speed up access tobiological information.biological clock The internal mechanismof an organism that regulates CIRCA-DIAN RHYTHMS and various other periodiccycles.biological conservation The preservationof the world’s natural biological diversityand natural habitats and themaintenance, as far as is possible, of theabiotic factors that influence these. Thismay involve human management of a habitat.Modern biological conservation mayalso include sustainable use of biologicalresources, which sometimes conflicts withthe conservation of maximum diversity.See World Conservation Union.biological control The use of naturalpredators or parasites, instead of chemicals,to control pests. The most famoussuccessful example was the introduction ofthe gray moth, Cactoblastis cactorum, intoAustralia to control the prickly pear, Opuntiainermis, which was overrunning vasttracts of land. The moth’s caterpillars eatthe shoots of the plant.biology The study of living organisms,including their structure, function, evolution,interrelationships, behavior, and distribution.biomass The weight or volume of livingorganisms of one particular species perunit area (species biomass), or of all thespecies in the community (community biomass).See also pyramid of biomass.biome A major regional community ofplants and animals with similar life formsand environmental conditions. It is thelargest geographical biotic unit, and isnamed after the dominant type of vegetation,such as tropical rain forest, grassland,or coral reef.biometry The application of statistics tothe analysis of biological phenomena.biorhythm A periodic physiological orbehavioral change that is controlled by abiological clock. Circadian rhythms are anexample.biosphere The part of the earth and itsatmosphere that is inhabited by living organisms.The Earth is made up of threeconcentric zones: the lithosphere (solidearth), the hydrosphere (water layer), andthe atmosphere. The biosphere includesparts of all three of these zones. It is some-29

iosystematicstimes considered to represent a singleecosystem, the ecosphere.biosynthesis Chemical reactions inwhich a living cell builds up its necessarymolecules from other molecules present.See anabolism.biosystematics The area of systematicsin which experimental taxonomic techniquesare applied to investigate the relationshipsbetween taxa. Such techniquesinclude serological methods, biochemicalanalysis, breeding experiments, comparativemorphology, cytological examination,and RNA analysis, in addition to the moreestablished procedures of comparativeanatomy. Evidence from ecological studiesmay also be brought to bear. See also molecularsystematics.biota The plants, animals, and other livingorganisms that live together in a particularplace.biotechnology The application of technologyto biological processes for industrial,agricultural, and medical purposes.For example, bacteria such as Penicilliumand Streptomycin are used to produce antibioticsand fermenting yeasts produce alcoholin beer and wine manufacture. Recentdevelopments in genetic engineering haveenabled the large-scale production of hormones,blood serum proteins, and othermedically important products. Geneticmodification of farm crops, and even livestock,offers the prospect of improved protectionagainst pests, or products withnovel characteristics, such as new flavorsor extended storage properties. See alsogenetic engineering.biotic environment The biological factorsacting on an organism, which arisefrom the activities of other living organisms,as distinct from physical factors.biotin A water-soluble vitamin generallyfound, together with VITAMINS in the Bgroup, in the vitamin B complex. It iswidely distributed in natural foods, eggyolk, kidney, liver, and yeast being goodsources. Biotin is required as a coenzymefor carboxylation reactions in cellular metabolism.biotrophic Obtaining nutrients fromthe living tissues of another organism.Compare autotrophism.biotype 1. A naturally occurring groupof individuals all with the same geneticcomposition, i.e. a clone of a pure line.Compare ecotype.2. A physiological race or form within aspecies that is morphologically identicalwith it, but differs in genetic, physiological,biochemical, or pathogenic characteristics.bipinnate Describing a pinnate leafwhose leaflets are pinnately subdivided,e.g. sensitive plant (Mimosa pudica).bisexualSee hermaphrodite.Biuret test A test for the presence ofproteins, in which sodium hydroxide isadded to the test solution, followed bydrops of 1% copper(II) sulfate solution. Aviolet color indicates the presence of proteinsor peptides (excluding dipeptides).Amino acids have no effect.bivalent Describing any pair of homologouschromosomes when they pair upduring MEIOSIS. Pairing of homologouschromosomes (synapsis) commences at oneor several points on the chromosome and isclearly seen during PACHYTENE of meiosis I.black earthSee chernozem.bladder 1. A modified leaf, found on thestems of members of the bladderwort family,that develops into a distended structurefor trapping small invertebrates. The bladderis thin-walled and closed by a one-wayvalve.2. An air-filled sac found in large numbersin the thalli of certain seaweeds (e.g. bladderwrack).blending inheritanceInheritance in30

acteolewhich the characters of the parents appearto blend into an intermediate form in theoffspring. It is seen in characters controlledby a single gene whose alleles are codominant,as in pink flowers of Antirrhinum,and also in characters such as yield, whichare determined by many genes. Until the rediscoveryof Mendel’s work, it wasthought that all characteristics becomeblended in the offspring. See Mendel’slaws.blepharoplastSee kinetosome.blight A plant disease in which seriousleaf damage appears suddenly. It is usuallydue to fungi or oomycetes, such as thepotato blight (Phytophthora infestans), oroccasionally to bacterial infections, as infireblight of pears, which is caused by Erwiniaamylovora.bloom A sudden growth of microorganismsin a freshwater or marine ecosystem,often known as an algal bloom. This mayoccur naturally in spring, when growth ofmicroorganisms is for a time unchecked byherbivores. It may also be brought on bynutrient enrichment of the water, often dueto fertilizers being washed off agriculturalland, or to seawater entering the marineenvironment.See Cyanobacte-blue-green bacteriaria.An order of basidiomycotefungi having fleshy mushroomshapedfruiting bodies in which the hymenium(spore-bearing surface) lines theinner surface of vertical tubes (pores) onthe underside of the cap (e.g. Boletus).Compare Agaricales.bolting Premature flowering and seedproduction. Biennial plants will sometimesbolt in their first year; this seriously reducestheir yield.borax carmine A dye used to stain nucleifor light microscopy.Boreal floral region (Holarctic floralprovince) A floral region that consists ofnorthern Eurasia and North America. Seefloral province.boron A MICRONUTRIENT found in verylow concentrations in plant tissues but essentialfor plant growth. It is involved inphenol metabolism and membrane function.It is implicated in the uptake of calcium,the development of apical meristems,and pollen germination.botanical 1. Relating to the study ofplants.2. A cultivated plant of an unaltered wildspecies.3. An insecticide derived from a plant, e.g.pyrethrum (from Chrysanthemum flowers).BODSee Biochemical Oxygen Demand.botanyThe scientific study of plants.bog A plant community found in wetacid areas where rainfall is high, the atmosphereis continually moist, and decompositionrates are slow, favoring PEATformation. Bogs are common in uplandsand western areas of north temperate andsome southern temperate regions. Typicalbog plants of north temperate regions includethe bog mosses (Sphagnum), cottongrasses (Eriophorum), rushes (Juncus),sedges (Carex), and certain heaths (e.g.Calluna vulgaris, Erica tetralix).Boletales (boletes)bracket fungi A member of the basidiomycoteorder Aphyllophorales, whosefruiting bodies (basidiomata) typically projectfrom the trunks of trees like shelves ordomed brackets. These fruiting bodies areusually tough, sometimes woody. Thespores are produced on small teeth orlamellae rather than gills.bract A modified leaf that developsbelow a flower or an inflorescence on itsaxis. It may be reduced or highly colored,as in the scarlet bracts of Poinsettia. In thedaisy family the numerous bracts which31

Bryophyta A phylum of simple, mainlyterrestrial, plants called mosses, commonlyfound in moist habitats. They show a heteromorphicalternation of generations, thegametophyte being the dominant generation.When mature the gametophyte showsdifferentiation into stem and leaves butthere are no roots or vascular tissues, butmosses do possess elongated conductingcells. The sporophyte, which is wholly dependenton the gametophyte, is simply aspore capsule borne on a stalk. The sporesgerminate to produce a slender, filamenbractscalesubtend the inflorescence are known as aninvolucre. In the grasses bracts are borne inpairs below the spikelets and are calledglumes. Each individual floret of thespikelet is also subtended by two bracts,the lower being called the lemma and theupper the palea.bracteole A small leaflike organ on thepedicel of a flower in an inflorescence thatis itself subtended by a bract, as in the bluebell.Compare bract.bract scale The structure found in largenumbers in the female cone of gymnosperms,each bearing an ovuliferousscale in its axil.branch A lateral stem arising from anotherstem of the same form.Brassica A genus of plants of the cabbagefamily (Brassicaceae) that includesmany vegetables and fodder plants.Brassicaceae (Cruciferae) A family ofannual, biennial, or perennial plants,mainly herbs, that includes many commerciallyimportant crops, especially the BRAS-SICA species; salad vegetables, such as cress(Sinapis), rocket (Eruca), and watercress(Rorippa); and ornamental plants, e.g.Alyssum, Aubretia, wallflower (Erysimum),stock (Matthiola), and honesty (Lunariaannua).breeding system The way in which aspecies breeds, including the method ofmate selection, genetic determination ofsex and incompatibility, method of mateselection or pollination, and other factorsthat place limits on the genetic diversity ofthe offspring. See self-incompatibility; sexdetermination; sexual reproduction.breeding true (pure breeding) The productionof offspring whose genetic makeupfor particular alleles is identical to that oftheir parents. This requires both parents tobe homozygous for same alleles in question.Brown, Robert (1773–1858) Britishbotanist. Brown studied medicine andserved as a military medical officer. Duringa visit to London he was introduced to SirJoseph Banks, who encouraged him toapply for the post of naturalist on an expedition(1801–06) to survey the coast ofAustralia in the Investigator. Having returnedwith some 4000 plant specimens, hespent the next few years on their classification.His painstaking observations led himto discover many important features ofplants. He was the first to observe andname the nucleus of cells and to recognizeits importance. He also observed that gymnospermovules were not enclosed in anovary wall, leading to the establishment ofone of the basic differences between angiospermsand gymnosperms. In 1827,upon observing a suspension of pollengrains in water, he discovered BROWNIANMOTION.brown algaeSee Phaeophyta.brown earth The type of soil foundunder deciduous forests in temperate climates.It is rich in humus and slightly acid,due to leaching of lime. When cleared, itprovides good fertile agricultural soil.Brownian motion The random motionof microscopic particles due to their continuousbombardment by the much smallerand invisible molecules in the surroundingliquid or gas. Particles in Brownian motioncan often be seen in colloids under specialconditions of illumination.32

uttress roottous haploid protonema, which producesbuds that develop into leafy moss plants.Formerly, the Bryophyta also included theliverworts and hornworts, which in theFive Kingdoms classification are nowplaced in separate phyla (HEPATOPHYTA andANTHOCEROPHYTA). The collective namefor mosses, liverworts, and hornworts isbryophytes.bud 1. A compacted undeveloped shootconsisting of a shortened stem and immatureleaves or floral parts. The youngleaves are folded about the growing tip,and the outermost leaves may be scaly andreduced to protect the growing point. Abud has the potential to develop into a newshoot or flower. Terminal buds are formedat the stem or branch tip. Axillary or lateralbuds develop in the leaf axils. Buds candevelop adventitiously on other parts of aplant and are sometimes a means of asexualreproduction.2. The swelling produced by a unicellularorganism that is reproducing asexually byBUDDING.budding 1. The production of buds onplants.2. A type of grafting in which the graftedpart is a bud.3. A type of asexual reproduction in whicha new individual is produced as an outgrowth(bud) of the parent organism. It occursin some unicellular fungi, such as theyeasts.buffer A solution that resists any changein acidity or alkalinity. Buffers are importantin living organisms because they guardagainst sudden changes in pH. They involvea chemical equilibrium between aweak acid and its salt or a weak base andits salt. In biochemistry, the main buffersystems are the phosphate (H 2 PO – 4 /HPO 2– 4 ) and the carbonate (H 2 CO 3 /HCO – 3 ) systems.bulb A modified shoot that acts as anorgan of perennation and often vegetativereproduction. The stem is reduced to adisklike structure, bearing concentric layersof fleshy leaf bases that comprise thefood store. Each leaf has a bud at its basethat is able to develop into a subsidiarybulb. The whole bulb is protected by scaleleaves and adventitious roots arise fromthe base of the stem. In spring or summerone or more buds grow and produce leavesand flowers, exhausting the food supply.The new leaves photosynthesize and foodis stored in their bases thus giving rise to anew bulb. If more than one bud develops,eventually more than one bulb develops.See perennating organ; asexual reproduction.bulbil A small bulblike organ of asexualreproduction that may form in a leaf axil,as in some lilies, or may replace flowersin an inflorescence, as in the wild leek (Alliumamploprasum). It may detach itselffrom the parent plant and become independent.bundle sheath The ring of parenchymatousor sclerenchymatous tissue, usuallyone cell thick, that surrounds the vascularbundle in an angiosperm leaf. The individualcells are closely packed with noapparent intercellular spaces, and conductwater and solutes from the vascularbundle to the surrounding tissues.Chloroplasts may be present in the bundlesheath, and are thought to be connectedwith starch storage in the tropicalgrasses.bunt (stinking smut) A seed-borne diseaseof wheat (Avena) and other grasses,caused by the basidiomycote Tilletia.bur A kind of pseudocarp (false fruit) inwhich the fruit is encased in a barbed involucre.The barbs catch in the fur of passinganimals and the fruits are thusdispersed. Examples include burdock (Arctium)and cocklebur (Xanthium).butenedioic acidSee succinic acid.33

CCactaceae (cacti) A large family of succulentxerophytic dicotyledonous plants,found mainly in warm dry parts of theAmericas; some species, notably theprickly pears (Opuntia), have been introducedto the Mediterranean and Australia.CaenozoicCainozoicSee Cenozoic.See Cenozoic.calcareous soil (basic soil; alkalinesoil) A soil with a pH greater than 6.0.Calcareous soils occur where lime accumulatesin the surface layers, usually as a resultof a calcareous bedrock such aslimestone, or a calcareous sand depositnear coasts. They are most common in regionsof light rainfall and are usually richin calcium ions and often other elements.See rendzina.calcicole Describing plants that thriveon neutral to alkaline soils rich in calciumcarbonate, i.e. chalk or limestone soils,such as marls. Compare calcifuge.calcifuge Describing plants that growon soils containing very little calcium carbonate,such as sandy or peaty soils andcannot grow on basic substrates such asmost chalk and limestone soils. Comparecalcicole.calcium Symbol: Ca. An essential mineralfor animal and plant growth. One ofits most important functions in plants is asan intracellular messenger with steep gradientsof calcium concentration formingacross membranes with a system of calciumpumps. It is an important constituentof the middle lamella between plant cellwalls as calcium pectates. The calcium ions(Ca 2+ ) cause cross-linking of adjacentchains of pectic acid, thus strengtheningthe cell wall.callose An insoluble polysaccharide thatis laid down around the perforations insieve plates, at wound or infection sites,and by growing pollen tubes. As sieve tubesage, the callose layers become thicker,eventually blocking the SIEVE ELEMENT eitherseasonally or permanently. At woundand infection sites callose provides a physicalbarrier. When pollen germinates on astigma the growing pollen tubes depositcallose as small plugs, but if the plant isself-incompatible, tubes from its ownpollen will deposit more callose, whichthen blocks the tube, preventing fertilization.See self-incompatibility.callus 1. A mass of undifferentiatedparenchyma. If a plant is injured, the surroundinguninjured parenchyma cells forma suberin-impregnated callus, sometimes,sealing off the wound.2. In tissue cultures, cells are first dedifferentiatedto form a callus that can thenbe induced to form adventitious shoots androots. See also graft.calmodulin A Ca 2+ -binding proteinfound in high levels in eukaryote cells. Itmediates many calcium-regulated processesin cells. When bound to calciumions, calmodulin changes shape and bindsto other proteins, such as kinases and phosphatases,that are involved in signalingpathways, activating or inactivating them.See cascade.Calvin, Melvin (1911–97) Americanbiochemist noted for his investigations ofthe DARK REACTIONS of photosynthesis34

Candolle, Augustin Pyrame de(using radioactive carbon). He wasawarded the 1951 Nobel Prize for Chemistry.Calvin cycleSee photosynthesis.calyptra 1. The root cap: a conicalshapedstructure that is formed by the activityof the meristem at the root apex andforms a protective cap around the root tip.It is constantly replaced by newly formedcells as the older tissue is sloughed off withgrowth of the root through the soil.2. A layer of cells derived from the venterof the archegonium that covers the developingsporophyte. In mosses and liverwortsit ruptures as the seta elongates,being taken up as a hood over the capsulein mosses, and forming the sheath of tissueat the base of the seta in liverworts. Thepresence of the calyptra is necessary for theproper development of the capsule inmosses and the embryo in ferns.calyptrogen A layer of meristematiccells covering the root apical meristem insome plants (e.g. grasses) that gives rise toand maintains the root cap.calyx (pl. calices) The outermost part ofa flower, enclosing the other floral partsduring the bud stage. It consists of sepals,often green but sometimes brightly colored.The symbol K denotes the calyx inthe floral formula. If the sepals are joinedat their lateral margins a calyx tube isformed, the mouth of which may be extendedinto lobes or teeth.CAMSee crassulacean acid metabolism.cambium The ring of dividing cells(meristem) responsible for lateral growthin plants. The primary cambium is found inthe stem and root between the phloem andxylem cells, and by division gives rise to thesecondary phloem and xylem in woody dicotyledonousplants. There are two zonesof cambium: the VASCULAR CAMBIUM, whichproduces secondary xylem and phloem,and the PHELLEM (cork cambium), whichproduces secondary cortex and cork orbark. See secondary growth; wood. Seealso annual ring; meristem.Cambrian The earliest period of the Paleozoicera, about 590–510 million yearsago. It is characterized by the appearanceof algae and a proliferation of marine invertebrateanimal forms, but no trueplants. In Britain, Cambrian rocks arefound in Wales and North-West Scotland.See also geological time scale.Camerarius, Rudolph Jacob (1665–1721) German botanist. Camerarius wasprofessor of medicine and director of thebotanic gardens at Tubingen. In his experimentson sexuality in dioecious plants(with male and female flowers on separateplants) and monoecious plants (with maleand female flowers on the same plant), hedemonstrated that if the female flowerswere isolated from the male flowers or themale flowers were removed, the seeds producedby the female flowers were alwayssterile. His results were published in Desexu plantarum (On the Sex of Plants) in1694. He also described the process of pollinationand showed that the stamens werethe male organs and that the stigma andstyle were the female organs of a plant.campylotropous (orthocampylotropous)See ovule.Canada balsam A yellowish resin withsimilar optical properties to glass, used as amounting medium for microscope slides.Candolle, Augustin Pyrame de (1778–1841) Swiss botanist. In 1806 the Frenchgovernment invited Candolle to carry out abotanical and agricultural survey of Francethat took him six years to complete.His Théorie élémentaire de la botanique(Elementary Theory of Botany; 1813) introducedthe term ‘taxonomy’ for classification.Candolle based his classification onresemblances between the plan of symmetryof the sex organs in various plants, eventhough these were often disguised by loss,fusion, or degeneration. His idea was thateven when these parts appeared to be dif-35

carbon Symbol: C. An essential elementin plant and animal nutrition that occurs inall organic compounds and thus forms thebasis of all living matter. It enters plants ascarbon dioxide and is assimilated into carbohydrates,proteins, and fats, forming thebackbones of such molecules. The elementcarbon is particularly suited to such a roleas it can form stable covalent bonds withother carbon atoms, and with hydrogen,oxygen, nitrogen, and sulfur atoms. It isalso capable of forming double and triplebonds as well as single bonds and is thus aparticularly versatile building block. Carcanesugarferent, they were in fact homologous andhad a common ancestry. This classificationreplaced that of LINNAEUS, eventually beingimproved upon by BENTHAM and HOOKER.He is also noted for having recognized theinfluence of the soil on the distribution ofvegetation.cane sugarSee sucrose.canker A localized plant disease inwhich there is considerable NECROSIS of thecortex tissue, especially in woody plants,which becomes surrounded by layers ofcallus tissue. An example is apple canker,caused by the fungus Nectria galligena.canopy In a woodland, forest, or shrubcommunity, the uppermost layer of vegetation,formed from the branches of thetallest plants. Compare ground layer.capillarity (capillary action) The rise orfall of liquids in narrow tubes as a result ofthe surface tension of the liquid, whichcauses the water to adhere to solid surfaces,such as soil particles or the walls ofxylem vessels. When water in the soil or inthe xylem is put under tension, due to theevaporation of water from a particular site(which creates a water potential gradient inthe direction of the evaporation), capillarityenables water to overcome the cohesiveforces within the water and move againstgravity in response to the water potentialgradient. It is a key feature in the movementof water through the xylem andthrough the finer pores of the soil.See inflores-capitulum (pl. capitula)cence.capsid The protein coat of a VIRUS,which surrounds its nucleic acid. Capsidproteins are involved in recognition and infectionof the host and can be used to identifythe virus by serological means oramino acid sequencing.capsule 1. A dry dehiscent fruit that isformed from several fused carpels. The numerousseeds may be released throughpores as in snapdragon (Antirrhinum), orby complete splitting of the capsule (e.g.Iris).2. The structure within which spores of thesporophyte generation of mosses and liverwortsare formed. It is borne at the end ofa stalk (the seta), and ruptures to releasethe spores.3. The mucilaginous covering often foundaround the cell membrane in bacteria.carbohydrates A class of compoundsoccurring widely in nature and having thegeneral formula type C x (H 2 O) y . (Note thatalthough the name suggests a hydrate ofcarbon these compounds are in no way hydratesand have no similarities to classes ofhydrates.) Carbohydrates are generally dividedinto two main classes: SUGARS andPOLYSACCHARIDES. Simple sugars are monosaccharides– single sugar units, usuallywith five (pentoses) or six (hexoses) carbonatoms. Ribose and glucose are examples.Oligosaccharides are polymers of two toten monosaccharide units. Polysaccharidesare long linear or branched chains ofmonosaccharide units, which have a structuralor storage role in the plant, since theytend to be insoluble and relatively unreactive.Carbohydrates are both stores of energyand structural elements in living systems.Living cells are able to build uppolysaccharides from simple units (anabolism)or break the larger units down tomore simple units for releasing energy (catabolism).36

carotenebon, like hydrogen and nitrogen, is farmore abundant in living materials than inthe Earth’s crust, indicating that it must beparticularly suitable to fulfill the requirementsof living processes. See also carboncycle.carbon cycle The circulation of carbonbetween living organisms and the environment.The carbon dioxide in the atmosphereis taken up by autotrophic organisms(mainly green plants) and incorporatedinto carbohydrates. The carbohydrates soproduced are the food source of the heterotrophs(mainly animals). All organismsreturn carbon dioxide to the air as a productof respiration and of decay. The burningof fossil fuels also releases CO 2 . Inwater, carbon, combined as carbonatesand bicarbonates, is the source for photosynthesis.See radiometric dat-carbon-14 datinging.Carboniferous The second most recentperiod of the Paleozoic era, some 355–280million years ago. It is characterized by theevolution on swampy land of amphibians,a few primitive early reptiles, and trees.Aquatic life included sharks and coelacanths.The period is named after the extensivecoal deposits that formed from theremains of vast forests of swamp plants.These were dominated by tree-sized lycophytes(related to present-day clubmosses)and sphenophytes (horsetails), ferns, andsome early seed plants (mainly pteridospermsor ‘seed ferns’) some of whichwhen rapidly buried and compressed overmany millions of years formed the coalmeasures. See also geological time scale.carboxylase An enzyme that catalyzesthe incorporation of carbon dioxide into asubstrate molecule. Carboxylases are foundin yeasts, bacteria, plants, and animals.The most important carboxylase in plantsis ribulose bisphosphate carboxylase (rubisco),which catalyzes the carboxylationof ribulose bisphosphate, the ‘carbonfixing’first step in the Calvin cycle.carboxylic acid An organic compoundof general formula RCOOH, where R is anorganic group and –COOH is the carboxylategroup. Many carboxylic acids are ofbiochemical importance. Those of particularsignificance are: 1. The lower carboxylicacids (such as citric, succinic,fumaric, and malic acids), which participatein the respiration in the KREBS CYCLE,and in CRASSULACEAN ACID METABOLISM.Some plants accumulate high concentrationsof certain carboxylic acids: citrusfruits contain a lot of citric acid, andrhubarb contains oxalic acid.2. The higher acids, which are bound inLIPIDS. These are also called fatty acids, althoughthe term ‘fatty acid’ is often used todescribe any carboxylic acid of moderateto-longchain length. The fatty acids containlong hydrocarbon chains, which maybe saturated (no double bonds) or unsaturated(C=C double bonds). Plant fattyacids are often unsaturated: oleic acid is thecommonest example.carcerulus (pl. carceruli) A type of capsularfruit that breaks up at maturity intoone-seeded segments or nutlets. Thecarcerulus is typical of the Lamiaceae family,e.g. deadnettle (Lamium).carnivorous plant (insectivorous plant)A plant adapted to supplementing photosynthesisby obtaining nutrients, especiallynitrates, from insects and other small animals.The plants have various means oftrapping and killing the insects, which arethen digested by enzymes secreted by theplant. Examples include butterworts (Pinguicula),with slippery inrolling leaves;sundews (Drosera), with sticky glandularhairs that close over the trapped insects;Venus’ fly trap (Dionaea muscipula), withhinged leaves that snap shut over the victim,triggered by sensitive hairs; andpitcher plants, with leaves modified toform pitchers into which small animals falland drown.carotene A carotenoid pigment, examplesbeing lycopene, and a- and b-carotene.The latter compounds are important in an-37

carotenoidsimal diets as a precursor of vitamin A. Red,yellow, or orange carotene pigments occurin all photosynthetic cells, where they functionas accessory pigments by screeningother photosynthetic pigments from photooxidation.Some carotenes give brightcolors to animal-dispersed fruits, e.g. lycopenein tomatoes.carotenoids A group of yellow, orange,or red pigments comprising the carotenesand xanthophylls. They are found in allphotosynthetic organisms, where theyfunction mainly as accessory pigments inphotosynthesis. They contribute, with anthocyanins,to the autumn colors of leavessince the green pigment chlorophyll, whichnormally masks the carotenoids, breaksdown first. They are also found in someflowers and fruits, e.g. tomato. Carotenoidshave three absorption peaks in theblue–violet region of the spectrum.carpel The female reproductive organ ofa flowering plant. It consists of an ovary,containing one or more ovules, a stalk, orstyle (absent in some plant groups), and aterminal receptive surface, the stigma.Each flower may have one or more carpelsborne singly giving an apocarpous gynoeciumor fused together giving a syncarpousgynoecium. The carpel is homologous withthe megasporophylls of certain spore-bearingvascular plants (pteridophytes) and theovuliferous scales of gymnosperms. It hasevolved by fusion of the two edges of themegasporophyll. This development can bemost clearly seen in simple carpels such asthose of the Fabaceae.carrier 1. An organism that carries a recessive,often harmful, gene masked in thephenotype by a normal dominant gene.2. An individual infected with pathogenicmicroorganisms without showing symptomsof disease. Such carriers can transmitthe infection to others.caruncle A fleshy outgrowth from aseed, similar to, but smaller than, an aril.Caruncles are often waxy or oily. Theymay arise from the placenta, micropyle, orfunicle and are seen in castor oil seeds(Ricinus) at the micropyle and in violetseeds (Viola) at the hilum. The oily carunclesof violet seeds attract ants, which helpto disperse them.caryopsis (grain) (pl. caryopses) A dryindehiscent fruit, typical of the grasses. It issimilar to an ACHENE except that the ovarywall is fused with the seed coat.cascade A sequence of biochemical reactions,each triggered by the previous one.Cascades are important in signaling pathways,where extracellular signals (such ashormones or ions) bind with receptors onthe cell membrane, triggering changes inmembrane proteins that activate proteinson the cytoplasmic side of the membrane,setting off a chain of reactions.Casparian strip An impervious band ofthickening on the radial and transversewalls of the endodermis. It consists of depositsof suberin and cutin and ensures thatall the water and solutes entering the stelepass through the cytoplasm of the endodermalcells, thus allowing for selective filteringof solutes and control of flow rate.catabolism Metabolic reactions involvedin the breakdown of complex moleculesto simpler compounds as, forexample, in respiration. The main functionof catabolic reactions is to provide energy,which is stored as ATP, for use in the synthesisof new structures, cell-to-cell signalling,or the movement of motile cells.See also metabolism. Compare anabolism.catalase An enzyme present in bothplant and animal tissues that catalyzes thebreakdown of hydrogen peroxide, a toxiccompound produced during metabolism,into oxygen and water. It also catalyzes theoxidation of substrates by hydrogen peroxide.Catalase is found in the cytoplasm insmall organelles called peroxisomes.catalyst A substance that increases therate of a chemical reaction without beingused up in the reaction. Enzymes are highly38

cell plateefficient and specific biochemical catalysts.They lower the activation energy needed toinitiate reaction by combining temporarilywith the reactants.cation A positively charged ion, formedby removal of electrons from atoms ormolecules. In electrolysis, cations are attractedto the negatively charged electrode(the cathode). Compare anion.catkinC 2 cyclecdlSee inflorescence.See photorespiration.See critical day length.cell The basic unit of structure of all livingorganisms, excluding viruses. Cellswere discovered by Robert HOOKE in 1665,but SCHLEIDEN and SCHWANN in 1839 werethe first to put forward a clear cell theory,stating that all living things were cellular.Prokaryotic cells (typical diameter 1 m m)are significantly smaller than eukaryoticcells (typical diameter 20 m m). The largestplant cells are the fibers of false nettles(Boehmeria nivea), up to 550 mm in lengthand stinging nettles (Urtica dioica) up to75 mm long; the smallest are the epidermalcells of roses (0.04 mm in diameter).Among algae, the internode cells ofstoneworts (Chara spp.) reach lengths upto 80 mm, while the individual independentcells of Chlamydomonas are only0.092 mm in diameter. All cells contain, oronce contained genetic material in the formof DNA, which controls the cell’s growthand some of its activities; in eukaryotes thisis enclosed in the membrane-bounded nucleus.All contain cytoplasm, containingvarious organelles and are surrounded by aplasma membrane. This controls entry andexit of substances. Plant cells and mostprokaryotic cells are surrounded by rigidcell walls. In multicellular organisms cellsbecome specialized for different functions;this is called differentiation. Within thecell, further division of labor occurs betweenthe organelles.cell cycleThe ordered sequence ofphases through which a cell passes leadingup to and including cell division (mitosis).It is divided into four phases G 1 , S, and G 2(collectively representing interphase), andM-phase, during which mitosis takes place.Synthesis of messenger RNA, transferRNA, ribosomes, and protein and generalgrowth of the cell occurs in G 1 , and replicationof DNA and synthesis of histonesoccurs during the S phase, as chromosomesdivide to form chromatids. The materialsrequired for spindle formation are formedin G 2 , when there is a further burst ofgrowth before prophase begins. The timetaken to complete the cell cycle varies indifferent tissues.See meiosis; mitosis; ami-cell divisiontosis.cell fractionation The separation of thedifferent constituents of the cell into homogenousfractions. This is achieved bybreaking up the cells in a mincer or grinderand then centrifuging the resultant liquid.The various components settle out at differentrates in a centrifuge and are thus separatedby appropriately altering the speedand/or time of centrifugation. The componentsare thus separated according to theirsize and weight.cell lineage The theory stating that cellsarise only from pre-existing cells. The celllineage of a structure traces the successivestages that the cells pass through from thetime of their formation in the zygote totheir appearance in the mature functionalstructure.cell membranecellobioseSee plasma membrane.See cellulase.cell plate A structure that appears inlate anaphase in dividing plant cells and isinvolved in formation of a new cell wall atthe telophase stage of mitosis. It is formedby fusion of vesicles from the Golgi apparatus,resulting in a flattened membraneboundedsac spreading across andeffectively dividing the cell. Cell-wall poly-39

cell theorysaccharides and pectic substances containedin the vesicles contribute to growthof the new wall and middle lamella insidethe sac. The process begins at the phragmoplast,a barrel-shaped region at the formersite of the spindle equator, wheremicrotubules remain and the vesicles cluster,possibly oriented by the microtubules.Endoplasmic reticulum and ribosomes arealso present. The cell-plate membraneseventually form the two new plasma membranesof the daughter cells. See cytokinesis.cell theory The theory that all organismsare composed of CELLS and cell productsand that growth and developmentresults from the division and differentiationof cells.See Acrasiomy-cellular slime moldscota.cellulase An enzyme that hydrolyzes1,4-glycosidic linkages in cellulose, yieldingcellobiose (a disaccharide) and glucose.It is important in the degradation of plantcell walls in living plants (e.g. in leaf abscission).cellulose A polysaccharide forming theframework of the CELL WALLS of all plants,many algae and other protoctists, andsome fungi. Cellulose molecules are unbranchedchains that together form a rigidstructure of high tensile strength. Bundlesof molecules form microfibrils, which maybe aligned in the primary cell wall eithertransversely or longitudinally. Celluloseforms an important source of carbohydratein the diets of herbivores, and is a majorconstituent of dietary fiber in human diets.The individual units in cellulose are b-1,4linked D-glucose molecules.cell wall A rigid wall surrounding thecells of plants, fungi, bacteria, and algae.Plant cell walls are made of cellulose fibersin a cementing matrix of other polysaccharides.The walls of some algae differ, e.g.the silica boxes enclosing diatoms, and thecalcareous layer on the cell walls of calcareousalgae. Fungi differ, with their wallsusually containing chitin. The walls ofsome fungal spores contain the pigmentmelanin, which protects against the damagingeffects of ultraviolet radiation. Bacterialwalls are more complex, containingpeptidoglycans – complex polymers ofamino acids and polysaccharides. Cellwalls are freely permeable to gases, water,and solutes. They have a mechanical function,allowing the cell to become turgid byosmosis, but preventing bursting. This contributesto the support of herbaceousplants. Plant cell walls can be strengthenedfor extra support by addition of lignin (asin xylem and sclerenchyma) or extra cellulose(as in collenchyma). They are an importantroute for movement of water andmineral salts. Other modifications includethe uneven thickening of guard cells, thesieve plates in phloem, the suberization ofendodermal cells, the waterproof coveringsof epidermal and cork cells.The cellulose in cell walls takes theform of microfibrils. At cell division inplants the primary wall is laid down on themiddle lamella of the cell plate as a loosemesh of cellulose microfibrils. This givesan elastic structure that allows cell expansionduring growth. Later the secondarywall grows and acquires greater rigidityand tensile strength. New cellulose fibersare laid down in layers, parallel withineach layer, but orientated differently in differentlayers. The Golgi apparatus providespolysaccharide-filled vesicles that depositwall material by exocytosis, guided by microtubules.Cenozoic (Caenozoic; Cainozoic) Thepresent geological era, beginning some 65million years ago, and divided into two periods,the Tertiary and the Quaternary. It ischaracterized by the rise of modern organisms,especially mammals and floweringplants. During this era, the world’s tallestmountain ranges such as the Rockies,Andes, European Alps and Himalayaswere formed, with dramatic effects on climateand plant distribution. See also geologicaltime scale.40

chaparralcentrarch Denoting a protostele inwhich protoxylem is at the center of theaxis. Compare endarch; exarch; mesarch.centric bundleSee concentric bundle.centrifugal Developing from the centeroutwards so the youngest structures are atthe outer edge. Centrifugal xylem differentiatesfrom the center outwards, and a centrifugalinflorescence (e.g. the dichasialcyme) is one in which the progression offlower opening is from the center to theperiphery. Compare centripetal. See alsoacropetal; basipetal.centriole A cell organelle consisting oftwo short tubular structures orientated atright angles to each other. Each tubularstructure is about 500 nm long and 150 nmin diameter. The centriole lies outside thenucleus of animal cells and many fungaland protoctist cells, but is absent in cells ofmost higher plants. Prior to cell division itreplicates, and the sister centrioles move toopposite ends of the cell to lie within thespindle-organizing structure, the centrosome.At the start of cell division, fibrilsappear to radiate from the centrioles,forming starlike ASTERS. These form thepoles of the developing spindle. However,the centriole is not essential for spindle formation,although an analogous structure,the basal body, is responsible for organizingthe microtubules of UNDULIPODIA.centripetal Developing from the outsideinwards so the youngest structures are atthe center. Centripetal xylem differentiatesfrom the outside toward the center, and acentripetal inflorescence (e.g. the capitulum)is one in which the outer flowers openbefore those in the center. Compare centrifugal.See also acropetal; basipetal.centromere The region of the chromosomethat contains the kinetochore, thestructure that becomes attached to the nuclearspindle during mitosis and meiosis. Itcontains DNA of no known significance.Following the replication of chromosomes,resultant chromatids remain attached atthe centromere. The centromere is a specificgenetic locus and remains relativelyuncoiled during prophase, appearing as aprimary constriction. It does not stain withbasic dyes.centrosome A structure found in all eukaryoticcells, except fungi, that forms theSPINDLE during cell division. It lies close tothe nucleus in nondividing cells. Microtubulesextend across the cell from the centrosome,which controls the distributionand assembly of the microtubules thatmake up the cytoskeleton, thus controllingthe cell’s shape and, in some cells, movement.The centrosome probably plays animportant role in the reorganization of thecell that occurs during differentiation. Atthe start of cell division the centrosome divides,and the sister centrosomes move toopposite ends of the cell, organizing thespindle. In animal and protoctist cells thecentrosome contains two short barrelshapedstructures, the CENTRIOLES, butthese are not directly involved in spindleformation. In fungi, the function of thecentrosome is served instead by the spindlepole body.chalaza (pl. chalazae or chalazas) Theregion of an angiosperm ovule where thenucellus and integuments merge. Whenovule orientation is orthotropous the chalazacorresponds to the point where the funicleis attached but in anatropous andcampylotropous ovules the chalaza is somedistance from the funicle.chalazogamy A method of fertilizationin angiosperms in which the pollen tube entersthe ovule by the chalaza instead ofthrough the micropyle, penetrating the placentaen route. Chalazogamy is seen in certaintrees and shrubs, e.g. beech. Compareporogamy.chamaephyte A perennial plant that isable to produce new growth from restingbuds near (within 0.25 m of) the soil surface.Chamaephytes are usually smallbushes (e.g. heather (Calluna vulgaris)).See also Raunkiaer’s plant classification.41

Charophyceaechaparral Sclerophyllous scrub vegetationof coastal and inland mountain regionsof southwestern North America,maintained by burning and grazing and, insome areas, possibly from past land clearance.It consists of dense thickets of broadleavedevergreen bushes and shrubs andtwisted dwarf trees up to 2.5 meters (8 ft)tall. The dominant plants are sages (Lamiaceae)and evergreen oaks and, in drierareas, chamise (Adenostoma fasciculatum),manzanita (Arctostaphylos pungens),and species of Ceanothus. Seemaquis. Compare garrigue.Charophyceae A class of the Chlorophyta(green algae) that includes thestoneworts, formerly placed in a separatephylum Charophyta. They resemble trueplants in many ways and are thought to bethe most closely related algal group toplants.CharophytaSee Charophyceae.chasmogamy The production of flowersthat open their petals so that cross pollinationis possible. Compare cleistogamy.See also anemophily; entomophily.chelation The process of forming a stablechemical complex (chelate) with agiven substance, usually a metal ion, thatinvolves the formation of at least twochemical bonds. Chelates are usually organicring compounds, the ion beingchelated to form the closure of the ring.Chelating agents can be used to removetoxic metal ions from soil or water. Someplants achieve tolerance to toxic ions bychelating them, usually by combinationwith an organic compound, such as citrateor malate; the chelate is then deposited inthe vacuole. Chlorophyll is also a chelate,the porphyrin part of the molecule beingbound to iron.chemical fossils Particularly resistantorganic chemicals present in geologicalstrata that are thought to indicate the existenceof life in the period when the rockswere formed. Chemical fossils (e.g. alkanesand porphyrins) are often the only evidencefor life in rocks of PRECAMBRIAN age.chemiosmotic theory The theory proposedby Peter Mitchell in 1961 to explainhow electron transport and ATP synthesisare coupled during photosynthesis and respiration.It requires a membrane that permitsthe movement of protons from insideto outside, but not vice versa. An energyrequiring‘proton pump’ (the ELECTRON-TRANSPORT CHAIN) causes movement ofprotons (H + ions) from the internal matrixof the organelle to the outside, creating apH gradient across the membrane. As theH + ions diffuse back across the membrane.This gradient of potential energy drives reactionsat specific sites on the membrane,where the enzyme ATPase uses the energyto drive ATP synthesis, so the energy canbe transferred to reaction sites elsewhere.chemoautotrophism (chemosynthesis)See autotrophism; chemotrophism.See chemotro-chemoheterotrophismphism; heterotrophism.chemoreceptor An organ or structurethat responds to a particular chemicalstimulant.See autotrophism; che-chemosynthesismotrophism.chemotaxis (chemotactic movement) ATAXIS in response to a chemical concentrationgradient. The spermatozoids of primitiveplants are often positively chemotactic,swimming toward the female organs in responseto a chemical secreted by the latter.For example, the archegonium (femaleorgan) of the moss Funaria secretes sucrose.Motile organisms may also be negativelychemotactic, swimming away fromconcentrations of toxic chemicals.chemotaxonomy (biochemical systematics;biochemical taxonomy) A system ofclassification of plants based on the natureof their metabolic products (metabolites).Techniques involved include chromatogra-42

chlorinated hydrocarbonphy and electrophoresis, and the sequencingof nucleic acids and proteins.chemotrophism A type of nutrition inwhich the source of energy for the synthesisof organic requirements is chemical.Most chemotrophic organisms are heterotrophic(i.e. chemoheterotrophic) andtheir energy source is always an organiccompound; animals, fungi, and most bacteriaare chemoheterotrophs. If autotrophic(i.e. chemoautotrophic or chemosynthetic)the energy is obtained byoxidation of an inorganic compound; forexample, by oxidation of ammonia to nitriteor a nitrite to nitrate (by nitrifyingbacteria), or oxidation of hydrogen sulfideto sulfur (by colorless sulfur bacteria).Only certain specialized bacteria are chemoautotrophic.Compare phototrophism.chemotropism (chemotropic movement)A TROPISM in which the stimulus is chemical.The hyphae of certain fungi (e.g.Mucor) are positively chemotropic, growingtoward a particular source of food.Pollen-tube growth down the style is bothpositively and negatively chemotropic: thetube grows toward chemicals secreted bythe pistil, and away from atmospheric oxygen.chernozem (black earth) A freely drainingCALCAREOUS SOIL found in regions withhigh summer rainfall, characterized byhumus distributed fairly evenly throughoutthe profile. The upper horizon of the soil isvery thick and black with humus; it mergesinto a brown lime-rich horizon below.Chernozems are rich in plant nutrients andhave a good crumb structure, allowingdrainage and aeration. They are associatedwith grasslands in temperate climates suchas the prairies of North America, the pampasof South America, and the Russiansteppes, where they support highly productiveagricultural systems.chiasma (pl. chiasmata) A connectionbetween homologous chromosomes seenduring the prophase stage of meiosis. Chiasmatarepresent a mutual exchange ofmaterial between homologous, nonsisterchromatids (CROSSING OVER) and provideone mechanism by which RECOMBINATIONoccurs, through the splitting of linkagegroups. Chiasmata also help to hold homologstogether until the start ofanaphase, thus helping to ensure the correctdistribution of chromosomes into thedaughter nuclei.chimera An individual or part of an individualin which the tissues are a mixtureof two genetically different tissues. It mayarise naturally due to mutation in a cellof a developing embryo, producing a lineof cells with the mutant gene, andhence different characteristics compared tosurrounding cells. It may also be inducedexperimentally. In plants, chimaeras producedfrom two different species areknown as graft hybrids. For example, abud may develop at the junction betweenthe scion and stock with a mixture of tissuesfrom both.chi-squared test A statistical test usedto compare observed frequencies of experimentalsampling results with expected(hypothetical) frequencies, usually calculatedfrom the results assuming no effect ofthe experimental variable. It is suitable forobservations that fall into discrete classes.The value:(observed – expected) 2 /expectedis calculated for each class, then the valuesare added together. The sum is checkedagainst a table of c 2 values at various degreesof freedom to determine the degree ofsignificance of the difference between observedand expected values is significant.chitin A nitrogen-containing polysaccharidefound in the cell walls of mostfungi and in some animals. Chitin is a polymerof N-acetylglucosamine. It consists ofmany glucose units, in each of which one ofthe hydroxyl groups has been replaced byan acetylamine group (CH 3 CONH).chlamydospore A thick-walled restingspore produced by certain fungi (e.g.43

chlorineChlorophyll: the structural formula of chlorophyll asmuts) and funguslike protoctists (e.g.Mucor).ChlorellaSee Chlorococcales.chlorenchyma A form of parenchymain which the cells contain many chloroplastsand relatively large intercellularspaces, as in the mesophyll of the leaf.chlorinated hydrocarbon A hydrocarbonin which one or more hydrogen atomsare replaced by chlorine atoms. Some chlorinatedhydrocarbons are used as insecticides,e.g. aldrin, dieldrin, and DDT.chlorine Symbol: Cl. An element foundin trace amounts in plants. The chlorideion (Cl-) can pass easily through cell membranes,and is involved in the maintenanceof osmotic and ionic balance in the plant.Chlorococcales An order of Chlorophyta(green algae) that contains singlecelledand colonial algae that lackundulipodia, e.g. Chlorella, Volvox, foundin freshwater plankton and most terrestrialhabitats. Only the gametes and zoosporesare motile. They reproduce asexually bymeans of zoospores or nonmotile sporesand sexually by fusion of similar-lookinggametes (isogamy).chlorophylls A group of photosyntheticpigments. They absorb blue-violet and redlight and hence reflect green light, impartingthe green color to green plants. Themolecule consists of a hydrophilic (waterloving)head, containing magnesium at thecenter of a porphyrin ring, and a long hydrophobic(water-hating) hydrocarbon tail(the phytol chain), which anchors the moleculein the lipid of the membrane. Differentchlorophylls have different chemicalgroups attached to the head. Chlorophyllsare located in the chloroplasts, and are involvedin the light reactions of photosynthesis,absorbing light energy and initiatingelectron transport. There are four kinds ofchlorophyll, a, b, c, and d, which differ intheir molecular structure. Chlorophyll a isuniversal in all photosynthesizing eukaryotes.It converts the absorbed light energyto chemical energy. Chlorophylls b, c, and44

chromatin loopd are accessory pigments. Chlorophyll boccurs in plants and in the Chlorophyta;the other chlorophylls occur in certaingroups of algae. See absorption spectrum;photosynthetic pigments. See also bacteriochlorophyll.Chlorophyta (green algae) A phylum ofprotoctists comprising mainly freshwateralgae with some marine and terrestrialforms. They contain the pigments chlorophylla and b together with carotenes andxanthophylls. The Chlorophyta store foodas starch and fat and have cell walls containingcellulose and hemicellulose. Someof the commoner orders include: Volvocales,unicellular and colonial plants, e.g.Chlamydomonas, Volvox; Chlorococcales,including unicellular and colonial plants,e.g. Chlorella, Pediastrum; Ulotrichales,filamentous, and thallose plants, or coenocytictubes, e.g. Ulothrix, Ulva, Enteromorpha;and Oedogoniales, e.g. thefilamentous Oedogonium.chloroplast A photosynthetic PLASTIDcontaining chlorophyll and other photosyntheticpigments. It is found in all photosyntheticcells of plants and protoctists butnot in photosynthetic prokaryotes. It has amembrane system containing the pigmentsand on which the light reactions of PHOTO-SYNTHESIS occur. The surrounding gel-likeground substance, or stroma, is where thedark reactions occur. The typical higherplantchloroplast is lens-shaped and about5 m m in length. Various other forms existin the algae, e.g. spiral in Spirogyra, stellatein Zygnema, and cup-shaped in Chlamydomonas.The number per cell varies, e.g.one in Chlorella and Chlamydomonas andin most hornworts (Antherocyta), two inZygnema, and about one hundred in palisademesophyll cells of leaves.Chloroplast membranes form elongatedflattened fluid-filled sacs calledthylakoids. The sheetlike layers of the thylakoidsare called lamellae. In all plants exceptalgae, the thylakoids overlap atintervals to form stacks, like piles of coins,called grana. In this way the efficiency ofthe light reactions seems to be improved.On the thylakoid membranes are sphericalstructures partially embedded in the membrane.These are the photosynthetic units(formerly called quantasomes), which containthe pigments and associated lipids andproteins of photosystems I and II. PhotosystemI units are about 17.5 nm in diameter,and photosystem II units about 11 nm.Other, smaller particles on the thylakoidmembranes are ATPases. The stroma maycontain storage products of photosynthesis,e.g. starch grains. Apart from enzymesof the dark reactions, the stroma also containstypical prokaryotic protein-synthesizingmachinery including circular DNA (seechloroplast DNA) and smaller ribosomes.There is now strong evidence that chloroplastsand other cell organelles, such asmitochondria, represent prokaryotic organismsthat invaded heterotrophic eukaryoticcells early in evolution and arenow part of an indispensable symbioticunion (see endosymbiont theory). In C 4plants there are two types of chloroplast.chloroplast DNA (cp-DNA) The DNAfound in a chloroplast. It is in circularform, rather like that of prokaryotes. Itcodes for only some of the proteins neededby the chloroplast.chlorosis The loss of chlorophyll fromplants resulting in yellow (chlorotic)leaves. It may be the result of the normalprocess of senescence, lack of key mineralsfor chlorophyll synthesis (particularly ironand magnesium), or disease.See Schultze’s solu-chlor-zinc-iodidetion.chromatid One of a pair of replicatedchromosomes found during the prophaseand metaphase stages of mitosis and meiosis.During mitosis, sister chromatids remainjoined by their centromere untilanaphase. In meiosis it is not untilanaphase II that the centromere divides,the chromatids being termed daughterchromosomes after separation.chromatin The loose network of45

chromatographythreads seen in nondividing nuclei that representsthe chromosomal material, consistingof DNA, protein (mainly histone), andsmall amounts of RNA. It is classified aseuchromatin or heterochromatin on thebasis of its staining properties, the latterstaining much more intensely with basicstains because it is more coiled and compact.Euchromatin is thought to be activelyinvolved in transcription and thereforeprotein synthesis, while heterochromatin isinactive. Euchromatin stains more intensivelythan heterochromatin during nucleardivision. See also chromosome.chromatin loop A region of uncoiledDNA, usually about 200 nm long, that extendsout in a loop from the chromosome.This uncoiled condition is thought to facilitateTRANSCRIPTION and probably indicatesan area where genes are active.chromatography A method of analyzingmaterials involving the separation byselective absorption of the various compoundsas identifiable bands. For instance,a mixture of substances in solution ispassed slowly down a long column packedwith alumina. The different compoundsmove at different rates and separate intobands. The medium, together with thebands of separated material, are collectivelycalled a chromatogram.In general chromatography involves atest material being carried by a movingphase (liquid or gas) through a stationaryphase (solid or liquid). Different substancesmove at different rates (dependingon their absorption–desorption) and aretherefore spatially separable, the leastreadily absorbed being carried the farthest.Colorless materials can be used if somemeans of detecting them is used (electronicdetection, radioactive labeling, or ninhydrindeveloper). See also paper chromatography;thin-layer chromatography; gaschromatography; gel filtration.chromatophore A name generally appliedto a pigment-bearing structure, usuallya membrane lamella or vesicle.chromomere A small beadlike deeplystaining structure seen in chromosomesduring prophase of mitosis and meiosis. Itis thought to represent an area where thechromosome is relatively tightly coiled.Homologous chromosomes have chromomeresin identical places along their length.chromoplast (chromatophore) A coloredplastid, i.e. one containing pigment.They include chloroplasts, which containthe green pigment chlorophyll and aretherefore photosynthetic, and nonphotosyntheticchromoplasts. The term is sometimesconfined to the latter, which are bestknown in flower petals, fruits (e.g.tomato), and carrot roots. They are yellow,orange, or red owing to the presence ofcarotenoid pigments.chromosome 1. One of a group ofthreadlike structures of different lengthsand shapes in nuclei of eukaryotic cells.They consist of DNA with RNA and protein(mostly histones) and carry the GENES.During nuclear division the chromosomesare tightly coiled and are easily visiblethrough the light microscope. After division,they uncoil and may be difficult tosee, the long narrow threads of uncoiledDNA being only about 2 nm in diameter.Along the length of the uncoiled chromosomesare beadlike structures called nucleosomes,highly organized aggregationsof DNA and histones.The number of chromosomes per nucleusis characteristic of the species exceptin aneuploid plants. Normally one set(haploid) or two sets (diploid) of chromosomesare present in the nucleus. However,many or perhaps a majority of plants arepolyploid in origin, i.e. they have multiplesets of chromosomes, though most act asdiploids in their reproduction. In somespecies there are diploid and polyploid varietiesthat look almost identical, thoughsometimes they may be referred to as separatespecies. In bryophytes the main plantbody is haploid, as is the case in manyalgae (see alternation of generations). Inearly prophase of mitosis and laterprophase of meiosis, the chromosomes46

ciliumsplit lengthwise into two identical chromatidsheld together by the CENTROMERE.The length of the arms of the chromosomeis determined by the position of thecentromere. The centromere contains thekinetochore, a structure involved in the attachmentof the chromosomes to the spindleduring nuclear division. In diploid cells,there is a pair of sex chromosomes; the remainderare termed autosomes. Each chromosomecontains one DNA molecule,which is folded and coiled. Histones play amajor architectural role, holding the coiled(helical) DNA in supercoils or beads, about10 nm in diameter and consisting of about170 base pairs. These in turn are coiled toform a hollow fiber, 30 nm in diameter,which itself forms a hollow coil measuringabout 240 nm in diameter. It is this that isvisible as the typical looped threadlikechromatin material of the nondividingchromosome. See chromomere; chromosomemap.2. See bacterial chromosome.chromosome map (genetic map) A diagramshowing the order of genes along achromosome. Such maps have traditionallybeen constructed from information gainedby linkage studies to give a linkage map.The techniques employed differ accordingto the type of organism being studied.Many plants and animals can be crossedexperimentally to study inheritance patternsof particular genes. However, the adventof new molecular techniques hasdramatically changed the nature of chromosomemapping in all organisms. Thegrowing science of PROTEOMICS seeks to analyzethe genome by starting with the proteinproducts of genes and working back tothe genome.Mapping a genome involves severalsteps. The first is to assign each gene toa particular chromosome. This can beachieved by, for example, somatic-cellhybridization or using a gene probe. Thenext step is to determine the relativepositions of the genes on a particular chromosome.This involves comparing RESTRIC-TION FRAGMENT LENGTH POLYMORPHISMSbetween individuals and constructing alinkage map of all restriction sites, i.e. sitesthat are cleaved by restriction enzymes.These restriction sites can then be used asmarkers for closely neighboring genes. Thelast step is to construct a physical map ofthe base sequence of the chromosomalDNA. One approach uses cloned DNAsegments obtained from a gene library ofthe chromosome. These clones can be fittedtogether to form a series of overlappingsegments (contig) that corresponds to aparticular region of the chromosome. Thebase sequence of the contig is then determined,and hence the base sequence of thechromosomal DNA. See gene library; restrictionmap.chromosome mutation A change inthe number or arrangement of genes in achromosome. If chromosome segmentsbreak away during nuclear division theymay rejoin the chromosome the wrongway round, giving an inversion. Alternatively,they may rejoin a different part ofthe same chromosome, or another chromosome,giving a translocation. If the segmentbecomes lost, this is termed a deficiency ordeletion; it is often fatal. A part of a chromosomemay be duplicated and occur eithertwice on the same chromosome or ontwo different nonhomologous chromosomes:this is a duplication. Chromosomemutations can occur naturally but their frequencyis increased by the effect of x-raysand chemical mutagens. Such aberrationsmay be visible by microscopy, especiallywith the use of certain stains. See also genemutation.chromosome number The number ofchromosomes possessed by a particularspecies. It may be expressed as either thehaploid (n) or diploid (2n) number. Formany species, chromosome number is auseful taxonomic character. Exceptions includespecies exhibiting APOMIXIS. Chromosomenumber also gives an indication ofpolyploidy, where numbers vary by exactmultiples between related species.Chrysomonada A phylum of goldenbrownalgae, formerly called the Chryso-47

circadian rhythmphyta, common in fresh temperate waters,with a few species in marine waters.Chrysomonads are mainly motile unicellularalgae, but there are some colonial formsand a few complex filamentous forms.ciliate 1. Describing a structure, such themargin of a leaf, fringed with fine hairs.2. A unicellular protoctist of the phylumCiliophora, whose members are usuallycovered in short undulipodia (cilia) andhave two different kinds of nuclei, micronucleiand macronuclei.cilium (pl. cilia) A whiplike extension ofcertain eukaryotic cells that beats rapidly,thereby causing locomotion or movementof fluid over the cell. Cilia are short UN-DULIPODIA, 2–10 m m long.The beat of each cilium comprises an effectivedownward stroke followed by agradual straightening (limp recovery). Ciliabeat in such a way that each is slightly outof phase with its neighbor (metachronalrhythm), thus producing a constant ratherthan a jerky movement or flow of fluid.The basal bodies of cilia are connected bythreadlike strands (neuronemes), which coordinatethe beating of neighboring cilia.circadian rhythm (diurnal rhythm) Adaily rhythm of various metabolic activitiesin animals and plants. Such rhythmspersist even when the organism is not exposedto 24-hour cycles of light and dark,and are thought to be controlled by anendogenous biological clock. Circadianrhythms are found in the most primitiveand the most advanced of organisms. ThusEuglena shows a diurnal rhythm in thespeed at which it moves to a light source.Stomata open and close in a daily cycle,which persists for several days in totaldarkness. Experiments have failed to revealthe type and location of the control mechanismsinvolved. See also biorhythm.circinateSee vernation.circumnutation The spontaneous spiralgrowth of the shoot tips of certainplants, particularly climbers. The directionof rotation is often constant in a species;for example, Convolvulus always rotatesin an anticlockwise direction. For climbersnutation increases the likelihood of a solidsupport being found. It is also marked inmany leaf tendrils and to a lesser extent inroots, flower stalks, and the sporangiophoresof some fungi. Gravity has beenshown to be involved in the response: if aplant is turned upside down, the tip and thelast few spirals unwind and straighten outbefore winding up in the characteristic wayin the new direction.cisterna (pl. cisternae) A flattened membrane-boundedsac of the endoplasmicreticulum or the Golgi apparatus, being thebasic structural unit of these organelles.cis–trans effect (complementation test)The phenomenon resulting from recombinationwithin a gene (cistron), in which amutation is only expressed in the phenotypeif the mutant pseudoalleles are on differenthomologous chromosomes.cis–trans test A test that determineswhether two mutations that have the sameeffect occur in the same gene or in differentgenes. The mutations may be in either thecis position (i.e. on the same chromosome)or the trans position (one on each homolog).If the mutations are in differentgenes then a normal phentotype resultswhether the mutations are in the cis ortrans position, since they are masked bycorresponding dominant genes on theother homolog. However, if the mutationsare in the same gene then a normal phenotypewill result only if the mutations are inthe cis position. In the trans position themutant phenotype is expressed, since bothalleles of the locus are mutants.cistron A unit of function, i.e. a segmentof DNA that determines a single polypeptidechain of a protein molecule or a messengerRNA molecule. Its boundaries maybe defined by the cis–trans test.CITES The Convention on InternationalTrade in Endangered Species of WildFauna and Flora: an agreement signed betweensome 160 countries, prohibiting48

cleavage polyembryonycommercial trade in endangered speciesand products derived from them. As morespecies have become endangered, the list ofprotected species has lengthened. CITESsets out degrees of protection for differentspecies, allowing threatened but not endangeredspecies to be exported and importedunder license.citric acid A six-carbon carboxylic acid,occurring in the juice of citrus fruits, particularlylemons, and present in manyother fruits. Citric acid is biologically importantbecause it participates in the KREBSCYCLE, where it is formed by the condensationof acetyl CoA and oxaloacetate,a rate-limiting step of the cycle. The systematicname is 2-hydroxypropane-1,2,3-tricarboxylic acidcitric acid cycleSee Krebs cycle.cladistics A method of classification inwhich the relationships between organismsare based on selected shared characteristics.These are generally assumed to havebeen derived from a common ancestor, inthe evolutionary process of cladogenesis,although the ‘transformed cladists’ believethat shared characteristics alone providea logical basis for classification withoutpostulating evolutionary relationships. Thepatterns of these shared characteristics aredemonstrated in a branching diagramcalled a cladogram. The branching pointsof the cladogram may be regarded either asan ancestral species (as in an evolutionarytree) or solely as representing shared characteristics.cladode A modified internode of thestem that functions as a leaf, being flattenedand highly photosynthetic. It may bea xerophytic adaptation and is seen inbutcher’s broom (Ruscus aculeatus) andprickly pear (Opuntia). The true leavesmay be reduced to spines or scales. Unlikea leaf, a cladode has buds on its surface.cladogramCladophoralesSee cladistics.An order of Chlorophyta(green algae), generally calledsiphonaceous algae. They are multicellularsaclike forms or branched filaments withbranching rhizoidlike holdfasts, e.g. Cladophora.class A taxonomic rank that is subordinateto a phylum (or sometimes a divisionin plant taxonomy) and superior to anorder. The Latin names of plant classes endin –ae, e.g. Dicotyledonae. Classes may bedivided into subclasses, with the Latin ending–idae, e.g. Rosidae.classification The grouping and arrangementof organisms into a hierarchicalorder. The aim is usually to aid in identificationof organisms or represent their phylogeneticrelationships, or, ideally, both.An important aspect of classifications istheir predictive value. For example if acharacteristic is found in one member of agroup of plants, then it is also likely to befound in the other members of that groupeven though the characteristic in questionwas not used in the initial construction ofthe classification.Artificial classification systems are designedto aid identification, and may notreflect true relationships. e.g. LINNAEUS’ssystem, which was based on the number ofstamens and carpels in the flower. Naturalclassification systems are based on morecharacteristics, and thus thus are better forpredicting relationships. Phenetic classificationsystems are based on many characters,and may also be weighted in favor ofparticular characters. Phylogenetic classificationsystems attempt to reflect the evolutionaryrelationships of taxa. The validityof these systems is now being tested bymRNA studies. Compare cladistics. Seetaxon; phylum; class; order; family; genus;species.clay Extremely fine-textured SOIL madeup of small mineral particles, less than0.002 mm in diameter, formed mainlyfrom aluminum and magnesium silicates.Clay soils become very sticky and difficultto work when wet and can easily becomewaterlogged. Nutrient availability to49

cleistogamyplants can be a problem as the nutrientsmay become chemically bound to the surfacesof the particles.clearing 1. In the preparation of permanentmicroscope slides, the stage betweendehydration and embedding, the purposeof which is to remove the dehydratingagent and replace it by a substance that ismiscible with the embedding substance.Clearing also renders the tissues transparent.Clearing agents include benzene andxylene.2. In the preparation of cell material forstaining, the dissolving away of cell contentsso that the distribution of tissues maybe better observed, using reagents such assodium hypochlorite.3. A treeless, usually grassy area in a woodlandor forest.cleavage polyembryony The splittingof an embryo into several identical parts,each of which may develop into a matureembryo. It is a form of natural cloning.Cleavage polyembryony occurs in somegymnosperms, e.g. in Pinus, but usuallyonly one embryo develops to maturity. Seeclone.cleistogamy The production of closedflowers. It is a method of ensuring selfpollinationand occurs toward the end ofthe flowering season in certain plants, e.g.wood sorrel (Oxalis acetosella) particularlywhen little or no seed has been set bythe cross-pollinating flowers. Comparechasmogamy.See as-cleistothecium (pl. cleistothecia)coma.climacteric The rise in respiration rate,found in the fruits of some species, associatedwith fruit ripening and senescence. Itmay be induced artificially by treatmentwith ethylene (ethene).climatic climax A plant communitythat is in equilibrium with a stable climate.It represents the CLIMAX of a succession.climatic factors Aspects of climate,such as temperature, rainfall, humidity, airmovement, and light, that affect living organisms.Compare biotic environment;edaphic factors.climax The final community in a successionof natural plant communities in onearea under a particular set of conditions.Mixed woodland with oak is the naturalclimax vegetation in much of lowlandBritain, but on chalk it is often beechwood.A climax is self-perpetuating, at least for atime, although there are constant smallchanges, e.g. when a tree dies pioneeringplants may invade or the tree may be replacedby a different species. Succession toa climax can be held at any stage by humanintervention, such as grazing on chalkdownland. Such an equilibrium is called asubclimax, plagioclimax, disclimax, or bioticclimax. See clisere.climbing ferns Ferns of the familySchizaeaceae, found in most parts of theworld in warm and tropical regions, includingNorth America. There are fourgenera: Anemia, Lygodium, Mohria, andSchizaea. Many climbing ferns have longcreeping rhizomes.cline A graded series of characters, usuallymorphological, exhibited by a speciesor other related group of organisms, usuallyalong a line of environmental or geographicaltransition. The populations ateach end of the cline may be substantiallydifferent from one another.clinostatSee klinostat.clisere A succession of climax communitiesin a given area, each giving way to thenext as a result of climatic changes. Seealso climax; sere.clone A group of organisms or cells thatare genetically identical. In nature, clonesare derived from a single parental organismor cell by mitotic cell division, asexualreproduction, or parthenogenesis. In geneticengineering, multiple identical copies50

coleoptileof a gene are produced in cloning vectors,such as plasmids and phages. See genecloning. See also apomixis.clubmossSee Lycophyta.clubroot A serious disease of the cabbagefamily (Brassicaceae) caused by Plasmodiophorabrassicae (phylum Plasmodiophora),in which the roots becomeswollen and misshapen, with characteristicgall-like swellings. Above-ground symptomsinclude wilting in bright sunlight anda reddish, bluish, or yellowish tinge to theleaves.CoA See coenzyme A.coadaptation The evolution and maintenanceof certain genetic characteristicsthat increase the effectiveness of a relationshipbetween two species. Flower/pollinatorrelationships are an example, in whichadaptations of flowers for pollination byanimals is to the mutual advantage of bothspecies (coevolution). See mutualism.cobaltSee micronutrient.coccus (pl. cocci) A spherical-shapedbacterium. Cocci may be found singly, inpairs (e.g. Diplococcus), or chains (e.g.Streptococcus), or in regularly or irregularlypacked clusters.See incomplete domi-co-dominancenance.codon A group of three nucleotide bases(i.e. a nucleotide triplet) in a messengerRNA (mRNA) molecule that codes for aspecific amino acid or signals the beginningor end of the message (start and stopcodons). Since four different bases arefound in nucleic acids there are 64 (4 ´ 4 ´4) possible triplet combinations. Thearrangement of codons along the mRNAmolecule constitutes the genetic code.When synthesis of a given protein is necessarythe segment of DNA with the appropriatebase sequences is transcribed intomRNA. When the mRNA migrates to theribosomes, its string of codons is pairedwith the anticodons of transfer RNA molecules,each of which is carrying one of theamino acids necessary to make up the protein.Most amino acids are coded for bymore than one codon. The genetic code istherefore said to be degenerate. The termcodon is sometimes also used for thetriplets on DNA, which are complementaryto those on mRNA, but contain thebase uracil instead of thymine. See transferRNA.coenocyte An area of cytoplasm containingmany nuclei and enclosed by a cellwall, typically found in many fungi, certaingreen algae (e.g. Cladophora), and somemembers of the Xanthophyta. Such a cell isdescribed as coenocytic.coenzymeSee cofactor.coenzyme A (CoA) A coenzyme that isimportant in the synthesis and reactions offatty acids. In the KREBS CYCLE it combineswith pyruvic acid, leading to loss of carbondioxide.coenzyme Q (ubiquinone) A coenzymethat is an essential component of the respiratoryelectron-transport chain.cofactor (coenzyme) A nonprotein substancethat helps an enzyme to carry out itsactivity. Cofactors may be cations or organicmolecules, known as coenzymes.Many enzymes require metal cofactors,e.g. ferredoxin requires ferrous and ferricions. The metal ion may be the catalyticcenter of the cofactor, or it may serve tobind the enzyme and substrate together tohold the enzyme in a catalytically activeconformation. Unlike enzymes cofactorsare, in general, stable to heat. When a catalyticallyactive enzyme forms a complexwith a cofactor a holoenzyme is produced.An enzyme without its cofactor is termedan apoenzyme. See also prosthetic group.cohesion theory A theory to explainhow sap rises up the xylem. It suggests thatthe lowering of water potential in the leaf51

An elongated thincoleorhizadue to an increase in evaporation causeswater to flow toward the leaf down a waterpotential gradient. Strong cohesive forcesbetween the water molecules in the xylemprevent the water column breaking.colchicine An alkaloid drug, obtainedfrom the autumn crocus Colchicum autumnale,which is used experimentally toprevent spindle formation in mitosis ormeiosis by preventing tubulin polymerizinginto microtubules. It has the effect of haltingcell division at metaphase, the stage atwhich the chromosomes have duplicated togive four homologs for each chromosome.See anther culture.cold hardening (cold acclimation)Processes taking place in plants before theonset of winter that prepare them to withstandlow temperatures. For example,plants may accumulate solutes, such asoligosaccharides and the amino acid proline,to make their tissues frost-resistant.coleoptile A sheathlike structure thatprotects the developing plumule in grasses.Some think that, together with the SCUTEL-LUM, it represents the COTYLEDON, whileothers believe it is the first plumular leaf.The coleoptile contains very little chlorophylland is usually light-sensitive.coleorhiza (pl. coleorhizae) The protectivesheath surrounding the radicle ingrasses.coliform bacteria Gram-negative rodshapedbacteria able to obtain energy aerobicallyor by fermenting sugars to produceacid or acid and gas. Most are found in thevertebrate gut (e.g. Escherichia coli), butsome are present in soil, water, or as plantpathogens. Many are pathogenic to humans(e.g. Salmonella).collateral bundle A kind of vascularbundle in which the phloem is external tothe xylem and on the same radius. Comparebicollateral bundle; concentric bundle.collenchyma A specialized type ofparenchyma, usually located just beneaththe epidermis, that functions as supportingtissue. The cell walls are irregularly thickenedwith cellulose and pectin, the thickeninggiving distinct patterns to the cells incross-section. The thickening is usuallygreatest in the corners of the cells or adjacentto the intercellular spaces. Collenchymais the first strengthening tissue tobe formed in young plants and is able toexpand as the young tissues continue development.Compare sclerenchyma.colloid A substance that is composed ofparticles dispersed through another substance.Such particles range from 10 –7 to10 –3 cm; they are too small to see with alight microscope, yet cannot pass througha selectively permeable membrane. Proteinsin the cytoplasm are an example.colony A body form in which many cellswith similar form and function aregrouped together, as in many algae, e.g.Volvox. Such a colony is often surroundedby mucilage.columella 1. The structure present insporangia of many zygomycete fungi (e.g.Mucor) produced by formation of a domeshapedseptum cutting off the sporangiumfrom the sporangiophore.2. The central column of sterile tissue in thesporangium of liverworts and mosses.commensalism An association betweentwo organisms in which one, the commensal,benefits and the other remains unaffectedeither way, e.g. an epiphyte on thebranch of a tree. Compare amensalism;mutualism; parasitism; symbiosis.community A general term coveringany naturally occurring group of differentorganisms living together in a certain environment,and interacting with each other.See association; consociation.community biomasscompanion cellSee biomass.52

confocal microscopywalled cell cut off longitudinally from thesame meristematic cell as the SIEVE EL-EMENT with which it is closely associated. Ithas a nucleus, dense cytoplasm, and manymitochondria, and is thought to provide,via PLASMODESMATA, the needs of the lessmetabolically active, enucleated sieve elementand perhaps plays a role in regulatingthe flow of metabolites through thesieve tube.compass plant A plant with its leafedges permanently aligned due north andsouth. Such plants thus avoid receiving thestrong midday rays of the sun directly onthe leaf blades, but are positioned to usefully the weaker rays of the morning andevening sun from the east and west. Thebest-known example is the compass plantof the prairies (Silphium laciniatum).compatibility The coexistence of differentbacterial plasmids in the same host cell.Plasmids that cannot exist in the same hostcell belong to the same incompatibilitygroup.compensation depthSee photic zone.compensation point The concentrationof carbon dioxide at which, in a closedsystem, the rate of photosynthesis is exactlybalanced by the combined rates ofrespiration and photorespiration, so thatnet exchange of oxygen and carbon dioxideis zero.The point at which photosynthesis doesnot increase with increased light intensityis termed the light saturation point. Thispoint occurs at much higher light intensitiesin C 4 plants than C 3 plants.competence A characteristic of embryoniccells and meristematic cells wherebythey have the potential to differentiate intoany of several different types of cell. Onceplant cells have differentiated, most losecompetence, except under special conditions.See totipotency.competition The utilization of the sameresources by one or more organisms of thesame or different species living together ina community, when the resources are notsufficient to fill the needs of all the organisms.Plants, particularly the dominantplants of a community, compete mainly asseedlings are establishing, but once this hasbeen achieved, the adult plant often outcompetesany potential competitor unlessaffected by herbivores or pathogens.competitive inhibition A form of enzymeinhibition in which the inhibitorcompetes with the substrate to bind withthe active site of the enzyme. A competitiveinhibitor is usually structurally similar tothe substrate, e.g. malonate is a competitiveinhibitor of the enzyme succinate dehydrogenase.It may bind to the site butwill not react to produce products.complementary DNA (cDNA) A formof DNA synthesized by genetic engineeringtechniques from a messenger RNA templateusing a reverse transcriptase enzyme.It is used in cloning to obtain gene sequencesfrom mRNA isolated from the tissueto be cloned. It differs from the originalDNA sequence in that it lacks intronand promoter sequences. Labeled singlestrandedcDNA is used as a gene probe toidentify common gene sequences in differenttissues and species. See gene cloning.complementary genes Genes that canonly be expressed in the presence of othergenes; for example if one gene controls theformation of a pigment precursor and anothergene controls the transformation ofthat precursor into the pigment, then bothgenes must be present for the color to developin the phenotype. Such interactionsbetween genes lead to apparent deviationsfrom the 9:3:3:1 dihybrid ratio in the F 2 .Compositaecomposite fruitcompound fruitSee Asteraceae.See multiple fruit.See aggregate fruit.concentric bundle (centric bundle) Avascular bundle in which the xylem and53

conjugation 1. A type of sexual reproductionfound in the conjugating greenalgae (phylum Gamophyta), and certainfungi, in which a conjugation tube formsbetween cells of two individuals and thegamete formed in one cell (the male gamete)moves through the tube and fuseswith the gamete of the other cell (the femalegamete) to form a zygote. Spirogyraand Rhizopus are examples.2. In prokaryotes, a means of transferringgenetic information from one bacterium toanother across a bridgelike structure. InEscherichia and related genera, two indiconformationphloem are arranged in rings, one completelysurrounding the other. Compare bicollateralbundle; collateral bundle.conceptacle A flask-shaped reproductivecavity that develops on the swollen tips(receptacles) of the thalli of certain brownalgae, (e.g. bladder wrack). Female conceptaclesare lined with unbranched sterilehairs (paraphyses) and the oogonia developon short stalks projecting from thechamber wall. Male conceptacles containbranched paraphyses that bear the antheridia.Both female and male conceptaclesopen to the exterior via a pore, theostiole.condensation reaction A chemical reactionin which two molecules are joinedtogether with the elimination of a simplermolecule or group. Many condensation reactionsinvolve the elimination of water,e.g. the formation of peptide bonds betweenamino acids in the formation of peptides.Compare hydrolysis.conduplicateconeSee strobilus.See ptyxis.confocal microscopy A form of lightmicroscopy involving the use of two microscopes,one on each side of the specimen.The first microscope focuses anilluminated aperture onto the specimen,while the other also focuses on the specimen,but receives the light transmittedthrough it. This produces an image of a singleplane in the specimen. It can be repeatedat different depths of focus to buildup a three-dimensional picture.conformation The spatial arrangementof atoms in a molecule. In affecting themolecule’s shape and the distribution ofchemical groups and electrostatic chargeson its surface, the conformation determinesthe potential energy and reactivityof the molecule that results from the attractiveand repulsive forces between thedifferent parts of the structure. The conformationof PROTEINS is critical to their effectivenessas enzymes. Their conformation isaffected by the sequence of amino acidsand the type, location, and nature of thebonds between different parts of the molecule,or between the polypeptide chainand various cofactors and prostheticgroups.conidiophoreconidiosporeSee conidium.See conidium.conidium (conidiospore) (pl. conidia)An asexual spore of certain fungi, especiallyof the Ascomycota and FungiAnamorphici e.g. Pythium and Albugo.Conidia are cut off externally in chainsat the apex of a specialized hypha, theconidiophore.Coniferophyta (conifers) The largestphylum of gymnosperms, comprisingabout 560 species of evergreen trees andshrubs, with many important species, e.g.Pinus (pine), Picea (spruce), Taxus (yew),and Abies (fir). They dominate the vast borealforests of the northern hemisphere.Most are evergreen, but Larix (larches)and Taxodium (swamp cypresses) are deciduous.conjugated protein A protein that onhydrolysis yields not only amino acids butalso other organic and inorganic substances.They are simple proteins combinedwith nonprotein groups (prostheticgroups). See also glycoprotein; lipoprotein.54

Convention on Biological DiversityConjugation: the stages occurring in Spirogyraviduals join by a conjugation bridge andpart of the genetic material of one, thedonor (or male) cell, is transferred to therecipient (or female) cell.connective The parenchymatous tissuethat joins the two lobes of the anther andcontains the vascular strand. See stamen.consociation A climax of natural vegetationdominated by one particular species,such as oakwood, dominated by the oaktree, or Calluna heathland dominated bythe heather, Calluna vulgaris. Many consociationstogether may form an association,for example oakwood, beechwood, andashwood consociations together make up adeciduous forest association. See also association.conspecific Describing individuals thatbelong to the same species.consumer An organism that feeds uponanother organism, e.g. all animals and parasiticand insectivorous plants. Compareproducer. See also trophic level.continental drift The theory that present-daycontinents have arisen by thebreaking up and drifting apart of a previouslyexisting ancient land mass (Pangaea).There is much evidence to support thetheory, and it serves to explain the distributionof contemporary and fossil plantsand animals. Continental drift is now believedto reflect the movement over geologicaltime of underlying plates in the earth’scrust – the theory of plate tectonics.continuous variation (quantitative vari-55

convergent evolutionation; multifactorial inheritance) A formof variation in which the characters showcontinuous variation within a certain rangeof values. Examples are grain yield inwheat and height in humans. Such traitsare typically controlled by many differentgenes (polygenes). The identification andmanipulation of genetic loci determiningquantitative traits (quantitative trait loci(QTLs)) is important in plant and animalbreeding.contractile root A specialized root developedby certain bulb- and corm-formingplants that serve to pull the bulb or cormdown to the appropriate depth in the soil,e.g. Crocus.Convention on Biological DiversityAn international convention, signed at theUnited Nations Rio Summit in 1992, thataims to conserve biodiversity, the sustainableuse of natural resources, and the equitablesharing of the benefits arising fromexploitation of genetic resources. It coversall aspects of biodiversity, from geneticresources to ecosystems, and aims to conserveresources while promoting sustainabledevelopment and to encourage thesharing of costs and benefits between developingand developed countries. TheUnited Nations Environment Program(UNEP) responded by commissioning theGlobal Biodiversity Assessment, to collectand review data, theories, and opinion relatingto current issues worldwide, whichprovided valuable information on therapid rate at which environmental changeis taking place and natural ecosystems arebeing modified.convergent evolution (convergence)The development of similar (analogous)structures in unrelated organisms as a resultof living in similar ecological conditions.An example is the evolution ofsimilar succulent forms in the Cactaceaeand the Euphorbiaceae, in which the leavesare reduced to pale spines. See also analogous.copperSymbol Cu. A metal trace element(micronutrient), essential for plantgrowth. It is found in the enzyme cytochromec oxidase, which is part of therespiratory electron-transport chain, andin the protein plastocyanin, which is involvedin photosynthetic electron transport.Copper deficiency results in chlorosis,suggesting that copper may be involvedin the synthesis of chlorophyll. See alsomicronutrient.coppice (copse) A woodland managedfor wood production by cutting trees backto ground level at regular intervals (usually10–15 years) and allowing adventitiousshoots to grow up from the base. Theyoung shoots, which are often verystraight, are used for fencing, charcoalburning, and firewood. Occasional treesmay be left to mature: these are termedstandards.coralloid roots A type of root regularlyproduced by cycads, which contains symbioticnitrogen-fixing cyanobacteria inroot nodules, giving the roots a knobblycoral-like appearance.cordate Heart-shaped, usually referringjust to the base of leaves, e.g. the leaves ofthe common blue violet (Viola papilionacea).corkSee phellem.cork cambiumSee phellogen.corm An organ of perennation and vegetativereproduction, consisting of a shorterect fleshy swollen underground stem,usually broader than high and coveredwith membranous scales. It stores foodmaterial in the stem and bears buds in theaxils of the scalelike leaf remains of theprevious years’ growth. One or more newcorms form from these buds, and lie abovethe shrivelled remains of the previousyear’s corm. The corm’s position in the soilis often maintained with the aid of contractileroots. Examples of corms are crocusand gladiolus. Compare bulb.56

Cretaceouscorolla A collective term for the petalsof a flower. The corolla is denoted in thefloral formula by the symbol C. Where themargins of the petals are partly or completelyfused, a corolla tube is formed.corona 1. In flowering plants, any typeof outgrowth from the petals or sepals,such as the trumpet of the daffodil flower(Narcissus).2. A group of cells at the tip of the oogoniumin certain green algae (Chlorophyta),such as the stonewort Chara and the mermaid’scup (Acetabularia).corpus The central region of the meristembelow the tunica where cell divisionsare in all directions giving both increasedwidth and length to the apex. The tissues ofthe stele and cortex are derived from thecorpus. See tunica–corpus theory.cortex (pl. cortices or cortexes) A primarytissue in roots and stems of vascularplants derived from the corpus meristem,that extends inwards from the epidermisto the phloem. It usually consists ofPARENCHYMA cells but other tissues (e.g.collenchyma and sclerenchyma) may bepresent.corymbcothallusSee inflorescence.See thallus.cotyledon (seed leaf) The first leaf of theembryo of seed plants, which is usuallysimpler in structure than later-formedleaves. Cotyledons play an important partin the early stages of seedling development.For example they act as storage organs insome seeds, such as peas and beans whenmature. They form the first photosyntheticorgan in seeds showing epigeal germination,e.g. sunflower (Helianthus) but inothers they remain underground. Monocotyledonsand dicotyledons are so termedbecause they normally contain one andtwo cotyledons respectively although thereare exceptions.covalent bond A chemical bond inwhich one or more pairs of electrons areshared between two atoms. It is often representedby a single line between the symbolsof the two atoms that are bondedtogether. Compare hydrogen bond; ionicbond.cp-DNASee chloroplast DNA.C 3 plant A plant in which the first productof photosynthesis is a 3-carbon acid,glycerate 3-phosphate (phosphoglycericacid). Most plants are C 3 plants. They arecharacterized by high carbon dioxide compensationpoints owing to PHOTORESPIRA-TION. Compare C 4 plant.C 4 plant A plant in which the first productof photosynthesis is a 4-carbon dicarboxylicacid, oxaloacetic acid. C 4 plantshave evolved from C 3 plants by a modificationin carbon dioxide fixation, leading tomore efficient carbon dioxide uptake. Themodified pathway is called the Hatch–Slack pathway or the C 4 dicarboxylic acidpathway. In the leaves, the mesophyll cellssurrounding the vascular bundles (bundlesheath cells) contain the carbon dioxidefixingenzyme phosphoenolpyruvate carboxylase(PEP carboxylase) in theircytoplasm. This has a higher affinity forcarbon dioxide than ribulose bisphosphatecarboxylase (RUBP carboxylase). The productof carbon dioxide fixation is oxaloacetate,which is rapidly reduced to the C 4acid malate or transaminated to the C 4 acidaspartate. These C 4 acids are transportedto the bundle-sheath cells encircling theleaf veins, where they are decarboxylated,releasing CO 2 , which is then refixed as inC 3 plants.C 4 plants are mainly tropical or subtropical,including many tropical grasses(e.g. maize, sugar cane, and sorghum).They are more efficient than C 3 plants,producing more glucose per unit leaf area.See also bundle sheath; crassulacean acidmetabolism.crassulacean acid metabolism (CAM)A form of photosynthesis first discoveredin the family Crassulacaeae and since57

cristafound in many other succulent plants, suchas cacti. It allows plants of arid environmentsto keep their stomata closed to conservewater during the day, opening themat night to exchange gases. Carbon dioxidetaken in through the stomata at night isconverted into an organic acid for storage.Instead of combining with ribulose bisphosphate,it combines with the 3-carbonphosphoenolpyruvate (PEP) to form oxaloaceticacid, which is then converted tomalic acid for storage in the vacuole. Thefollowing day, when the stomata areclosed, the malic acid is broken down andthe carbon dioxide is released to take partin the light reactions of photosynthesis.When stressed by drought, plants withcrassulacean metabolism (CAM plants)can last for days or even weeks withoutopening their stomata, conserving waterand living on stored resources. CompareC 3 plant; C 4 plant.cremocarp A dry fruit splitting into twoone-seeded portions. The portions aretermed mericarps.Cretaceous The most recent period ofthe Mesozoic era, 145–66 million yearsago. The dominant vegetation on land inthe early Cretaceous was forests of cycads,conifers, gingkgoes and ferns. Most modern-typeferns, gymnosperms, and angiospermsarose during the Cretaceous,and by the end of the period the floweringplants had replaced the gymnosperms asthe dominant terrestrial vegetation, formingvast broad-leaved forests with magnolias,figs, poplars, sycamores, willows, andherbaceous plants and a great variety of insectpollinators. The Cretaceous is namedafter the large amounts of chalk (fossilizedplankton) found in rocks of the period. Seealso geological time scale.crista (pl. cristae) The structure formedby folding of the inner mitochondrialmembrane, giving a large surface area forelectron transport.critical day length (cdl) The amount oflight per day that is the maximum a shortdayplant may receive and still flower, andconversely, the minimum a long-day plantneeds to flower. Cocklebur, a short-dayplant, will not flower if given more than15fi hours light per day; i.e. its cdl is 15fihours. Henbane, a long-day plant, willonly flower if given more than 11 hourslight per day; i.e. the cdl is 11 hours. In reality,however, it is the length of the darkperiod that it critical, short-day plants requiringmore than a critical length of darkperiod before they will flower, and viceversa. See photoperiodism.cross 1. The act of cross-fertilization.2. The organism resulting from crossfertilization.crossing over The exchange of materialbetween homologous chromatids by theformation of chiasmata. This results in therecombination of linked genes. Such reassortmentof genes is the main source of geneticvariation during sexual reproduction.Unequal crossing over, in which one chromatidends up with one copy of a chromosomesegment while the other has threecopies, results from crossing over afterpairing between homologous chromosomesthat are not properly aligned. Seealso chiasma; independent assortment;tetrad.crossover frequencymap.See chromosomecross-pollination The transfer of pollenfrom the anthers of one individual to thestigma of another individual, usually of thesame species, with subsequent germinationof the pollen and growth of the pollen tube.This transfer is achieved by intermediaryagents such as birds (ornithophily), bats,insects (entomophily), wind (anemophily),or water (hydrophily). See incompatibility.Compare self-pollination.crown gall A disease of plants caused bythe soil-borne bacterium Agrobacteriumtumefasciens. It causes galls both andabove and below ground on a wide rangeof plants, especially fruit trees.58

cypselaCruciferaeSee Brassicaceae.crumb structure The texture of a soil interms of the size of the soil particles andhow they interact, often determined simplyby rubbing the soil with fingers. Some soils,such as sandy soils, have a loose crumbstructure with good drainage, whereas inclay soils the particles are small and sticktogether to give a dense crumb structureand poor drainagecryptogam In early classifications, anyplant that reproduces by spores or gametesrather than by seeds. Cryptogams werethus named because early botanists consideredtheir method of reproduction to behidden (cryptic). They included the algae,fungi, mosses, liverworts, hornworts,ferns, clubmosses, and horsetails, the vascularplant group often being termed vascularcryptogams. Compare phanerogam.cryptophyte (geophyte) A plant inwhich the resting buds are below the soilsurface or in water. See also Raunkiaer’splant classification.cultivar Any agricultural or horticultural‘variety’. The term is derived from thewords cultivated variety.culture A population of microorganismsor dissociated cells of a tissue grown on orwithin a solid or liquid medium for experimentalpurposes. This is done by inoculationand incubation of the nutrientmedium. See also tissue culture.culture medium A mixture of nutrientsused, in liquid form or solidified with agar,to cultivate microorganisms, such as bacteriaor fungi, or to support tissue cultures.cushion plant A plant that forms a tightlow hummock as an adaptation to cold anddry or windy situations. Cushion plantsusually have small thick leaves to minimizewater loss, or hairy leaves to reduce transpiration.Cushion plants are common inarctic–alpine habitats.cuticle A waterproof, protective layer ofwax secreted by the epidermis that coversthe external surface of the plant, with theexception of the stomata and lenticels.Some seeds also have a cuticle. Its thicknessvaries with the species and environment.Xerophytes tend to have thick cuticles, anddry conditions often induce cuticular thickening.Its main function appears to be restrictionof water loss from the plant. Onaverage, only about 5% of the water lostfrom a plant is via the cuticle (cuticulartranspiration).cutin A group of substances chemicallyrelated to fatty acids forming a continuouslayer called the cuticle on the epidermis ofplants, interrupted only by stomata orlenticels. Being fatty in nature, cutin iswater-repellent, therefore helping to reducetranspiration. It is also protective, forexample preventing invasion by parasites.See suberin.cutinization The impregnation of aplant cell wall with cutin.cutting A part of a plant that is removedfrom the parent and encouraged to growinto another plant. Cuttings are a means ofasexual propagation and may vary in sizefrom buds, leaves or root segments to largeshoots.Cyanobacteria A phylum of Eubacteriacontaining the blue-green bacteria (formerlycalled blue-green algae) and thegreen bacteria (chloroxybacteria). Bothgroups convert carbon dioxide into organiccompounds using photosynthesis,generally using water as a hydrogen donorto yield oxygen, like green plants. Cyanobacteriaare an ancient group, and theirfossils (stromatolites) have been dated atup to 3500 million years old. They arespherical (coccoid) or form long microscopicfilaments of individual cells.Cycadophyta (cycads) A phylum ofcone-bearing gymnosperms (about 75 livingspecies) with palmlike compoundleaves and special CORALLOID ROOTS at or59

cystnear the ground surface, which containsymbiotic nitrogen-fixing cyanobacteria.cyclic AMP (cAMP; adenosine-3¢ ,5¢ -monophosphate) A form of adenosinemonophosphate (see AMP) formed fromATP in a reaction catalyzed by the enzymeadenyl cyclase. It has many functions,acting as an enzyme activator, geneticregulator, chemical attractant, secondarymessenger, and as a mediator in the activityof many hormones, including epinephrine,norepinephrine, vasopressin, ACTH,and the prostaglandins.cyclic photophosphorylation In PHO-TOSYNTHESIS, the flow of electrons fromPhotosystem I, after excitation by absorbedlight energy, along the ferredoxin/cytochrome b/f/plastocyanin electrontransportchain and back to Photosystem Ito be reenergized. Cyclic photophosphorylationdrives ATP synthesis, but does notproduce NADPH.cyclins Regulatory proteins whose concentrationvaries with certain stages of theCELL CYCLE. Cyclins help control progressthrough the various stages of the cell cycleby activating protein kinases (cyclindependentprotein kinases).cyclosis (cytoplasmic streaming) Thestreaming of cytoplasm in a circular motionaround the cell observed in someplants, particularly young sieve tube elements.cycyanocobalamin (vitamin B 12 ) Oneof the water-soluble B-group of VITAMINS.cymose inflorescence (cyme; definite inflorescence)See inflorescence.cypsela An achenelike fruit that developsfrom an inferior ovary and containssome tissues not derived form the carpel. Itis common in the Asteraceae, e.g. dandelion(Taraxacum), whose fruits bear a ringof hairs (the pappus) derived from thecalyx.cystA thick-walled resting spore.cysteine A sulfur-containing AMINO ACIDsynthesized from methionine and serine. Itis involved in the synthesis of biotin. It alsoacts as a store of sulfur for biosynthesis,and can be broken down to pyruvate.cystine A compound formed by the joiningof two cysteine AMINO ACIDS through a–S–S– linkage (a cystine link). Bonds of thistype are important in the conformation ofproteins, acting as cross-links between differentparts of a polypeptide or betweendifferent polypeptide chains, thus determiningthe pattern of folding. It is also requiredto the synthesis of the coenzymethiamin pyrophosphate, which is involvedin many decarboxylation reactions.cystolith A deposit of calcium carbonatearising internally on a stalk from thecell walls of large modified epidermal cellsin some flowering plants, e.g. stinging nettle(Urtica dioica).cytidine (cytosine nucleoside) A NUCLE-OSIDE formed when cytosine is linked to D-ribose via a b-glycosidic bond.cytochrome oxidase (cytochrome aa 3 )The enzyme that catalyzes the final step inthe respiratory electron transport chain, inwhich water is formed from hydrogen ionsand oxygen. See respiration.cytochromes Conjugated proteins containingheme, that act as intermediates inthe electron-transport chains of photosynthesisand respiration. The iron ion in theporphyrin prosthetic group of cytochromeschanges its oxidation state from ferric(Fe 3+ ) to ferrous (Fe 2+ ) when the cytochromeis reduced. There are four mainclasses, designated a, b, c, and d, mostbound to membranes.cytogamy The fusion of gametes duringfertilization. Compare karyogamy.cytokinesis The division of the cytoplasmafter nuclear division (mitosis or60

cytotaxonomymeiosis). In the cells of plants and mostalgae it involves formation of a new plantcell wall by means of a structure called thecell plate. In primitive protoctists, as in animals,the cell membrane ingrows underthe control of a ring of actin microfilaments,and a furrow constricts the cellequator until it pinches the cell in two. Seecell plate.cytokinin One of a class of plant hormonesconcerned with the stimulationof cell division, nucleic acid metabolism,and root–shoot interactions. Cytokininsare often purine derivatives: e.g. kinetin (6-furfuryl aminopurine), an artificial cytokinincommonly used in experiments topromote cell division in tissue cultures; andzeatin, found in maize cobs. Natural cytokininsare all derivatives of adenine.Cytokinins are produced in roots,where they stimulate cell division. They arealso transported from roots to shoots in thetranspiration stream, where they are essentialfor healthy leaf growth. Subsequentmovement from the leaves to youngerleaves, buds, and other parts may occur inthe phloem and be important in sequentialleaf senescence up the stem.cytologyThe study of cells; cell biology.cytoplasm The living contents of a cell,excluding the nucleus and large vacuoles,in which many metabolic activities occur.It is contained within the plasma membraneand comprises a colorless substance(hyaloplasm) containing organelles andvarious inclusions (e.g. crystals and insolublefood reserves). The cytoplasm is about90% water. It is a true solution of ions (e.g.potassium, sodium, and chloride), smallmolecules (e.g. sugars, amino acids, andATP), and a colloidal solution of large molecules(e.g. proteins, lipids, and nucleicacids). It can be gel-like, usually in its outerregions, or sol-like. A complex cytoskeletonof F-actin filaments and microtubulesis often present, and appears to control thestructure of the cytoplasm, cell movements,and movements of the cytoplasmand conformation of the membranes. Seecytoskeleton; organelle; protoplasm.cytoplasmic inheritance (extrachromo-61

D2,4-D (2,4-dichlorophenoxyacetic acid)A synthetic AUXIN used as a potent selectiveweedkiller. Monocotyledenous specieswith narrow erect leaves (e.g. cereals andgrasses) are generally resistant, while dicotyledonousplants are often very susceptible.The compound is thus particularlyeffective in controlling weeds in cerealcrops and lawns.dark-ground illumination A light microscopytechnique used to study transparentmaterial, such as living cells ormicrobes. The specimen is lighted from theside, so that only diffracted light from thespecimen passes through into the objective.The specimen thus appears bright against adark background. The difference in refractiveindex between organelles and the surroundingcytoplasm causes the boundariesbetween organelles and cytoplasm to reflectmore light.dark reactions (light-independent reactions)A group of reactions that follow theLIGHT REACTION in PHOTOSYNTHESIS andform glucose and other reduced productsfrom carbon dioxide. They are not dependenton light, although they can take placein the light. The dark reactions use energy(as ATP) and reducing power (as NADPH)generated by the light reaction of photosynthesisto reduce carbon dioxide. In eukaryotes,the dark reactions take place inthe stroma of the chloroplast by one of twopathways, according to whether the plantis a C 3 PLANT or a C 4 PLANT. The reducedcarbon dioxide is incorporated into carbohydratesin the Calvin cycle. See also crassulaceanacid metabolism.Darwin, Charles Robert (1809–82)British naturalist noted for his theory ofevolution by NATURAL SELECTION. Darwinfirst became interested in natural history atCambridge University. He was inspired bythe geologist Adam Sedgwick and encouragedby the professor of botany at Cambridge,John Henslow, to apply for theposition of naturalist on a survey of the Pacificand South America on HMS Beagle in1831. On this voyage, he observed manydifferent animal species and was struck bythe way in which they gradually changedfrom one location to another, in particular,the finches of the Galápagos Islands. Herecorded 14 species of finches on these islands,each species dominating a particularpart of the islands in different conditions.He speculated that it was probably unlikelythat they could all have arisen spontaneouslyas separate species and thoughtthat it was more probable that they hadevolved from a mainland species of finch,possibly from Ecuador. It took 20 years forDarwin to collect enough evidence to publishhis seminal work On the Origin ofSpecies by Means of Natural Selection(1859), having been stimulated to publishby an unexpected letter in 1858 from theBritish naturalist Alfred Russel WALLACE,outlining theories similar to Darwin’s own.Darwin and Wallace read a joint paper ontheir theories to the Linnaean Society inLondon in the summer of 1858. The publicationof Darwin’s book led to a storm ofcontroversy among scientists and churchmenas his theory challenged the biblicalaccount of the Creation. Darwin continuedto publish more works, including his TheDescent of Man (1871), in which he appliedhis theory of evolution to humans,implying that they had descended fromapelike creaturesDarwinismDarwin’s explanation of62

deficiency diseasethe mechanism of evolutionary change,namely, that in any varied population oforganisms only the best adapted to that environmentwill tend to survive and reproduce.Individuals that are less well adaptedwill tend to perish without reproducing.Hence the unfavorable characteristics, possessedby the less well-adapted individuals,will tend to disappear from a species, andthe favorable characteristics will becomemore common. Over time the characteristicsof a species will therefore change, eventuallyresulting in the formation of newspecies. Darwin called this process of selectivebirth and death NATURAL SELECTION.See also neo-Darwinism.dating techniques Methods used to determinethe age of rocks, fossils, or archaeologicalremains. There are two mainmethods. Relative dating assesses the ageof a specimen in comparison to other specimens.Absolute dating involves assessingthe actual age of a specimen by using somereliable measure of time. See also dendrochronology;radiometric dating.day-neutral plant A plant that requiresno particular photoperiod to flower. Seephotoperiodism.deamination A type of chemical reactionin which an amino group (NH 2 ) is removedfrom a compound. Amino acids canbe broken down by oxidative deamination.Transamination of many amino acidsyields glutamate, which can be deaminatedto a-ketoglutarate, which can then enterthe Krebs cycle to be further broken downwith the release of energy. See transamination.De Bary, Heinrich Anton (1831–88)German botanist. De Bary abandoned a careerin medicine in Frankfurt to devote therest of his life to his botanical studies, becomingprofessor of botany at a number ofGerman universities. His work on fungirepresents a major landmark in the understandingof and fight against plant diseases.His observations of fungi in theirnatural habitat and his methods of culturingthem enabled him to work out their developmentand life cycles, thus layingdown the foundations of modern mycology.In his Researches on Fungal Blights(1853) outlining certain rust and smut diseases,he explained that fungi are the causeand not the effect of diseases. He alsoshowed that a lichen was an association betweenan alga and a fungus and coined theterm ‘symbiosis’ for dissimilar plants livingtogether to their mutual benefit.decarboxylase An enzyme that catalyzesthe decarboxylation of carboxylicacids, including the conversion of aminoacids to amines.deciduous Denoting plants that seasonallyshed all their leaves, for example beforethe winter, or dry season. It is anadaptation to prevent excessive water lossby transpiration when water is scarce.Compare evergreen.decomposer An organism that feedsupon dead organisms, breaking themdown into simpler substances. Decomposersrecycle nutrients, making themavailable to producer organisms. Bacteriaand fungi are important decomposers inmost ecosystems. See also food chain;trophic level.decussate Describing leaves in oppositepairs and with alternating pairs at right anglesto each other up the stem.dedifferentiation The reversion of adifferentiated cell to the meristematic (undifferentiated)state, which is capable ofdifferentiating again. It occurs naturallyjust before the production of secondarymeristems or in response to wounding orstimulation by growth factors. See callus;totipotency.deficiency disease A disease caused bydeficiency of a particular essential nutrient(such as a MICRONUTRIENT or trace element),usually with a characteristic set ofsymptoms. For example, iron deficiency resultsin CHLOROSIS between the veins, par-63

definite growthticularly in young leaves that are still synthesizingchlorophyll. Nitrogen deficiencycauses stunting of growth and chlorosis ofolder leaves, as chlorophyll pigments arebroken down so their components can bereused in the young growing parts of theplant.definite growth See determinate growth.See cymose in-definite inflorescenceflorescence.definitive nucleusSee polar nuclei.deforestation (disafforestation) The permanentremoval of forests, especially bymeans of logging for commercial timberand clearing for agriculture and human settlement.A major cause of loss of naturalhabitats and biodiversity worldwide, it canlead to soil erosion, flooding downstream,and increasing drought in neighboringareas.degenerate code A part of the geneticcode in which some amino acids are codedfor by more than one triplet of nucleotidebases (codons). For some amino acids, onlythe first two bases appear to be important,and it matters little what the third base is.For example, leucine is coded for by CUU,CUC, CUA, and CUG, and by UUA andUUG.dehiscent Describing a fruit or fruitingbody that opens at maturity to release theseeds or spores. Dehiscence may be violentto aid seed dispersal. Compare indehiscent.dehydration A process followed whentissues are prepared for permanent microscopeslides. Water is removed by immersingthe tissue in increasing strengths ofethyl alcohol. The alcohol concentrationmust be increased gradually as otherwisethe cells would dehydrate too quickly andshrink. Dehydration is necessary becausewater does not mix with the chemicals usedin clearing and mounting sections.dehydrogenaseAn enzyme that catalyzesthe removal of certain hydrogenatoms from specific substances in biologicalsystems. Dehydrogenases are usuallycalled after the name of their substrate, e.g.lactate dehydrogenase. Some dehydrogenasesare highly specific, with respect toboth their substrate and coenzyme, whilstothers catalyze the oxidation of a widerange of substrates. Many require the presenceof a coenzyme, which is ofteninvolved as a hydrogen acceptor. Dehydrogenasescatalyze the transfer of two hydrogenatoms from substrates to NAD andNADP. See also alcohol dehydrogenase.deletionSee chromosome mutation.deme A subpopulation of a species: adiscrete interbreeding group of organisms,with recognizable cytological or geneticcharacters, that is spatially distinct fromother such groups, although it may be adjacentto them. Compare subspecies.denaturation A process that causes unfoldingof the peptide chain of proteins orof the double helix of DNA. These changesmay be brought about by a variety of physicalfactors: change in pH, temperature, violentshaking, and radiation. The primarystructure remains intact. Denatured proteinsand nucleic acids show changes inphysical and biological properties; denaturedproteins, for example, are often insolublein solvents in which they wereoriginally soluble.dendrochronology A method of archaeologicaldating by the ANNUAL RINGS oftrees, used when the lifespans of living andfossil trees in an area overlap. Exact datesfor sites can be calculated and the methodis more accurate than radiometric datingtechniques. Bristlecone pines, which canlive for up to 5000 years, have been used insuch work.denitrification The chemical reductionof nitrate by soil bacteria. Denitrifying bacteriasuch as Pseudomonas, Micrococcus,and Clostridium use nitrate as the terminalelectron acceptor in anaerobic respiration.64

De Vries, HugoThe process is important in terms of soilfertility since the products of denitrification(e.g. nitrites and ammonia) cannot beused by plants as a nitrogen source. Comparenitrification. See nitrogen cycle.dentate Toothed: describing leaf marginsthat have a series of outward-pointingnotches.denticulate Describing a leaf marginthat is finely toothed.deoxyribonucleasedeoxyribonucleic acidSee DNase.See DNA.deoxyribose A pentose (five-carbon)sugar that is a component of the nucleotidesthat make up DNA.deoxy sugar A SUGAR in which oxygenhas been lost by replacement of a hydroxylgroup (OH) with hydrogen (H). The mostimportant example is DEOXYRIBOSE.dermatogen (protoderm)theory.See histogendesert An area in which the rate of evaporationexceeds the rate of precipitationfor most of the time. A desert may form inany climate where average annual precipitationis less than 250 mm and intermittent.A ‘true desert’ has no plant life at all.The term is also used for semideserts inwhich plant and animal life is sparsely distributed,and adapted to long periods ofdrought. EPHEMERAL plants are commonand there may also be succulent or xerophyticperennials. Cold deserts includeparts of the TUNDRA and may also form inhot regions with annual rainfall of less than400 mm.desiccation The drying out of an organismdue to evaporation.desmids A group of green algae of thephylum GAMOPHYTA that are basically single-celledbut in some species form filamentousor irregular colonies.determinate growth (definite growth) Aform of growth that has a finite limit, i.e.once a maximum size is reached, no furthergrowth occurs. It occurs, for example, inannual and biennial plants and plant organssuch as internodes. The term is sometimesapplied to cymose inflorescences, inwhich growth ceases with the productionof a terminal flower bud. Compare indefinitegrowth.detritus Fragments of dead material,such as leaf litter and products of thebreakdown of organic material by decomposers.Deuteromycota See Fungi Anamorphici.Devonian The geological period knownas the ‘Age of Fish’, some 405–355 millionyears ago, between the Silurian and theCarboniferous periods of the Paleozoic era.It was also the time when the first major invasionof the land by plants occurred. Thevery first land plants date from the late Silurian,but during the Devonian many vascularland plants appeared, such as therhyniophytes (all now extinct), clubmosses(Lycophyta), horsetails (Sphenophyta),and eusporangiate ferns (early members ofthe Filicinophyta. The gymnosperms probablyarose in the Late Devonian. See alsogeological time scale.De Vries, Hugo (1848–1935) Dutchplant physiologist and geneticist. An experton Netherlands flora, De Vries showedthat turgor pressure of the fluid in plantcells was partially responsible for growthand coined the term ‘plasmolysis’ for theeffect on cells of water loss during wilting.His work in this area formed a basis fortheories of osmosis.In the 1880s he developed an interest inheredity, proposing that there were structuresin the nucleus (which he called ‘pangenes’)that determine characteristics in theoffspring. He carried out plant breedingexperiments, and the results of his crossesshowed segregation of characters in theprogeny in a ratio of 3:1. He came across65

dextrinMENDEL’s work in 1900 and immediatelypublished his results.In The Mutation Theory (1901–03), DeVries postulated that new species couldarise from a single dramatic mutation. Hehad observed different types of eveningprimrose within a single colony and believedthem to be the result of mutations. Ithas subsequently been discovered that theplants De Vries believed to be mutantswere triploids or tetraploids, but his essentialpremise continues to be important,showing that variation is essential for evolutionwithin a species.dextrin Any of a class of intermediatesproduced by the hydrolysis of starch. Furtherhydrolysis eventually produces themonosaccharide glucose.dextrose (grape sugar)name for GLUCOSE.An outdateddiakinesis The last stage of thePROPHASE in the first division of MEIOSIS.Chiasmata are seen during this stage, andby the end of diakinesis the nucleoli andnuclear membrane have disappeared. Duringdiakinesis the sister chromatids of homologouspairs of chromosomes completetheir separation, and the chromosomes coiltightly, shortening and thickening.dialysis The separation of large moleculesfrom smaller ones by DIFFUSIONthrough a selectively permeable membranethat allows through only molecules up to acertain size. For example, proteins can beseparated from amino acids or glucose inthis way. Compare osmosis.Diatoms A phylum of the Protoctistawhose members are unicellular algae foundin freshwater, the sea, and soil. Much ofplankton is composed of diatoms and theyare thus important in FOOD CHAINS. Diatomsare diploid unicells with silica cellwalls (frustules) composed of two overlappingvalves ornamented with perforations,which are arranged differently in eachspecies. The frustules of dead diatoms arehighly resistant to decay, and form deepsediments on the floors of oceans and lakescalled diatomaceous earth or kieselguhr,which is used in insulating materials, fireproofcements, filters, and in the manufactureof explosives.diatropism (diageotropism)gravitropism.dicaryonSee dikaryon.See inflo-dichasial cyme (dichasium)rescence.See tropism;dichogamy The condition in which theanthers and stigmas mature at differenttimes thus helping prevent self-pollination.Compare homogamy. See protandry; protogyny.dichotomy Forked branching producedby division of the growing point into twoequal parts, seen for example in Fucus.Dicotyledonae A class of floweringplants (phylum Angiophyta) characterizedby having two cotyledons in the seed. Theyinclude herbs, shrubs, and trees and secondarygrowth is normal. Examples are thesunflower and the oak. The flower partsare arranged in fours or fives, or multiplesthereof, and the leaf veins are branched.The vascular bundles of the stem arearranged in a ring within a single endodermisand pericycle, giving a eustele. CompareMonocotyledonae.dictyosome A stack of membraneboundedsacs (cisternae) that, togetherwith associated vesicles (Golgi vesicles),forms the GOLGI APPARATUS. The term isusually only applied to plant cells, wheremany such stacks are found. In contrast,the Golgi apparatus of most animal cells isa continuous network of membranes. Theterm dictyosome is little used today.dictyostele A modified solenostele thatis broken up by large leaf gaps so crowdedtogether that they overlap. The tube of astelar tissue is thus broken up into a mesh,each small segment of remaining vascular66

diplobiontictissue being called a meristele. It is found incertain fern stems (e.g. Dryopteris).differentially permeable membraneSee osmosis.differentiation A process of changeduring which cells with generalized formbecome morphologically and functionallyspecialized to produce the different celltypes that make up the various tissues andorgans of the organism. See also totipotency.diffuse-porous Describing wood inwhich vessels of approximately equal diametertend to be evenly distributed sothere is no obvious growth ring. Diffuseporouswood is seen for example in yellowbirch (Betula lutea). Compare ring-porous.diffusion The movement of moleculesalong a concentration gradient from areasof high concentration to areas of lowerconcentration as a result of random movement.Gaseous exchange in plants takeplace by diffusion, and some of the movementof ions through plant fluids is alsodue to diffusion. See dialysis; transpiration.Compare active transport; osmosis.diffusion pressure deficit (DPD) Thenet force that causes water to enter a plantcell. It is the difference between OSMOTICPRESSURE (P) and TURGOR PRESSURE:DPD = P – TP.See water potential.dihybrid (dihybrid cross) An organismthat is heterozygous at two loci, formed bycrossing homozygous parents with differentalleles at two given loci: for example,Mendel’s cross between yellow round(YYRR) and green wrinkled (yyrr) gardenpeas to give a yellow round dihybrid(YyRr). When a dihybrid is selfed, a characteristicdihybrid ratio of 9:3:3:1 is obtainedin the offspring. Nine plants exhibitboth dominant characters, six plants showone dominant and one recessive character,and one plant exhibits both recessives.Compare monohybrid.dikaryon (dicaryon) A fungal hypha ormycelium whose cells each contain two differentnuclei, arising from the fusion of twocompatible cells, each with one nucleus.The nuclei do not fuse immediately, insteaddividing independently but simultaneously.Dikaryosis is common inascomycetes and basidiomycetes, and resultsfrom the fusion of two monokaryoticmycelia of different mating types.dimorphism The existence of two distinctforms. Examples include male and femaleplants in dioecious species, aerial andsubmerged leaves, alternation of gametophyteand sporophyte stages of a life cycle,and mesophyll and bundle-sheath chloroplasts.dinitrogenaseThe enzyme that catalyzesNITROGEN FIXATION.dinoflagellateSee Dinomastigota.Dinomastigota (dinoflagellates) A phylumof the Protoctista containing mainlysingle-celled marine or freshwater algaethat swim in a twirling manner by means oftwo UNDULIPODIA that lie at right angles toeach other in two grooves within the organism’srigid body wall (test). The bodywall is often stiffened by cellulose platesunder the cell membrane. Many dinoflagellatespossess stinging organelles (trichocysts)that they discharge to catch prey;and some produce potent toxins that arecapable of killing fish. Planktonic dinoflagellatescan multiply rapidly to form redtides. The phosphorescent seas of the tropicsare usually due to bioluminescent dinoflagellates.Roughly half of all knowndinoflagellates are capable of photosynthesis.Many form symbioses with marine invertebratessuch as corals and clams.dinucleotide A compound of two nucleotideslinked by their phosphate groups.Important examples are the coenzymesNAD and FAD.dioecious Denoting a plant species inwhich male and female reproductive or-67

diploidgans are borne on separate individuals.Compare hermaphrodite; monoecious.diplobiontic Describing life cycles showinga typical alternation of generationswith haploid and diploid somatic bodies.Ferns and mosses are diplobiontic organisms.Compare haplobiontic.diploid A cell or organism containingtwice the haploid number of chromosomes(i.e. 2n). In plants exhibiting an alternationof generations the sporophyte is diploid,and the gametophyte haploid. In lowerplants such as mosses and liverworts thepersistent vegetative plant is haploid, thesporophyte being a relatively short-livedphase. Higher plants are normally alwaysdiploid. Exceptions are those species inwhich polyploidy occurs.diplont A diploid organism that representsthe vegetative stage in life cycles inwhich haploidy is restricted to the gametes.Such diplontic life cycles are found in someprotoctists, such as the diatoms and certainPhaeophyta (brown algae), including thewracks. Compare haplont; alternation ofgenerations.diplotene In MEIOSIS, the stage in late PRO-PHASE I when the pairs of chromatids beginto separate from the tetrad formed by theassociation of homologous chromosomes.Chiasmata can often be seen at this stage.disaccharide A SUGAR with moleculescomposed of two monosaccharide units.Sucrose and maltose are examples. Theseare linked by a –O– linkage (glycosidiclink). See also glycosidic bond.discontinuous variation (qualitativevariation; qualitative inheritance) A formof variation in which a character has twoor more distinct forms in a population. Examplesare Mendel’s pea characters. It generallyoccurs when there are two or moreallelic forms of a major gene in a population.See polymorphism.disjunctDenoting a species or genuswhose distribution is not continuouswithin its range. Examples are species thatare found in the Alps, arctic regions, andnorth European mountains and magnoliasthat occur in eastern North America, CentralAmerica, and southeast Asia, but notin the intervening regions. Such disjunctionsmay arise where climate change reducesthe range of a species, restrictingthem to certain isolated areas (refugia)where the local climate is more suitable forthem, i.e. since the last Ice Age for the coldlovingarctic–alpine species now confinedto isolated mountain tops. The other majorcause of disjunction is CONTINENTAL DRIFT.disk floretSee inflorescence.distal Denoting the part of an organ,limb, etc., that is furthest from the origin orpoint of attachment. Compare proximal.distely Having two steles, e.g. the stemof Selaginella kraussiana. The steles arejoined only at branches. See also polystely.disulfide bridge In PROTEINS, a covalentbond formed between the sulfhydryl (–SH)groups of different cysteine molecules.Such bridges are important in stabilizingthe tertiary structure of peptides and proteins.diurnal 1. Describing an event or activityoccurring during the day or a species orprocess that is active only by day and notby night.2. Occurring at daily intervals. See circadianrhythm.divergent evolution The evolution ofdifferent forms from a single basic structurein response to different selection pressures.For example, the basic angiospermstem is vertical and elongated, but somecacti have swollen water-storing stems,while other xerophytic plants that havelost their leaves during the course of evolutionhave evolved flattened leaflike stemsfor photosynthesis, e.g. butcher’s broom(Ruscus aculeatus).68

DNAdivision The equivalent of a phylum inthe Five Kingdoms classification. The termdivision was retained for the plant kingdomuntil recently. Division names ofplants (and hence phylum names too) endin -phyta (e.g. Bryophyta, Filicinophyta),while those of fungi end in –mycota (e.g.DNA69

DNA hybridizationAscomycota, Basidiomycota). Divisionsmay be divided into subdivisions.DNA (deoxyribonucleic acid) A nucleicacid, mainly found in the CHROMOSOMES,that contains the hereditary information oforganisms. The molecule is made up of twohelical polynucleotide chains coiled aroundeach other to give a double helix. Phosphatemolecules alternate with deoxyribosesugar molecules along both chains,linked by phosphodiester bonds. Eachsugar molecule is also joined to one of fournitrogenous bases – adenine, guanine, cytosine,or thymine. The two chains arejoined to each other by hydrogen bondingbetween bases. The sequence of basesalong the chain makes up a code – the geneticcode – that determines the precise sequenceof amino acids in proteins (seemessenger RNA; protein synthesis; transcription).In DNA, the two purine bases (adenineand guanine) always bond with the pyrimidinebases (thymine and cytosine), and thepairing is quite specific: adenine withthymine and guanine with cytosine. DNAis the hereditary material of all organismswith the exception of RNA viruses. Togetherwith RNA and histones it makes upthe chromosomes of eukaryotic cells. SomeDNA viruses contain single-stranded DNArather than the double helix. See also junkDNA; replication; selfish DNA; RNA.DNA hybridizationhybridization.See nucleic acidDNA ligase An enzyme, used in DNAREPLICATION and the in vitro synthesis ofDNA, that joins together segments ofnewly synthesized polynucleotide chainsusing energy from ATP. See replication.DNA polymeraseSee polymerase.DNA: the double helixDNA probe (gene probe) A nucleic acidconsisting of a single strand of nucleotideswhose base sequence is complementary tothat of a particular DNA fragment beingsought, for example a gene on a chromosomeor a restriction fragment in a DNAdigest. The probe is labeled (e.g. with a radioisotopeor a fluorescent compound) sothat when it binds to the target sequence,both it and the target can be identified(by autoradiography or FLUORESCENCE MI-CROSCOPY). DNA probes are used to analyzefragments of DNA or RNA generatedby restriction enzymes and determine their70

dwarfismsequence in the original nucleic acid, aprocess called restriction mapping (see restrictionmap). They are also used to detectgene and chromosome mutations. See alsoFISH.DNase (deoxyribonuclease) Any enzymethat hydrolyzes the phosphodiesterbonds of DNA. DNases are classified intotwo groups, according to their site of actionin the DNA molecule (see endonuclease;exonuclease).domain 1. A discrete part of the tertiarystructure of a PROTEIN that has a particularfunction, e.g. as the binding site for thesubstrate of an enzyme.2. A taxonomic grouping of organisms thatranks higher than kingdom. There are generallyconsidered to be three domains: AR-CHAEA, BACTERIA, and EUKARYA. The FIVEKINGDOMS CLASSIFICATION recognizes onlytwo ‘superkingdoms’: Prokarya (which includesthe single kingdom Bacteria comprisingboth archaea and the bacteria) andEukarya.dormin (abscisin II)ABSCISIC ACID.A former name fordorsal 1. Designating the lower or abaxialsurface of a lateral organ such as a leaf.2. Designating the upper surface awayfrom the substrate in thallose plants suchas liverworts. Compare ventral.dorsifixed A stamen in which the filamentis fused to the back of the anther.Compare basifixed; versatile.dorsiventral Describing a structure thathas distinct upper and lower surfaces. Adorsiventrally flattened structure has flatdorsal and ventral surfaces and very narrowlateral surfaces.double fertilization The fusion of onepollen nucleus with the egg to form the zygoteand of the other pollen nucleus with apolar nucleus to form the triploid endospermnucleus. The process is restrictedto certain angiosperms but a primitiveform is known from some gymnosperms.domatium (pl. domatia)See ant plant.double helixSee DNA.dominant 1. An allele that, in a heterozygote,prevents the expression of another(recessive) allele at the same locus.Organisms with one dominant and one recessiveallele thus appear identical to thosewith two dominant alleles, the difference intheir genotypes becoming apparent only onexamination of their progenies. The dominantallele usually controls the normalform of the gene, while mutations are generallyRECESSIVE.2. A plant species that is the most importantmember of a particular communityand affects the structure of that community.dormancy A period of minimal metabolicactivity of an organism or reproductivebody. It is a means of surviving aperiod of adverse environmental conditions,e.g. cold or drought. Seeds, spores,cysts, and perennating organs of plants arepotentially dormant structures. See estivation.double recessive An organism containingboth recessive alleles of a particulargene and thus expressing the recessive formof the gene in its phenotype. Double recessives,being of known genotype, are oftenused in test crosses to establish whether theorganism to which it is crossed is heterozygousor homozygous for the same gene. Seebackcross.DPDSee diffusion pressure deficit.drupe (pyrenocarp) A fleshy indehiscentfruit containing one or more seeds eachsurrounded by a hard stony wall, the endocarp.Drupes with one seed include plumsand cherries while many-seeded drupes includeholly and elder fruits. Blackberriesand raspberries are collections of smalldrupes or drupelets.duplex Double, or having two distinctparts. The term is particularly used to describethe double helix of the Watson–71

Eecocline A CLINE that is due to a specificenvironmental factor, such as a gradient ofheavy metal concentration in the soil.ecological niche The functional role ofan organism in a community. It includesthe habitat in which the organism lives, theresources it uses, and the periods of time inwhich it is present and/or active there andits interactions with other organisms. Itcan normally be defined only by the presenceof the organism and is used more extensivelyfor animals than for plants. If twospecies occupy the same niche, then competitionmay occur until either one has replacedthe other or the two divide the nichebetween them to some extent. A similarniche may be occupied by different speciesin different areas. For example differentspecies of grasses are dominant in grasslandsin different parts of the world.ecology The study of the relationshipsof organisms to one another and to theirliving (biotic) and nonliving (abiotic) environment.ecosystem (ecological system) A unitmade up of all the living and nonlivingcomponents of a particular area that interactand exchange materials with eachother. The concept of the ecosystem differsfrom that of the community in that moreemphasis is placed on abiotic factors. Variousstudies have been made to attempt toitemize the energy flow of an entire ecosystem,taking into account factors such asincoming radiation, photosynthetic efficiency,etc. The term can be applied on variousscales, from small ponds to the wholeplanet.ecotype A population of individualswithin a species adapted genetically to thecombination of environmental factors intheir local habitat, but still able to reproducewith other ecotypes belonging tothe same species and produce fertile offspring.Differences between ecotypes maybe physiological or morphological. Comparebiotype. See also adaptive radiation;speciation.ectomycorrhiza (ectotrophic mycorrhiza)(pl. ectomycorrhizae) See mycorrhiza.ectoparasiteSee parasitism.ectoplasm The outer gel-like layer ofthe cytoplasm in the cells of plants andsome protoctists, which lies immediatelybeneath the cell membrane and contains adense array of microtubules.See mycor-ectotrophic mycorrhizarhiza.edaphic factors The physical, chemical,and biological characteristics of the soilthat together form an important componentof the habitat because of their influenceon plant distribution. The mainedaphic factors are water content, pH, organicmatter, and soil texture.EDTA Ethylenediamine tetra-acetic acid,a compound that acts as a chelating agent,reversibly binding iron, magnesium, andother positive ions. It is used as a competitiveinhibitor of certain enzymes that havemetal ion cofactors, and as a source ofiron, which it releases slowly into the solution.effector A molecule that can combinewith a repressor in an OPERON, making the72

elongationREPRESSOR inactive and thus enabling MES-SENGER RNA and protein to be produced.egg apparatus The three haploid nucleithat are situated at the micropylar end ofthe embryo sac in most flowering plants.The central nucleus is the female gameteand those to either side of it are called thesynergids. See embryo sac. See also antipodalcells.egg cellSee ovum.elaioplast (lipidoplast; oleoplast) A colorlessplastid (leucoplast) storing lipids(fats or oils).elaiosomeSee oil.elater One of many elongated spirallythickened cells that are formed withinthe capsule of certain liverworts (e.g.Marchantia and Pellia). When the elatersare exposed to the air they dry out unevenlybecause of the differential thickening.The resulting twisting movements ofthe cells help in dispersing the spores.electron carrier A molecule that canfunction as both an acceptor and a donorof electrons and/or protons in an electrontransportsystem. See coenzyme Q; cytochromes;electron-transport chain; ferredoxins.electron micrographelectron microscopeSee micrograph.See microscope.electron-transport chain (respiratorychain) A chain of membrane-linked oxidation–reductionreactions involving proteinsand enzymes, during which electronsare transferred from an initial electrondonor through a series of intermediates toa final electron acceptor, resulting in theformation of ATP. The main electrontransportsystems in plants are involved inrespiration and the light reaction of PHO-TOSYNTHESIS. Heterotrophic bacteria havea similar respiratory electron-transportchain.In eukaryotic cells, the enzymes andother components of the respiratory electron-transportchain are located in theinner membrane of the MITOCHONDRIA.During aerobic respiration the reducedcoenzyme NADH, produced by the KREBSCYCLE in the mitochondrial matrix gives uptwo electrons to the first component in thechain, NADH dehydrogenase, and two hydrogenions (H + ) are discharged from thematrix of the mitochondrion into the intermembranespace. The electrons are transferredalong the chain to a carrier molecule(COENZYME Q), and then in sequence to aseries of cytochromes, finally acting withthe enzyme cytochrome oxidase to reducean oxygen atom, which combines with twoH + ions to form water. During this electrontransfer, a further two pairs of H + ions arepumped into the intermembrane space,making a total of six per molecule ofNADH. If FADH 2 is the electron donor,only four H + ions are pumped across.The function of electron transport inthe mitochondrion is to phosphorylateADP to ATP.electrophoresis The migration of electricallycharged particles toward oppositelycharged electrodes in solution underan electric field – the positive particles tothe cathode and negative particles to theanode. The rate of migration varies withmolecular size and shape. The techniquecan be used to separate or analyze mixtures(e.g. of proteins or nucleic acids).elicitors Molecules that initiate cell signalingpathways leading to the activationof plant defense genes and the productionof PHYTOALEXINS and other defensivechemicals in response to infection by apathogen.elongation The enlargement of cellsafter mitotic division that results from theuptake of water by OSMOSIS until the cellsbecome turgid. Elongation also involvesthe enzyme-mediated stretching of the cellwalls. Hormones, especially AUXINS, are involvedin the promotion of elongation.TROPISMS are due to differential elongation73

elongation factorof cells on different sides of root or shoot,resulting in curvature.elongation factorSee translation.elution The removal of an adsorbedsubstance in a CHROMATOGRAPHY columnor ion-exchange column using a solvent(eluent), giving a solution called the eluate.The chromatography column can selectivelyadsorb one or more componentsfrom the mixture to ensure efficient recoveryof these components graded elution isused. The eluent is changed in a regularmanner starting with a nonpolar solventand gradually replacing it by a more polarone. This will wash the strongly polar componentsfrom the column.emasculation The removal of the stamensof a plant before the anthers burstto prevent self-pollination or unwantedcrosses to nearby plants.Embden–Meyerhof–ParnasSee glycolysis.pathwayembedding The sealing of tissue preparedfor permanent microscope slides in asolid block of paraffin wax prior to sectioning.After CLEARING, tissues are placedin two or three baths of molten paraffinwax. When the tissue is completely infiltratedby the wax it is allowed to harden.As the wax is opaque the block must bemarked to insure correct orientation whensectioning. Tissue prepared for electronmicroscopy may be embedded in tougherepoxy resins such as Araldite and cut witha diamond knife. See also fixation.embryo The organism that developsfrom the zygote of a vascular plant afterthe proembryo has differentiated into embryoand SUSPENSOR but before germination,i.e. while the young plant is stillnutritionally dependent on parental tissues.In the Filicinophyta, Sphenophyta,and Lycophyta the embryo sporophyteproduces a footlike structure that remainsembedded in the tissues of the parent gametophyteplant, absorbing nutrients likethe suspensor of higher plants. See embryosac.embryogeny (embryony) 1. The developmentof an EMBRYO from the momentthe ZYGOTE divides.2. The development of embryos fromdiploid somatic cells in embryo culture.Such embryos are often termed embryoids,to distinguish them from embryos formedfrom zygotes. They are capable of givingrise to normal plants. Embryoids that developfrom pollen grains grow into haploidplants.embryology The study of the developmentof embryos, usually from fertilization.embryonySee embryogeny.embryo sac A large oval thin-walled sacin the nucellus of flowering plants in whichegg fertilization and subsequent embryodevelopment occurs. It corresponds to thefemale GAMETOPHYTE of lower plants andcontains a number of nuclei derived by divisionof the MEGASPORE nucleus.There are commonly eight nuclei in theembryo sac: the egg apparatus at the micropylarend, made up of an egg nucleusand two synergid nuclei; three antipodalcells at the opposite chalazal end that probablyaid embryo nourishment; and twopolar nuclei in the center that fuse to formthe primary endosperm nucleus. Otherplants have four nuclei or sixteen nuclei intheir embryo sacs. At fertilization one maleEmbryo sac74

endoplasmic reticulumnucleus fuses with the egg nucleus to formthe zygote, while the second male nucleusfuses with the primary endosperm nucleusto form a triploid cell, or more rarely a seconddiploid or a pentaploid cell that latergives rise to the ENDOSPERM. In the gymnospermsthe megaspore gives rise to a cellthat is termed the embryo sac because of itssimilarity to the angiosperm structure. Seenucellus; micropyle.Emerson effect The observation byRobert Emerson in 1957 that PHOTOSYN-THESIS proceeds much faster when chloroplastsare illuminated with light at awavelength of 650 nm as well as at 700 nm(the absorption wavelength of chlorophylla molecules). It contributed to the discoveryof PHOTOSYSTEMS I AND II.endangered speciesData Book.See CITES; Redendarch Denoting a stele in which metaxylemdevelops to the outside of protoxylem.Compare centrarch; exarch; mesarch.endemic Describing a population orspecies that is restricted geographically.Some endemics have evolved in geographicalisolation on islands or mountain tops;others may be relics of once widespreadspecies that have become restricted owingto climate change, geological change, orhuman activity.endocarpSee pericarp.endocytosis The bulk transport of materialsinto cells across the plasma membraneby processes not involving diffusionor active transport. It is described aspinocytosis (cell drinking) or phagocytosis(cell eating) depending on whether the materialis fluid, containing molecules in solution,or solid respectively. The processinvolves extension and invagination of theplasma membrane to form small vesicles inpinocytosis (pinocytotic vesicles) or vacuolesin phagocytosis (food vacuoles). Thecontents are often digested by enzymesfrom lysosomes. Compare exocytosis. Seelysosome.endodermis The innermost part of thecortex, consisting of a single layer of cellsthat controls the passage of water andsolutes between the cortex and the stele. Aclearly defined endodermis is seen in allroots and in the stems of the non seed-bearingvascular plants and some dicotyledons.See Casparian strip; passage cells.endogenous Produced or originatingwithin an organism. Compare exogenous.endomitosis The duplication of chromosomeswithout division of the nucleus,causing polyploidy and leading to an increasein nuclear and cytoplasmic volume.Endomitosis occurs regularly in certain tissues,such as the phloem cells of some leguminousplants. Compare amitosis; mitosis.endomycorrhiza (endotrophic mycorrhiza)(pl. endomycorrhizae) See mycorrhiza.endonuclease An enzyme that catalyzesthe hydrolysis of internal bonds of polynucleotidessuch as DNA and RNA, producingshort segments of linked nucleotides(oligonucleotides). See also DNase; restrictionendonuclease.endophyte A bacterium, protoctist, orfungus that lives inside a plant or otherphotosynthetic organism, but is not a parasite.Compare epiphyte.endoplasm The inner layer of cytoplasmin plant cells, which contains themain organelles. Compare ectoplasm.endoplasmic reticulum (ER) A systemof membranes forming tubular channelsand flattened sacs (cisternae), runningthrough the cytoplasm of all eukaryoticcells and continuous with the nuclear envelope.Although often extensive, it was onlydiscovered with the advent of electron microscopy.Its surface is often covered withribosomes, forming rough ER. The pro-75

endopolyploidyteins they make can enter the cisternae fortransport to other parts of the cell or for secretionvia the GOLGI APPARATUS. ER lackingribosomes is called smooth ER and isinvolved with lipid synthesis, includingphospholipids and steroids.endopolyploidySee endomitosis.endoscopic Describing the type of developmentof a plant embryo in which theinner cell formed by the first division of thezygote develops into the embryo, while theouter cell develops into the suspensor. It isseen in many ferns and in all seed plants.Compare exoscopic.endosperm The nutritive tissue that surroundsthe embryo in angiosperms. Innonendospermic seeds most of the endospermis absorbed by the developing embryoand the food stored in the cotyledons.In endospermic seeds the endosperm replacesthe nucellus and is often a richsource of growth-regulating substances.Many endospermic seeds (e.g. cereals andoil seeds) are cultivated for their food reserves.The endosperm is usually triploidand develops from the primary endospermnucleus, the triploid nucleus resulting fromthe fusion of one of the male gametes fromthe pollen tube with the polar nuclei in theembryo sac.endospore A resting stage produced bycertain bacteria under unfavorable conditions.Endospores are formed within thecell, one in each parent cell, and are surroundedby a thick coat containing dipicolinicacid. On germination the wall is lysedand one vegetative cell is produced. Endosporescan remain viable for several centuriesand are resistant to heat, desiccation,and x-rays.endosporium (intine)See pollen.endosymbiont theory The theory thateukaryotic organisms evolved from symbioticassociations between bacteria. It proposesthat integration of photosyntheticbacteria, for example purple bacteria andcyanobacteria, into larger bacterial cellsled to their permanent incorporation asforerunners of the plastids (e.g. chloroplasts)seen in modern eukaryotes. Similarly,other symbiotic aerobic bacteria gaverise to the mitochondria.endotrophic mycorrhiza (pl. endotrophicmycorrhizae) See mycorrhiza.entire Describing the margin of a leaf,sepal, or petal that is smooth and undivided.entomophily Pollination by insects.Various structures and mechanisms haveevolved to attract insects (e.g. showy petalsand nectar) and to insure that they carrypollen away on their bodies.environment The complete range of externalconditions under which an organismlives, including physical, chemical, and biologicalfactors, such as temperature, light,the availability of food and water, and theeffects of other organisms.enzyme A compound that catalyzes biochemicalreactions. Enzymes are proteins,which act with a given compound (the substrate)to produce a complex, which thenforms the products of the reaction. The enzymeitself is unchanged in the reaction; itspresence allows the reaction to take placeby lowering the activation energy. Thenames of most enzymes end in -ase, addedto the substrate (e.g. lactase) or the reaction(e.g. hydrogenase).Enzymes are extremely efficient catalystsfor chemical reactions, and specific toparticular reactions. They may have a nonproteinpart (cofactor), which may be aninorganic ion or an organic constituent(coenzyme). The mechanism of action ofmost enzymes appears to be by active siteson the enzyme molecule. These have veryspecific shapes and charge distributions.The substrate acting with the enzymechanges shape to fit the active site, and thereaction proceeds. Enzymes are very sensitiveto their environment – e.g. temperature,pH, and the presence of other76

Equisetalessubstances. See activation energy. See alsoribozyme.Eocene The second oldest epoch of theTertiary period, 55–38 million years ago. Itwas a period of widespread temperate andsubtropical forests. The extensive grasslandsof the late Tertiary had not yet developed,but large-hoofed grazing mammals,such as horses and elephants, were evolvingrapidly. See also geological time scale.eosin An acid stain that colors cellulosered and cytoplasm pink. See staining.ephemeral A plant that has a very shortlife cycle and may complete more than onelife cycle within a year. Such plants areoften referred to as annuals. Examples areshepherd’s purse (Capsella bursa-pastoris)and certain desert plants that grow, flower,and set seed in brief periods of rain. Compareannual; biennial; perennial.epicarpSee pericarp.epicotyl The part of the plumule abovethe cotyledons. Compare hypocotyl.epidermis The outer protective layer ofcells in plants. In aerial parts of the plantthe outer wall of the epidermis is usuallycovered by a waxy cuticle that preventsdesiccation, protects the underlying cellsfrom mechanical damage, and increasesprotection against fungi, bacteria, etc. Thecells are typically platelike and closelypacked together except where they aremodified for a particular function, as areguard cells. The epidermis arises from thetunica meristem. In damaged stems androots, and in those undergoing secondarygrowth, it is replaced by a secondary layer,the periderm. The specialized epidermalarea of the roots from which the root hairsarise is termed the piliferous layer. Theterm epiblem is sometimes used instead todenote the outermost layer of cells in theroot.epigeal germination Seed germinationin which the cotyledons form the first photosyntheticorgans above the ground, e.g.sunflower (Helianthus). Compare hypogealgermination.epigyny The type of flower structure inwhich the perianth and androecium are insertedabove the gynoecium, giving an inferiorovary, fused with the receptacle. It isseen in the Asteraceae and Rosaceae. Comparehypogyny; perigyny.epilimnion In a stratified lake (a lake inwhich there are distinct layers of differenttemperature densities), the upper layer ofwarm water. See stratification.epinasty (epinastic movements) A nasticmovement that involves the curving of aplant organ away from the axis (i.e. downward)as a result of greater growth on theupper surface, as in the opening of flowers.See nastic movements; leaf mosaic.epiphylly 1. The growth of a plant onthe leaf of another plant. Such a plantis termed an epiphyll, or is described asepiphyllous. Examples include manymosses.2. The production of adventitious buds ona leaf, e.g. Bryophyllum, where the budsform along the leaf margins, developinginto small plantlets with adventitious rootsthat eventually become detached from theleaf.epiphyte Any plant growing upon or attachedto another plant or object merelyfor physical support. Examples of tropicalepiphytes are ferns, orchids, and bromeliads.In temperate regions vascular epiphytesare rare: here most epiphytes arelichens, mosses, liverworts, and algae.epithelium (pl. epithelia) A tissue consistingof a sheet (or sheets) of cells thatcovers a surface. Epithelia may line resincanals in gymnosperms, or, sometimes,gum ducts in dicotyledons, and usuallyhave a secretory role.equatorial plate An arrangement of thechromosomes in which the centromeres becomealigned in a single plane at the center77

ERof the spindle during metaphase of mitosisand meiosis.Equisetales (horsetails) The single orderof the phylum SPHENOPHYTA, which containsthe horsetails.ERSee endoplasmic reticulum.era A unit of the geological time scalethat is made up of several periods, e.g.the Mesozoic Era comprises the Triassic,Jurassic, and Cretaceous periods. Whenused formally, the initial letter is capitalized.ergot A fungal disease of many cereals.The hard black sclerotia of Claviceps purpurea,an ascomycete, replace the grains(see sclerotium). They contain alkaloidsthat can cause severe poisoning and evendeath if taken in with food by humans andother animals.Escherichia coli A rod-shaped Gramnegativebacterium, some strains of whichare normally present in animal intestines,soil, or water. It is widely used in geneticresearch and genetic engineering as arepository for fragments of DNA incorporatedinto plasmids in its cytoplasm.essential amino acidSee amino acids.essential element An element that is indispensablefor the normal growth, development,and maintenance of a livingorganism. Some, the major elements, arerequired in relatively large quantities andmay be involved in several different metabolicreactions (see carbon; hydrogen;oxygen; nitrogen; sulfur; phosphorus;potassium; magnesium; calcium). Others,such as iron, manganese, molybdenum,boron, zinc, copper, cobalt, iodine, and selenium,are required in only small orminute amounts. See micronutrient.essential oil Any of a number of volatileOILS secreted by aromatic plants that arethe source of characteristic odors or tastes.esterA compound formed by the condenstionreaction of a carboxylic acid withan alcohol:RCOOH + HOR 1 ® RCOOR 1 + H 2 O.estivation The way in which sepals andpetals are folded in the flower bud beforeexpansion. Compare ptyxis; vernation.ethanedioic acidethanoic acidetheneSee ethylene.See oxalic acid.See acetic acid.ethylene (ethene) A gaseous hydrocarbon(C 2 H 4 ), produced in varying amounts bymany plants, that functions as a plant HOR-MONE. Its production may be stimulated byother plant hormones. The amino acid methionineis a precursor. Ethylene is involvedin the control of germination, cellgrowth, fruit ripening, abscission, andsenescence, and it inhibits longitudinalgrowth and promotes radial expansion. Inripening fruit a rapid rise in ethylene productionprecedes respiration to reach theclimacteric ripeness: production of ethylenestimulates further production. This iswhy ripe fruit stimulate other fruits toripen quickly and this process can be controlledto some extent for fruit storage andtransport.etiolation The type of growth exhibitedby plants grown in darkness, usually fromseed. They lack chlorophyll and thereforeappear white or yellow. They show less differentiationand contain reduced amountsof supporting material, such as lignin, concentratingresources on elongation of internodes.The plant can then reach the lightfaster and photosynthesize and synthesizemore chlorophyll.etioplast A modified chloroplast formedfrom proplastids in leaves grown in totaldarkness.Eubacteria A major subkingdom of theBacteria, containing a large and diversegroup of bacteria, principally distinguishablefrom the other major subkingdom, theArchaea, by differences in the base se-78

eutrophicquences of RNA subunits of the ribosomes,which are thought to reflect the very earlyevolutionary divergence between the twogroups. Most are unicells that divide by binaryfission. The cells can be spherical,rod-shaped, or helical, and some formassemblages of cells, such as branchingfilaments. Most are immotile, but somepossess flagella. They are a ubiquitousgroup, some being found in extreme conditions.Cell-wall structure, morphology,and metabolic features are used to distinguishthe different phyla. The Eubacteriacontain both Gram-negative and Grampositivebacteria, and bacteria with no cellwalls at all.eucaryoteeuchromatinSee eukaryote.See chromatin.Euglenophyta Formerly, a phylum ofaquatic single-celled protoctists that swimusing one or more UNDULIPODIA (flagella),now classified as a class, the Euglenida, ofthe phylum Discomitochondria. It containsboth photosynthesizing and nonphotosynthesizingmembers. For example, membersof the genus Euglena possess a flexiblepellicle surrounding the cell, and a singleundulipodium. Many species containchloroplasts, with pigments similar tothose found in plants, although these organismsmay also consume dissolved orparticulate food from their surroundings.Eukarya In taxonomy, a DOMAIN of livingorganisms characterized by having eukaryoticcells (See eukaryote). In the FiveKingdoms classification, the Eukarya constitutesa ‘superkingdom’ containing thekingdoms Animalia, Fungi, Plantae, andProtoctista.eukaryote (Eukaryotae) All the livingkingdoms except the Bacteria (Archaeaand Eubacteria) are the eukaryotes. Theyare defined by the presence of a much moreelaborate cell than the prokaryotes. Eukaryotesare characterized by: the geneticmaterial packaged in chromosomes withina membrane-bound nucleus; possession ofmitochondria, and, in photosynthetic eukaryotes,chloroplasts; a quite differentand much elaborated membrane structure,including internal membranes such as theendoplasmic reticulum and Golgi apparatus;different-sized ribosomes; and complexUNDULIPODIA composed of arrays ofMICROTUBULES. Some of these featuresprobably arose through endosymbiosis ofprokaryotes. See endosymbiont theory.Euphorbiaceae A large family of dicotyledonousplants that include thespurges. There are over 8000 species. Theyare mostly trees and shrubs, with but a fewherbaceous species, and are found worldwide,but especially in the tropics. Commerciallyimportant species include Heveabrasiliensis, the main source of naturalrubber; cassava (Manihot esculenta); andcastor oil (Ricinus communis).euphotic zoneSee photic zone.euploidy The normal state in which anorganism’s chromosome number is anexact multiple of the haploid number characteristicof the species. For example, if thehaploid number is 7, the euploid numberwould be 7, 14, 21, 28, etc., and therewould be equal numbers of each differentchromosome. Compare aneuploidy.eusporangiate Describing the condition,found in clubmosses, horsetails, andcertain ferns in which the sporangia developfrom a group of initial cells and donot have an elaborate spore dispersalmechanism. Compare leptosporangiate.See also Filicinophyta.eustele The STELE arrangement found inmost gymnosperms and dicotyledons inwhich the vascular tissue is arranged into aring of discrete bundles separated bymedullary rays, all contained within a singlering of endodermis and pericycle. It is aform of SIPHONOSTELE.eutrophic Describing lakes or pondsthat are rich in nutrients and consequentlyare able to support a dense population of79

evergreenplankton and littoral vegetation. Eutrophicationis the process that results when anexcess of nutrients enters a lake, for exampleas sewage or from water draining offland treated with fertilizers. The nutrientsstimulate the growth of the algal populationgiving a great concentration or‘bloom’ of such plants. When these die theyare decomposed by bacteria, which use upthe oxygen dissolved in the water, so thataquatic animals such as fish are deprived ofoxygen and die from suffocation. Compareoligotrophic.evergreen Describing plants that retaintheir leaves throughout the year or throughseveral years. Many tropical species ofbroadleaved flowering plants are evergreenand their leaves are thicker and more leatherythan those of deciduous trees. Waterconservation is not a problem in tropicalevergreen forests, so there is no need toshed leaves in a particular season. In polarand cold temperate regions many of theevergreens are shrubs or trees with needlelikeor scalelike leaves, adaptations thatprevent water loss by transpiration. Comparedeciduous.evolution The process of genetic changethat occurs in populations of organismsover a period of time. It manifests itself asnew characteristics in a species, and eventuallythe formation of new species. SeeDarwinism; natural selection.exarch Denoting a stele in which themetaxylem develops to the inside of theprotoxylem. Compare centrarch; endarch;mesarch.excretion The process by which excess,waste, or harmful materials, resulting fromthe chemical reactions that occur withinthe cells of living organisms, are eliminatedfrom the body. In plants most waste productsaccumulate in the vacuoles, whose selectivelypermaeable membranes preventthem interfering in cell reactions. Gaseousexcretion takes place by diffusion throughthe stomata and lenticels. In fungi andsmall protoctists waste products are excretedby diffusion through the cell or bodysurface into the external medium.exine (exosporium)exocarp (epicarp)See pollen.See pericarp.exocytosis The bulk transport of materialsout of the cell across the plasma membrane.It involves fusion of vesicles orvacuoles with the plasma membrane in areversal of endocytosis. See also lysosome.Compare endocytosis.exodermis An outermost layer of thickenedor suberized cortical cells that sometimesreplaces the epidermal layer in theolder parts of roots if the epidermal cellshave died.exogamy The fusion of gametes producedby organisms that are not closely related.See outbreeding.exogenous Produced or originating outsidean organism. Compare endogenous.exon A segment of a gene that is bothtranscribed and translated and hence carriespart of the code for the gene product.Most eukaryotic genes consist of exonsinterrupted by noncoding sequences (seeintron). Both exons and introns are transcribedto heterogeneous nuclear RNA(hnRNA), an intermediary form of messengerRNA (mRNA); the introns are then removedleaving mRNA, which has only theessential sequences and is translated intothe protein. Bacteria do not have introns.exonuclease An enzyme that catalyzesthe hydrolysis of the terminal linkages ofpolynucleotides such as DNA and RNA,thereby removing terminal nucleotides. Seealso DNase; restriction endonuclease.exoscopic Describing the type of developmentof a plant embryo in which theapex of the sporophyte develops from theouter cell formed by the first division of thezygote, the inner cell giving rise to the foot.Compare endoscopic.80

eyespotexosporic The formation of spores outsidethe spore-producing organ. In thebasidiomycete fungi, for example, the basidiosporesare borne on the tips of thesterigmata, which are outgrowths of thebasidium.exosporium (exine)See pollen.explantation The culture of isolated tissuesof adults or embryos in an artificialmedium for the study of maintenance,growth, and/or differentiation. Such a pieceof transplanted tissue is called an explant,and its placement on the culture medium iscalled inoculation. Explants are also ameans of plant propagation, producingclones and offering a more rapid means ofmultiplication than seed production.exponential growth A type of growthin which the rate of increase in numbers ata given time is proportional to the numberof individuals present. Thus, when thepopulation is small multiplication is slow,but as the population gets larger, the rateof multiplication also increases. An exponentialgrowth curve starts off slowly andincreases faster and faster as time goes by(called the log phase). However, at somepoint factors such as lack of nutrients, accumulatedwastes, etc., limit further increase,when the curve of number againsttime begins to level off and the curve becomessigmoid (S-shaped). When the populationremains stable, it has reached thestationary phase. If organisms are introducedinto a new medium, there is often aperiod during which the population remainsconstant before it begins to increase(called the lag phase). During this phase thecells are increasing in size and synthesizingmaterials necessary for growth under thenew conditions.extensinOccurring or situated out-extracellularside a cell.See glycoprotein.extrachromosomal DNA In eukaryotes,DNA found outside the nucleus of thecell and replicating independently of thechromosomal DNA. It is contained withinorganelles in the cytoplasm, e.g. mitochondria,chloroplasts, and plastids, and is responsiblefor cytoplasmic inheritance. Seecytoplasmic inheritance.extrorse Denoting anthers in which dehiscencelines are to the outside of theflower, promoting cross-pollination. Compareintrorse.eyespot (stigma) A light-sensitive structureof certain protoctists and invertebrateanimals. The eyespot of unicellular andcolonial algae and their gametes and81

FF 1 The first filial generation; i.e. the firstgeneration that results from a particularcross.F 2 The second filial generation, obtainedby crossing or selfing within the F 1 generation.If the characteristic is governed bysimple Mendelian genes, the typical monohybridand dihybrid ratios become apparentin the F 2 generation.Fabaceae A large family of dicotyledonousflowering plants, commonly knownas legumes. There are about 18,000 species,which include herbs, trees, shrubs,climbers, and some aquatics. Most specieshave root nodules containing nitrogenfixingbacteria. The leaves are usually compoundand pinnate, with stipules. TheFabaceae have a worldwide distributionand many are of economic importance,including peas (Pisum sativum), beans(Phaseolus and Vicia), lentils (Lens culinaris),and peanuts (Arachis hypogea).Others are used for forage, e.g. clovers(Trifolium) and lucerne (Medicago sativa).There are also ornamental species such aslupins (Lupinus).facilitated diffusion A passive transportof molecules across a cell membranealong a concentration gradient, mediatedby carrier molecules or complexes, usuallyproteins. No energy is expended in thisprocess, but it enables the passage throughthe membrane of molecules that otherwisecould not pass through.facultative Describing an organism thatcan utilize certain conditions but is not dependenton them, i.e. an organism that canadopt an alternative mode of living. For example,a facultative anaerobe is an organismthat can grow under anaerobic conditionsbut is also able to survive in aerobicconditions. Compare obligate.FAD (flavin adenine dinucleotide) A derivativeof riboflavin that is a coenzyme inelectron-transfer reactions. It acts as aprosthetic group to various dehydrogenaseenzymes, the substrate being oxidized asFAD accepts electrons and is reduced toFADH 2 . See also flavoprotein.Fagaceae A family of deciduous or evergreentrees and shrubs that includes theoaks (Quercus), beeches (Fagus), southernbeeches (Nothofagus), and chestnuts (Castanea).Members of this family dominatebroad-leaved forests, especially of temperateregions.false fruitSee pseudocarp.family A taxonomic category involvinga collection of similar genera. Families maybe subdivided into subfamilies, tribes, andsubtribes. Plant family names generally endin aceae. Similar families are grouped intoorders.far-red light Electromagnetic radiationof wavelength approximately 740 nm. Seephytochrome; photoperiodism.fascicular cambium (pl. cambia)intrafascicular cambium.fast greenSee staining.Seefat A triglyceride of a long-chain carboxylicacid (fatty acid) that is solid below20°C. Fats commonly serve as energy storagematerial in some plants, especially inseeds. See also lipid.82

field capacityfatty acidSee carboxylic acid.feedback inhibition The inhibition ofthe activity of an enzyme (often the first)in a multienzyme reaction sequence bythe product of that sequence. When theproduct accumulates beyond an optimalamount it binds to a site (ALLOSTERIC SITE)on the enzyme, changing the shape so thatit can no longer react with its substrate.However, once the product is utilized andits concentration drops again, the enzymeis no longer inhibited and further formationof product results. The mechanism isused to regulate the concentration of certainsubstances within a cell.feedback loop (feedback mechanism) Aregulatory mechanism in a system. Thismay involve negative feedback, for example,an increase in the number of herbivoresin an ecosystem leads to overgrazingand reduction of the herbivores populationby starvation or increased susceptibility todisease, which allows the plants to growback, and so on – the population of bothspecies fluctuates about a mean. Positivefeedback has the opposite effect, reinforcinga change in the system. In certain metabolicpathways, for example, the substrateof the first enzyme in the pathway stimulatessubsequent reactions in the sequence.fermentation The breakdown of organicsubstances, particularly carbohydrates,under anaerobic conditions. It is aform of anaerobic respiration and is seen incertain bacteria and yeasts, and also inplants under waterlogged conditions. Seealcoholic fermentation. See also glycolysis;lactic acid bacteria.fernsSee Filicinophyta.ferredoxins A group of red-brown proteinsfound in green plants, many bacteria,and certain animal tissues. They containnonheme iron in association with sulfur atthe active site. They are strong reducingagents (very negative redox potentials) andfunction as electron carriers, for examplein photosynthesis and nitrogen fixation.They have also been isolated from mitochondria.In photosynthesis ferredoxin isinvolved in electron transfer between photosystemI and the final electron acceptor,NADP.fertilization (syngamy) The fusion of amale gamete with a female gamete to forma zygote, the essential process of sexual reproduction.External fertilization occurswhen gametes are expelled into a waterysubstrate from the parental bodies beforefusion; it is typical of sporebearing plants.Internal fertilization takes place in the seedplants and some clubmosses and ferns,often by means of a pollen tube. Internalfertilization is an adaptation to life in a terrestrialenvironment.Feulgen’s stain A test used to show thepresence of DNA, especially in dividing nuclei.A sample of tissue is placed in dilutehydrochloric acid at 60 o C to hydrolyze theDNA. This exposes the aldehyde groups ofdeoxyribose. The tissue is then soaked inSchiff’s reagent: the development of a magentacolor indicates the location of DNA.See staining.fiber A form of SCLERENCHYMA cell thatis often found associated with vascular tissue.Fibers are long narrow cells, withthickened walls and finely tapered ends.Their function is more as supporting tissuethan as conducting tissue. Where theyoccur interspersed with the xylem theymay be distinguished from tracheids bytheir narrower lumen.fiber-tracheid An elongated cell withbordered pits found in wood, intermediatein form between a fiber and a tracheid.fibrous root system A branching rootsystem in which there is no main root. Infibrous root systems the radicle or primaryroot either branches or dies and is replacedby adventitious roots, e.g. grasses(Poaceae). Fibrous root systems are characteristicof monocotyledons. Compare taproot.83

field layerfield capacity The point at which thesoil contains all the water it can hold bycapillary and chemical attraction. Anymore water added to soil at field capacitywould drain away by gravity. Soil at fieldcapacity has a high water potential. Watercontent at field capacity is usually expressedas a percentage of the weight orvolume of oven-dry soil. This is affected bythe soil texture, clay having a high field capacityand sand a low field capacity.field layer The herb layer of a plantcommunity. Compare canopy; groundlayer.filament 1. The stalk of the stamenbearing the anther in angiosperms.2. The vegetative body of the filamentousalgae (e.g. Spirogyra), composed of a lineof similar cells joined by their end walls.3. A strand of protein of diameter 4–15 nmfound inside a cell. Such filaments are usuallycomposed of the contractile proteinsactin and/or myosin, and are thought toplay a role in cell motility, and perhaps alsochange in shape.Filicinophyta (ferns) The largest phylumof spore-bearing vascular plants comprisingthe ferns and including about12,000 species. It is divided into two maingroups, the LEPTOSPORANGIATE ferns andthe EUSPORANGIATE ferns. In leptosporangiateferns, the sporangia are thin-walledand develop from one cell. In the Filicalesthe leptosporangium has a specializedspore dispersal mechanism involving anannulus of cells with thick walls that ejectss pores explosively as it dries out. Leptosporangiateferns include the typicalferns (Filicales) and the water ferns (Marsilealesand Salviniales). The Filicales ismuch the largest living group of ferns, andthey normally have large spirally arrangedleaflike fronds (MEGAPHYLLS) derived fromlateral branches of the main axis, some orall of which bear sporangia on their marginsor undersurfaces. In eusporangiateferns, the sporangia are thick-walled anddevelop from a number of cells. Eusporangiateferns include the adderstongues(Ophioglossales), Marattiales, and royalferns (Osmundales), which are to some extentintermediate between leptosporangiateand eusporangiate ferns. The whiskferns (Psilotales) may also be includedhere, but in the Five Kingdoms classificationthey have the status of a phylum(Psilophyta). There are also a number offossil genera, leptosporangiate ferns datingback to the Cretaceous era, eusporangiateferns to the Devonian. Ferns do not showsecondary thickening, but in large fernssuch as tree ferns bands of sclerenchymaand the overlapping bases of the frondshelp to support the plant.fimbriae (sing. fimbria)fine structureSee pili.See ultrastructure.FISH (fluorescence in situ hybridization)A technique for locating specific sequencesof DNA using fluorescent probes that areviewed under the MICROSCOPE. The probesbind to (hybridize with) specific sequencesof DNA; different colors of probe may beused to distinguish different DNA sequences.See DNA probe.fission A type of asexual reproductionin which a parent cell divides into twodaughter cells (binary fission) geneticallyidentical to the parent. Binary fission occursin many unicellular organisms, such asprotoctists, bacteria, and fission yeasts.Fission begins with division of the nucleusby mitosis, followed by cytoplasmic division,and sometimes sporulation. Comparebudding; fragmentation.fitness In an evolutionary context, theability of an organism to produce a largenumber of offspring that survive to a reproductiveage.Five Kingdoms classification A classificationsystem that recognizes five kingdomscontaining at least 96 phyla. Itcomprises two superkingdoms, the Prokarya(PROKARYOTES) and Eukarya (EU-KARYOTES). The Prokarya contains a singlekingdom, the BACTERIA, which is divided84

floral formulainto two subkingdoms, the ARCHAEA andthe EUBACTERIA. The Eukarya contains fourkingdoms, the Animalia, Plantae, Fungi,and Protoctista.fixation 1. The situation in a usuallysmall population when an allele reaches a100% frequency due to the complete lossof all other alleles of that gene. All membersof the population are homozygous forthis particular allele.2. In the preparation of microscope slides,the process by which tissues are rapidlykilled and preserved by chemicals to preventdecay or autolysis with minimaldistortion of structure. Fixing agents (fixatives)should also render cell organelles andinclusions more visible and harden the tissueto prevent shrinkage and distortionduring dehydration, embedding, and sectioning.Examples of fixatives areformaldehyde, osmium tetroxide, ethanol,and acetic acid. See also dehydration; embedding.3. The incorporation of inorganic materialssuch as carbon and nitrogen into organiccompounds by living organisms. Anexample is the incorporation (fixation) ofcarbon from carbon dioxide into new organiccompounds during photosynthesis.See also nitrogen fixation.flaccid Lacking turgor. When a plantloses water to the extent that its cells becomeflaccid, wilting occurs. See plasmolysis.flagellum (pl. flagella) A whiplike extensionof prokaryote cells, with a basalbody at its base, whose beat causes locomotionof the cell. Strictly the term is nowreserved for the bacterial flagellum. Theflagella and cilia of eukaryote cells have aquite different structure and are called UN-DULIPODIA. Bacterial flagella are much simplerthan undulipodia, being hollowcylinders about 15 nm in diameter, consistingof subunits of a protein (flagellin)arranged in helical spirals.flavin A derivative of riboflavin occurringin the flavoproteins; i.e. FAD or FMN.Flavins are light-sensitive yellow pigmentswith an absorption peak around 370 nm.They are involved in phototropism, and inPHOTOPERIODISM, where they are thoughtto affect phytochrome by transferring lightFlavonoid structureenergy from blue to red wavelengths. Seephytochrome.flavin adenine dinucleotideflavin mononucleotideSee FAD.See FMN.flavonoid One of a common group ofplant compounds having the C 6 –C 3 –C 6chemical skeleton in which C 6 is a benzenering. They are an important source of nonphotosyntheticpigments in plants.flavoprotein A conjugated protein inwhich a FLAVIN (FAD or FMN) is the prostheticgroup joined to a protein component.Flavoproteins are enzymes and manyare dehydrogenases. They act as electrontransportagents and play an importantrole in the electron-transport chains ofrespiration, photosynthesis, and the microsomalsystem responsible for the desaturationof fatty acids in heterotrophicbacteria.flocculation The aggregation of soilparticles into crumbs. Compacted structurelessclay soils can be flocculated by theaddition of neutral salts, particularly ofcalcium. The addition of lime to saturatedclay soils (liming) is a common agriculturalpractice, improving soil structure by en-85

floral provincecouraging crumb formation and makingheayy soils more workable.floral formula A series of symbols usedto describe the structure of a flower. Thevarious whorls of structures are abbreviatedas follows: K (calyx), C (corolla), P(perianth), A (androecium), and G (gynoecium).These letters are each followed by anumber indicating the number of parts inthe whorl. If the number exceeds 12 thenthe symbol ¥ is used to denote an indefinitenumber. Fusion of parts of the whorl is indicatedby placing the number of parts inbrackets. The position of the gynoecium isshown by a line above or below the letter Gdenoting an inferior or superior ovary respectively.The formula is preceded by Å toindicate actinomorphic flowers and ·|· forzygomorphic flowers. The floral formulafor the buttercup would be written:Å K5 C5 A¥ G¥floral province (floral kingdom; floral region)One of the six main regions and afew minor ones into which the world is dividedon the basis of plant distribution.Each province has one or more centers ofdiversity and each contains unique familiesand genera of plants. The main provincesare the Boreal, which covers all of thenorth temperate; the Neotropical, coveringtropical Central and South America; thePaleotropical, covering tropical Africa andAsia; the Australian; the Cape Province,covering a small area around the Cape ofGood Hope; and the Antarctic, includingNew Zealand and temperate South America.Oceanic islands, particularly in the Pacific,form minor provinces. There arefloral affinities and connections betweenthe provinces, particularly near theirboundaries.floret 1. An individual small flower of aclustered inflorescence, e.g. daisy (Bellis).2. In grasses, the lemma, palea, and theflower they enclose.florigen (flowering hormone) A hypotheticalplant hormone that has been postulatedto account for the transfer of thephotoperiod stimulus from the leaves tothe apex where flowering is induced. Thestimulus can be transferred from one plantto another by grafting, but attempts to isolateflorigen have so far been unsuccessful.See photoperiodism.flower The characteristic reproductivestructure of an angiosperm. It usually consistsof an axis or receptacle bearing thesepals, petals, and stamens. The gynoeciumis borne either above the receptacle or enclosedwithin it. One or more of thesewhorls may be absent. Flower structuresare extremely variable and show numerousadaptations to promote pollination andseed dispersal. See also floral formula; inflorescence.flowering hormoneflowering plantsSee plasma mem-fluid-mosaic modelbrane.See florigen.See angiosperms.fluorescence in situ hybridizationFISH.Seefluorescence microscopy A form ofmicroscopy in which fluorescent probesare added to the material being investigated.These probes bind to specific partsof cells or to particular molecules, such asparticular sequences of DNA or protein receptors.FMN (flavin mononucleotide) A derivativeof riboflavin that is a coenzyme inelectron-transfer reactions. It acts as aprosthetic group to various dehydrogenases,such as NADH dehydrogenase,which catalyzes the transfer of electronsfrom NADH to coenzyme Q in the respiratoryelectron-transport chain. See alsoflavoprotein.folic acid (pteroylglutamic acid) One ofthe water-soluble B-group of VITAMINS.Folic acid is important in metabolism invarious coenzyme forms, all of which arespecifically concerned with the transfer86

freeze fracturingand utilization of the single carbon (C 1 )group. It is important in the growth and reproductionof cells, participating in thesynthesis of purines and thymine.follicle A dry dehiscent fruit formedfrom one carpel that splits along one edgeto release its seed, for example Delphiniumfruit.food chain The chain of organisms existingin any natural community, throughwhich food energy is transferred. Each linkin the food chain obtains energy by eatingthe one preceding it and is in turn eaten bythe organisms in the link following it. Ateach transfer a large proportion (80–90%)of the potential energy is lost as heat, thereforethe number of links in a sequence islimited, usually to 4 or 5. The shorter thefood chain, the greater the available energy,so total energy can be increased bycutting out a step in the food chain, for exampleif people consume cereal grain insteadof consuming animals that eat cerealgrains.Food chains are of two basic types: thegrazing food chain, which goes from greenplants, to grazing herbivores, and finally tocarnivores; and the detritus food chain,which goes from dead organic matter,to microorganisms, and then to detritusfeedingorganisms. The food chains in acommunity are interconnected with oneanother, because most organisms consumemore than one type of food, and the interlockingpattern is referred to as a food webor food cycle. See trophic level.food webSee food chain.foot The basal portion of an embryo,sporophyte, or spore-producing body,which is embedded in the tissue of the parent.forest 1. A plant formation dominatedby trees whose crowns touch, forming acontinuous canopy.2. A major plant community in which thedominant plants are trees.form The lowest taxonomic group,ranking below the variety level, normallyreferring to the recognizably different morphologyfound in several members within aplant species. Subforms may also be recognized.formalin A mixture of about 40%formaldehyde, 8% methyl alcohol, and52% water (the methyl alcohol is presentto prevent polymerization of the formaldehyde).It is a powerful reducing agent andis used as a disinfectant, germicide, andfungicide and also as a general preservingsolution.fossil The remains of, or impressions leftin rocks by, long-dead animals and plants.Most fossils consist of hard skeletal materialbecause soft tissues and organs rot awayvery quickly. Plants seldom contain hardmineral material, so they are preservedwell only under certain conditions. Withrapid burial in a moist anoxic environment,such as the coal swamps of the Carboniferousperiod, carbonaceaous films ofplant material like fern fronds may survive.Alternatively, mineral salts from surroundingrocks gradually replace the hard organicmaterial, to give a cast in a processtermed petrification, as in the various petrifiedforests dotted around the globe. Thehard coats of spores and pollen grains arepreserved better, forming microfossils. Alternativelythe organic material dissolvesaway leaving an impression or mold in thesurrounding rocks. See also chemical fossils;palynology.fragmentation 1. A form of ASEXUALREPRODUCTION in which the plant or algabreaks into fragments, each fragmentgrowing into a new, independent plant.Examples include filamentous algae suchas Spirogyra, and waterweeds like theCanadian pondweed (Elodea). Comparebudding; fission.2. The separation through human activityof once continuous habitats into fragmentsthat may or may not contain viable populationsof the organisms present.87

Fries, Elias MagnusSee gene muta-frameshift mutationtion.freeze drying A technique for dehydratinga specimen by the sublimation of iceunder a vacuum. It avoids damage thatwould result from heating.freeze fracturing A method of preparationof material for electron microscopy,particularly useful for studying membranes.Material is frozen rapidly (e.g. byimmersion in liquid nitrogen) thus preservingit in lifelike form. It is then fractured,usually with a sharp knife. The fractureplane tends to follow lines of weakness,such as between the two lipid layers ofmembranes, revealing their internal surfaces.Carbon or platinum replicas made ofthe surfaces are shadowed for examinationin the electron microscope. In freeze etchingthe fractured surface is etched, i.e. someice is allowed to sublime away beforeSHADOWING. This exposes more detailedstructure, such as the outer surface of themembrane.Fries, Elias Magnus (1794–1878)Swedish mycologist. Fries, working atLund University, published Systema Mycologicum,his three-volume work on theclassification of fungi, in 1921–22. He alsopublished flora covering all the Scandinaviancountries and an important work onEuropean lichens.frond A term usually applied to largewell-divided leaves as found in ferns,palms, and cycads. The leaflike thalli ofcertain algae and lichens may also betermed fronds. See also megaphyll.fructification A seed- or spore-bearingstructure. The term is used especially forthe aerial fruiting bodies of fungi, e.g.mushrooms.fructose A sugar (C 6 H 12 O 6 ) found infruit juices, honey, and cane sugar. It is aketohexose, existing in a pyranose formwhen free. In combination (e.g. with glucosein the disaccharide sucrose) it exists inthe furanose ring form. Fructose formspolymers called fructosans, which includeINULIN, an important storage polysaccharidein the Asteraceae.fruit The ripened OVARY of a flower thatis usually formed following fertilization ofthe ovule. It may consist of the ripenedovary only or include other parts of theflower. Fruits vary according to themethod of seed dispersal, succulent fruitsnormally being distributed by animalswhile dry fruits may be dispersed by windor water, and dehiscent fruits split to releasethe seeds with the fruit remaining onthe plant. Fruits are classified according tohow the ovary wall (pericarp) develops, dependingon whether it becomes fleshy orhard. Fruits are further classified accordingto whether or not the fruit wall opens to releasethe seeds. See also dehiscent; indehiscent;multiple fruit; pseudocarp.Fucales An order of Phaeophyta (brownalgae) in which the haploid generation consistsonly of the gametes. The gametes areproduced in special cavities called conceptacles,often in the inflated tips of branchesof the thallus, e.g. wracks (Fucus). See alternationof generations.Fuchs, Leonhard (1501–66) Germanbotanist and physician. His beautifully illustratedherbal manual De historia stirpumwas published in 1542. It containedabout four hundred German plants andabout a hundred foreign plants, arrangedin alphabetical order. The illustrationswere accompanied by a description of eachplant and its habitat and even informationabout the best time to collect them. Thebook also included a glossary of terms.Fuchs is best known for the genus of ornamentalshrubs Fuchsia, which was namedfor him.fucoxanthin A brownish xanthophyllpigment found in Diatoms, Chrysomonada,Haptomonada, Phaeophyta, andsome Dinomastigota. The light absorbed isused with high efficiency in photosynthesis,the energy first being transferred to88

Fruit: a classification of types of fruit89

Fungichlorophyll a. It has three absorptionpeaks covering the blue and green parts ofthe spectrum, which penetrate to greaterdepths under water than the red end of thespectrum. See carotenoids.fumaric acid (trans-butenedioic acid) Anunsaturated dicarboxylic acid, which occursin many plants. The fumarate ion participatesin several important metabolicpathways, e.g. the Krebs cycle, purinepathways, and the urea cycle.Fungi A kingdom of nonphotosyntheticmainly terrestrial organisms that are quitedistinct from plants and animals, andplaced in their own kingdom, Fungi. Theyare characterized by having cell walls madechiefly of chitin, not the cellulose of plantcell walls, and they all develop directlyfrom spores without an embryo stage.Moreover, undulipodia are never found inany stage of their life cycles. Fungi are generallysaprophytic or parasitic, and may beunicellular or composed of filaments(termed hyphae) that together comprise thefungal body or mycelium. Hyphae maygrow loosely or form a compacted mass ofpseudoparenchyma giving well-definedstructures, as in toadstools. Fungi aremajor decomposers in soil, and freshwater. Some form symbiotic associationswith algae or cyanobacteria to formlichens, while others form extremely importantmycorrhizae with the roots ofmany plants, including most forest trees.Some parasitic fungi cause diseases inplants and animals, while others aresources of antibiotics. Yeasts and otherfungi are used in commercial fermentationprocesses such as brewing. There are threemain phyla of Fungi, based on the natureof their fruiting structures: the Zygomycota,Ascomycota, and Basidiomycota. Inthe modern Five Kingdoms classification,certain funguslike organisms that were previouslyconsidered to be fungi, are nowassigned to phyla in the Kingdom Protoctista.Fungi Anamorphici (Deuteromycota;Fungi Imperfecti; mitosporic fungi) Aphylum of so-called ‘imperfect’ fungi inwhich sexual reproduction is unknown orhas been lost during evolution. Its members,for example the Penicillium molds,are assigned as a taxonomic convenience,rather than by any true criteria, and someauthorities prefer to allocate the deuteromycotesto either the Ascomycota (the majority)or the Basidiomycota, on the basisof available evidence. Indeed Penicillium isnow known to have a sexual stage, formerlyregarded as a quite distinct ascomycetefungus (Talaromyces). Most ofthese fungi reproduce by conidia producedby mitosis (see conidium). There are about15 000 species.fungicide A chemical that inhibits fungalgrowth. They are used to prevent fungalinfection as well as to deal with existinginfections.90

Ggalactose A sugar found in lactose andmany polysaccharides. It is an aldohexose,isomeric with glucose. See also sugar.gall An abnormal outgrowth or swellingon a root, stem, or leaf in response toattack by a parasite or to invasion by asymbiont. ROOT NODULES induced by thenitrogen-fixing bacterium Rhizobium arepart of a symbiotic relationship (see symbiosis).gametangium (pl. gametangia) A cell ororgan in which sexual cells (gametes) areproduced. When the gametes are readilydistinguished as male or female, otherterms may be used such as ANTHERIDIUM,ARCHEGONIUM, or OOGONIUM.gamete A cell capable of fusing with anothercell to produce a zygote, from whicha new individual organism can develop.Gametes may have similar structure andbehavior (isogametes), as in many simpleorganisms, but are usually dissimilar in appearanceand behavior (anisogametes).The typical male gamete is small, motile(by means of UNDULIPODIA), and producedin large numbers. The typical female gameteis large because of the food reserves itcontains, and is produced in smaller numbersthan the male gametes. In all exceptthe most primitive organisms, the femalegamete is nonmotile (OOGAMY). Gametesare usually haploid, so fusion of gametesresults in the nucleus of the zygote havingthe diploid number of chromosomes.gametogenesiscells or gametes.The formation of sexgametophyte The gamete-producing generationin the life cycle of a plant or multicellularalga. The gametophyte is HAPLOIDand produces sex organs. It derives fromthe germination of haploid spores producedby the SPOROPHYTE generation. It isthe main generation in the life cycle of theBryata, in which the sporophyte is completelyor partially dependent upon it. ThePROTHALLUS of Lycophyta (clubmosses),Sphenophyta (horsetails), and Filicinophyta(ferns), which may be a green structureon the soil surface or a subterraneanmycorrhizal structure, is the gametophyte.In these groups and in the bryophytes,some species produce unisexual gametophytesand some produce hermaphroditegametophytes. In the Coniferophyta the femalegametophyte is reduced to a nonphotosynthetictissue with up to 2000 nuclei inthe ovule, which produces archegonia (seearchegonium). While in the Angiophytathe female gametophyte is simply the EM-BRYO SAC. The male gametophytes ofconifers and angiosperms are the POLLENgrains. See also alternation of generations.Gamophyta A phylum of green algae inwhich sexual reproduction is by conjugation,e.g. Spirogyra and DESMIDS. Amoeboidgametes are produced, which cross toadjacent filaments through conjugationbridges and fuse to form a zygote, whichusually forms a resistant zygospore. Ongermination, the zygospore undergoesmeiosis to produce haploid cells that giverise to a new thallus. Asexual reproductionis by FRAGMENTATION.garrigue Scrub woodland, a secondaryformation derived from the original mixedforest, characteristic of limestone areas ofthe Mediterranean region, with low rainfalland thin, dry soils. It is dominated bylow-growing prickly dwarf shrubs and91

gene In classical genetics, a unit ofhereditary material located on a chromosomethat, by itself or with other genes, determinesa characteristic in an organism. Itcorresponds to a segment of the geneticmaterial, usually DNA (although the genesof some viruses consist of RNA). Genesmay exist in a number of forms, termed alleles.For example, a gene controlling thecharacteristic ‘height’ in peas may havetwo alleles, one for ‘tall’ and another for‘short’. In a normal diploid cell only twoalleles can be present together, one on eachof a pair of homologous chromosomes: thealleles may both be of the same type, orthey may be different. The segregation ofalleles at meiosis and their dominance relationshipsare responsible for the particulatenature of inheritance. Genes can occasiongaschromatographyaromatic herbs (most commonly of theLamiaceae), such as lavender (Lavandulavera), sage (Salvia officinalis), and thymes(Thymus), many with drought-resistantfoliage. There are also bulbous plants suchas species of Iris. Compare chaparral;maquis.gas chromatography A technique widelyused for the separation and analysis ofmixtures. Gas chromatography employs acolumn packed with either a solid stationaryphase (gas–solid chromatography orGSC) or a solid coated with a nonvolatileliquid (gas–liquid chromatography orGLC). The whole column is placed in athermostatically controlled heating jacket.A volatile sample is introduced into the columnusing a syringe, and an unreactive carriergas, such as nitrogen, passed throughit. The components of the sample will becarried along in this mobile phase. However,some of the components will clingmore readily to the stationary phase thanothers, either because they become attachedto the solid surface or because theydissolve in the liquid. The time taken fordifferent components to pass through thecolumn is characteristic and can be used toidentify them. The emergent sample ispassed through a detector, which registersthe presence of the different components inthe carrier gas.gas–liquid chromatographychromatography.gas–solid chromatographychromatography.See gasSee gasgel filtration (gel-permeation chromatography)A chromatographic method usinga column packed with porous carbohydrate-gelbeads of standard size and porosity.It is a standard technique used forseparating and identifying macromoleculesof various sizes, e.g. proteins or nucleicacids. A solution of the mixture of macromoleculesis added to the top of the columnand allowed to flow through by gravity.The smaller molecules are hindered in theirpassage down the column because they arebetter able to penetrate the hydrated poreswithin the particles of the gel. Moleculestoo large to penetrate the pores are excluded,and thus flow more rapidlythrough the column. By analyzing the liquidthat drips from the bottom of the column(the eluate) at set intervals andcomparing it with a standard (obtained byrunning a known macromolecule throughthe column) information about the sizesand molecular weights of the componentsof the mixture is gathered. The most frequentlyused commercial gel is Sephadex.gemma (pl. gemmae) 1. A vegetativepropagule produced by mosses, liverworts,and certain Lycophyta (clubmosses). Gemmaeare small dispersal units consisting ofone to several cells, often forming disks,plates, or filaments. They often form ingroups in receptacles called gemma-cupsand eventually become detached from theparent to form new plants.2. See chlamydospore.gemmation A type of asexual reproduction,seen in mosses and liverworts, involvingthe production of a group of cells (agemma) that develops into a new individual,before or after separation from theparent.92

generative cellally undergo changes, called MUTATIONS, tonew allelic forms.Although the DNA molecules of thechromosomes account for the great majorityof genes, genes are also found asPLASMAGENES in certain DNA-containingcytoplasmic bodies (e.g. mitochondria,plastids). A gene can be defined as thesmallest hereditary unit capable either ofrecombination or of mutation. alternativelyit may be described as a unit of function,a CISTRON, which may be much largerthan a unit of recombination or mutation,and controls the synthesis of a singlepolypeptide or a messenger RNA moleculewhose genetic code is contained within thegene. Research with bacteria has shownthat the smallest unit of recombination ormutation is one base pair, while the size ofa unit of function can be determined by theCIS–TRANS TEST.gene bank 1. A place where plant materialis stored in a viable condition inorder to preserve plants that are in dangerof becoming extinct in the wild or cultivarsthat are being lost from cultivation. Thematerial also provides a source of genes forbreeding new varieties.2. See gene library.gene cloning (DNA cloning) A techniqueof GENETIC ENGINEERING whereby agene sequence is replicated, giving manyidentical copies. The gene sequence is isolatedby using restriction endonucleases, orby making a complementary DNA from amessenger RNA template using a reversetranscriptase. It is then inserted into the circularchromosome of a cloning vector, i.e.a plasmid or a bacteriophage. The hybrid isused to infect a bacterium, usually Escherichiacoli, and is replicated within thebacterial cell. A culture of such cells producesmany copies of the gene, which cansubsequently be isolated and purified.genecology The study of ecological genetics,or population genetics in relation toenvironment.gene flowThe movement of alleleswithin and between populations of thesame species through interbreeding.gene frequency The proportion of anallele in a population in relation to otheralleles of the same gene.gene library A collection of clonedDNA fragments derived from the entireGENOME of an organism. The genetic materialof the organism is first broken uprandomly into fragments using restrictionenzymes, for example. Then each fragmentis cloned using a VECTOR (e.g. plasmid orbacteriophage) inside a suitable host, suchas the bacterium E. coli. Particular genes orDNA sequences are identified by a suitableDNA PROBE. See also gene bank; genecloning.gene mutation An alteration in a singlegene resulting from a change in the number,type, or sequence of nucleotide bases.Unlike CHROMOSOME MUTATIONS, gene mutationsare not visible under the microscope.A point mutation is a change at asingle locus. It may be an addition, deletion,inversion, or substitution of one ormore bases. A substitution of one or twobases may still result in the same aminoacid, or at worst to just one amino acidchange, and may have little effect on theresulting protein. However, if the mutationresults in the loss or gain of one ortwo bases, it causes the remainder of thegene to be misread during translation, as itchanges the ‘reading frame’; for example,CUC/CAG/GCA becomes UCC/AGG/CA.Such mutations are called frameshift mutations,and are more likely to be deleterious.See mutation; chromosome mutation.gene pool The total number and varietyof genes existing within a breeding populationor species at a given point in time.gene probeSee DNA probe.generative cell One of the haploid cellsin the POLLEN TUBE of seed plants. In angiospermsit gives rise directly to the twomale gametes or generative nuclei. In gym-93

generative nucleinosperms it gives rise to the body cell andstalk cell.generative nuclei The two gametic nucleiformed by the division of the generativecell in the pollen tube of angiosperms.One fuses with the egg cell to form a zygotewhile the other either degenerates or, incertain angiosperms, fuses with the polarnuclei to give the primary endosperm nucleus.gene sequencing Determination of theorder of bases making up a gene on a DNAmolecule. Sequencing requires multipleclonedcopies of the gene; long DNA sequencesare cut into more manageablelengths using restriction enzymes. Sincethese cleave DNA at specific points, it ispossible to reconstitute the overall sequenceonce the constituent fragmentshave been analyzed individually.gene splicing 1. The joining of EXONSafter the INTRON sequences have been removed,to produce functional messengerRNA that leaves the nucleus to undergoTRANSCRIPTION. In the nucleus this is performedby a special assemblage of RNAand proteins called a spliceosome.2. In GENETIC ENGINEERING, the joining ofDNA fragments by the action of the enzymeDNA ligase.genetic code The sequence of bases ineither DNA or messenger RNA that conveysgenetic instructions to the cell. Thebasic unit of the code consists of a group ofthree consecutive bases, the base triplet orCODON, which specifies instructions for aparticular amino acid in a polypeptide, oracts as a start or stop signal for translationof the message into polypeptide assembly.For example, the DNA triplet CAA (whichis transcribed as GUU in mRNA) codes forthe amino acid valine. There are 64 differenttriplet combinations but only 20 aminoacids; thus many amino acids can be codedfor by two or more triplets. The code issaid to be degenerate, since some aminoacids are coded for by more than onetriplet: in certain cases only the first one ortwo bases, are necessary to insure the codingof a specific amino acid. Three triplets,termed ‘nonsense triplets’, do not code forany amino acid and have other functions,e.g. marking the beginning and end of apolypeptide chain.genetic drift (Sewall Wright effect) Thefluctuation of allele frequencies in a smallpopulation due entirely to chance. If thenumber of matings is small, then the actualnumbers of different types of pairing maydepart significantly from the number expectedon a purely random basis. Geneticdrift is one of the factors that can disturbthe HARDY–WEINBERG EQUILIBRIUM.genetic engineering The direct introductionof foreign genes (from other individualsor species or artificially synthesized)into an organism’s genetic material by micromanipulationat the cell level. Geneticengineering techniques bypass crossbreedingbarriers between species to enable genetransfer between widely differing organisms.The commonest method of geneticengineering is RECOMBINANT DNA TECHNOL-OGY. Genes may be take from living tissuesby isolating the RNA produced when thegene is active, and recreating the DNA sequenceof the gene using the enzyme reversetranscriptase. Gene transfer can beachieved by various methods, many ofwhich employ a replicating infective agent,such as a virus, plasmid, or phage for DNAsegments up to 24 000 base pairs in length,a cosmid for segments up to 45 000 basepairs, or a YAC (yeast artificial chromosome)for pieces up to 50 000 base pairs, asa VECTOR (see gene cloning). Other methodsinclude microinjection of DNA intocell nuclei and direct uptake of DNAthrough the cell membrane. Recognizingwhether or not transfer has occurred maybe difficult unless the new gene confers anobvious visual or physiological characteristic.Consequently the desirable gene maybe linked to a marker gene, e.g. a gene conferringresistance to an antibiotic in thegrowth medium. The transferred genemust also be linked to appropriate regulatoryDNA sequences to insure that it works94

genetic engineeringin its new environment and is regulatedcorrectly and predictably.Initial successes in DNA transfer wereachieved with bacteria and yeast. Humangenes coding for medically useful proteinshave been transferred to bacteria. Humaninsulin, growth hormone, and interferonare now among a wide range of therapeuticsubstances produced commerciallyfrom genetically engineered bacteria. Geneticallyengineered vaccines have alsobeen produced by transfer of antigencodinggenes to bacteria. Genes have beenintroduced to crop plants for variousreasons, for instance to reduce damageduring harvest or to make them resistantto the herbicides used in controlling weeds.Genetically modified tomatoes and soyabeans are now widely available. Thereis also hope that in future many geneticdiseases will be treatable by manipulatingthe faulty genes responsible. However, ge-Genetic code95

genetic fingerprintingnetic engineering raises many legal and ethicalissues, and the introduction of geneticallymodified organisms into theenvironment requires strict controls andmonitoring.genetic fingerprinting A technique foridentifying individuals by means of theirDNA. The DNA being tested is extractedfrom cells (from blood, semen, tissue fragments,etc.) and broken into fragments of600–700 bases each, using restriction enzymes.The human genome contains manyloci where short base sequences are repeatedin tandem, with great variation betweenindividuals in the number of suchrepeats. These so-called variable numbertandem repeats (VNTR) can be identifiedusing special DNA probes, thus providinga virtually unique set of markers for anygiven individual. This technique is used inveterinary and human medicine to establishthe parentage of individuals, and inforensic science to identify individualsfrom traces of body tissue or fluids. Evenminute amounts of DNA can now be amplified,using the POLYMERASE CHAIN REAC-TION, to provide sufficient material forgenetic fingerprinting.genetic map A map showing the sequenceof particular genes or segments ofDNA on a chromosome. Restriction enzymesand DNA probes are used to determinethe exact sequence, but it is possibleto get a rough estimate by analyzing thefrequency of recombination between thealleles of linked genes. See chromosomemap; DNA probe; restriction endonuclease.genetics The term coined by the Britishbiologist Bateson (1861–1926) to describethe study of inheritance and variation andthe factors controlling them. Today thesubject has four main subdivisions –Mendelian genetics (classical genetics),population genetics, cytogenetics, and molecularor biochemical genetics.genome A complete haploid chromosomeset. Haploid organisms have onegenome, diploid organisms have two homologoussets, polyploid organisms havemany sets sometimes from the same ancestor(autopolyploids) and sometimes fromdifferent ancestors (allopolyploids). Interspecifichybrids also have two or more differencegenomes.genotype The genetic make-up of an organism.The actual appearance of an individual(the phenotype) depends on thedominance relationships between alleles inthe genotype and the interaction betweengenotype and environment. Compare phenotype.genus (pl. genera) A taxonomic categoryinvolving a collection of similar species.Genera may be subdivided into subgenera,and also, especially in plant taxonomy,into sections, subsections, series, and subseries.The scientific name of a species alwaysincludes the genus name (or itsabbreviation) as the first word of the binomial.Similar genera are grouped into families.See binomial nomenclature.geological time scale A system of measuringthe history of the earth by studyingthe rocks of the earth’s crust. Since newrocks are generally deposited on top of existingmaterial, those lower down are oldest,although this is often disrupted by e.g.volcanic activity, tectonic movements anderosion. The strata of rock are classified accordingto their age, and a time scale correspondingto this can be constructed. Themain divisions (eras) are the PALEOZOIC,MESOZOIC, and CENOZOIC. These are furthersubdivided into periods and epochs.geophyte A plant whose perennatingbuds are situated below ground as, for example,in plants with bulbs, corms, tubers,or rhizomes. See Raunkiaer’s plant classification.geotropismSee gravitropism. cellgerm cell Any cell in a cell lineage thateventually produces gametes.96

Geological time scale97

germ linegermination The first outward sign ofgrowth of a reproductive body, such as aspore or pollen grain. The term is mostcommonly applied to seeds, in which germinationinvolves the emergence of theradicle, plumule, or coleoptile through thetesta. Both external conditions (e.g. wateravailability, temperature, and light) and internalbiochemical status must be appropriatebefore germination can occur. Seedgermination may be either epigeal, inwhich the cotyledons appear aboveground, or hypogeal, in which the cotyledonsremain below ground.germ line The lineage of cells fromwhich gametes arise, continuous throughgenerations.Gibberellic acid: the skeleton of themolecular structuregerm plasm The part of an organismthat, according to Weissmann at the beginningof the 20th century, passed its characterson to the next generation. It is nowknown that most of this information is carriedby DNA in the chromosomes.giant fernsSee Marattiaceae.gibberellic acid (GA 3 ) A common gibberellinand one of the first to be discovered.Together with GA 1 and GA 2 it wasisolated from Gibberella fujikuroi, a fungusthat infects rice seedlings causing abnormallytall growth. Gibberellic acid is aterpenoid synthesized from mevalonicacid. Some growth retardants are thoughtto act by blocking this synthesis pathway.See gibberellin.gibberellin A plant hormone involvedchiefly in shoot extension. Gibberellins arediterpenoids chemically related to gibberellicacid; their molecules have the gibbaneskeleton. More than thirty have beenisolated, the first and one of the most commonbeing GIBBERELLIC ACID, GA 3 . Gibberellinsstimulate elongation of shoots ofvarious plants, especially the extension tonormal size of the short internodes of geneticallydwarf pea or maize plants. This isachieved by increasing cell-wall expansibility,but it requires the presence of auxin,too. Such dwarf varieties may be used inbioassays for gibberellin. Increased gibberellinlevels can mimic or mediate theeffect of long days. Thus they stimulate internodeextension and flowering in LONG-DAY PLANTS such as lettuce and spinach.They are also effective in inhibiting tuberdevelopment or breaking tuber dormancy,e.g. in the potato, and breaking bud dormancyin woody species. Gibberellins caninduce PARTHENOCARPY, and are used in theproduction of seedless fruits. They may beproduced in both shoots and roots andtravel in both xylem and phloem. Theywork antagonistically with ABSCISIC ACID,and in some situations work cooperativelywith AUXINS as, for example, in the controlof sex expression in dioecious plants.gill One of many thin platelike sporeproducingstructures radiating outwardfrom the stalk on the undersurface of thecap (see pileus) of agaric fungi such asmushrooms and toadstools (see Basidiomycota).The spores (basidiospores) areproduced on the outer layer (hymenium).Ginkgophyta (ginkgoes) A phylum ofdeciduous gymnosperms with fan-shaped,stalked leaves that have dichotomouslybranching veins, a feature unique amongliving gymnosperms. There is a single class,order, genus and species, the maidenhairtree or ginkgo (Ginkgo biloba) fromChina, known only in cultivation. Fossilremains of other species have been found inmany parts of the world, dating back to theMiddle Jurassic period. The ginkgo is dioecious.glabrous Smooth, with no hairs or otherprojections.98

glycerate 3-phosphategland A specialized cell or group of cellsconcerned with the secretion of varioussubstances produced as byproducts ofplant metabolism. The secretions may passto the exterior or be contained in cavitiesor canals in the plant body.GLCSee gas–liquid chromatography.gley (glei) A waterlogged soil lacking inoxygen, in which raw humus accumulatesas a result of lack of decomposition by bacteria.The defining feature is the gley horizonbelow the humus – blue-grey claywhose color is due to ferrous iron compoundsthat have been reduced by microorganisms,flecked with localized areasof rust-colored oxidized ferric compounds.The formation of a gley is known as gleying.Gley soils are typical of tundra, meadows,and boggy areas.global warmingSee greenhouse effect.globulin A kind of globular protein thatdissolves in dilute salt solutions but is relativelyinsoluble in pure water. Globulinsare found in plant seeds as storage proteins,particularly in the seeds of legumes(Fabaceae).glucan A polysaccharide made up entirelyof glucose units, e.g. cellulose, starch.See polysaccharide.gluconeogenesis The synthesis of glucosefrom noncarbohydrate precursors,such as pyruvate, amino acids, and fattyacids (which are converted to glucose bythe KREBS CYCLE and GLYOXYLATE CYCLE),glycerol, and intermediates of the Krebscycle. In plants, glucose is also formed byphotosynthesis.glucose A monosaccharide (C 6 H 12 O 6 )occurring widely in nature as a storagecompound: it occurs as glucose units in sucrose(which is hydrolyzed to glucose andfructose) and GLYCOGEN (which is hydrolyzedto glucose), as well as in STARCHand CELLULOSE. It is the main energy sourceof most organisms, and the starting pointfor synthesis of carbohydrates and nucleotidesugars. Glucose is synthesized directlyfrom carbon dioxide in the Calvincycle, and from fatty acids via b-oxidationto acetyl CoA, which then enters the GLY-OXYLATE CYCLE (a synthetic pathway ingerminating oil-rich seeds). Glucose is oxidizedin respiration through glycolysis andthe KREBS CYCLE. See glycolysis; photosynthesis.glumeSee bract.glutamic acidSee amino acids.glutamine An amino acid formed by theaddition of ammonia to glutamic acid,using energy from ATP. Glutamine is importantin the storage and neutralization offree ammonia. It also donates an aminogroup in synthesis reactions, e.g. the synthesisof purines and pyrimidines. Seeamino acids.gluten A mixture of proteins found inwheat flour. It is composed mainly of twoproteins (gliaden and glutelin), the proteinsbeing present in almost equal quantities.Certain people are sensitive to gluten(celiac disease) and must have a gluten-freediet.glycan 1. A POLYSACCHARIDE made up ofa single type of sugar unit (i.e. >95% ). Asa class the glycans serve both as structuralunits (e.g. cellulose in plants and chitin infungi) and energy stores (e.g. starch). Themost common homoglycans are made upof D-glucose units and called GLUCANS.2. Any polysaccharide.glyceraldehyde (CHOCH(OH)-CH 2 OH)A simple monosaccharide bearing an aldehydegroup. The phosphorylated form,glyceraldehyde 3-phosphate is derivedfrom fructose 1,6-bisphosphate duringGLYCOLYSIS, and from glycerate 3-phosphatein the Calvin cycle (See photosynthesis).Glyceraldehyde 3-phosphate isinvolved in many metabolic pathways.glycerate 3-phosphate (phosphoglyceric99

glycerideacid) The phosphorylated form of glycericacid, a 3-carbon atom molecule. Glycerate3-phosphate is the first product of thedark reactions of PHOTOSYNTHESIS, formedby the carboxylation and cleavage of ribulosebisphosphate. It is also formed fromglyceraldehyde 3-phosphate in GLYCOLYSIS,with the release of ATP, after which it maybe converted to phosphoenol pyruvate.Glycerate 3-phosphate is a precursor of theamino acid serine.glycerideSee acylglycerol.glycerol (glycerin; 1,2,3-propanetriol)An alcohol with three OH groups. Glycerolis biologically important as the alcoholinvolved in LIPID formation (these particularlipids being called acylglycerols). It isalso used to protect living tissues such ascorneas and red blood cells for freezing,and as a mounting medium in microscopy.See acylglycerol; phosphoglyceride.glycogen A polysaccharide composed ofmany GLUCOSE units, with a branchingstructure similar to that of amylopectin. Itis used as a storage compound in fungi andcertain bacteria as well as in animals, beinghydrolyzed to glucose with the aid of amylase.glycolipid (glycosyldiacylglycerol) Anacylglycerol containing a carbohydrategroup or an amino sugar. Glycolipids arethe major lipids in chloroplasts. See glycerolipid.glycolysis (Embden–Meyerhof–Parnaspathway) The conversion of glucose intopyruvate, with the release of some energyin the form of ATP. Glycolysis occurs incell cytoplasm. In ANAEROBIC RESPIRATION,glycerolipid Lipids derived from GLYC-EROL. Glycerolipids are synthesized fromglycerol 3-phosphate by the addition offatty acid chains and a head group. Plantssynthesize glycerolipids in mitochondria,plastids and the endoplasmic reticulum.Glycerolipids include triacylglycerols (glycerollinked to three fatty acid chains), importantstorage compounds in seeds, beingbroken down with the aid of lipases intofatty acids, which can then undergo b-oxidation. Compare glycolipid.glycine (aminoethanoic acid; aminoaceticacid) The simplest AMINO ACID, with theformula NH 2 CH 2 COOH. It may be synthesizeddirectly from ammonia, carbondioxide, and a donated methyl group; derivedfrom serine; or derived from glycerate3-phosphate by TRANSAMINATION ofglyoxylate. It is broken down by a reversalof the first two pathways, or by deaminationto glyoxylate, followed by conversionto malic acid and further breakdown in theKrebs cycle. Glycine is necessary for thesynthesis of purines, and is a basic componentof PORPHYRINS.Glycolysis100

Golgi apparatusbreakdown proceeds no further and pyruvateis converted into ethanol (or lacticacid in animals and some bacteria) for storageor elimination. In AEROBIC RESPIRATION,glycolysis is followed by the KREBS CYCLE.Glycolysis alone yields only two moleculesof ATP per molecule of glucose in anaerobicrespiration. In aerobic respiration thereis a net yield of six (the conversion ofNADH back to NAD yields a further fourATP molecules, and can occur only whenoxygen is present). The main regulatoryenzyme in glycolysis is phosphofructokinase,which catalyzes the phosphorylationof fructose 6-phosphate to fructose 1,6-bisphosphate,and is inhibited by high concentrationsof ATP. See also respiration;alcoholic fermentation.glycoprotein A conjugated proteinformed by the combination of a proteinwith carbohydrate side chains. Certainantigens, enzymes, and hormones are glycoproteins.One of the most importantglycoside A derivative of a pyranosesugar (e.g. glucose) in which there is agroup (R) attached to the carbon atomthat is joined to the –CHO group. In a glycosidethe C–OH is replaced by C–OR.The linkage –O– is a GLYCOSIDIC BOND,joining monosaccharides in polysaccharides.Glycosides are formed by the reactionof a pyranose sugar with an alcohol orphenol.glycosidic bond (glycosidic link) Abond formed between monosaccharides byremoval of water (condensation reaction).The most common glycosidic bond occursbetween the carbon-1 of one sugar and thecarbon-4 of the other (a 1-4 glycosidicbond); sucrose has a 1-2 glycosidic bondbetween the carbon-1 of glucose and carbon-2of fructose. If the bond lies belowthe plane of the glucose ring, the bond istermed an a-glycosidic bond (e.g. starch), ifabove this plane, a b-glycosidic bond (e.g.cellulose).glycosyldiacylglycerolSee glycolipid.glyoxylate cycle A modification of theKREBS CYCLE occurring in plants, protoctists,and some microorganisms, in regionswhere fats are being rapidly metabolized,e.g. in germinating fat-rich seeds. Acetylgroups formed from the fatty acids arepassed into the glyoxylate cycle, with theeventual formation of mainly carbohydrates.The glyoxylate cycle bypasses thecarbon dioxide-releasing steps of the Krebscycle, allowing carbohydrates to be synthesizedfrom fatty acids via succinate. See illustrationoverleaf.glyoxysomeSee microbody.Glycosidic bond: the glycosidic bond insucroseplant glycoproteins is extensin, a structuralcomponent of cell walls, in which the sidechains are tetrasaccharides of arabinose.Gnetophyta A phylum of gymnospermswith compound cones, cells in the secondaryxylem almost identical to angiospermvessels, and the tendency for thetip of the nucellus to be drawn out into amicropylar tube; there are no resin canals.The embryo has two cotyledons. Oftenmore than one embryo is produced initially,but usually only one reaches maturity.There are three very different orders,each with only one genus: Gnetales (Gnetum),Ephedrales (Ephedra or joint-firs),and Welwitschiales (Welwitschia).Golgi apparatus (Golgi body; Golgi complex)An organelle of eukaryotic cellsdiscovered by Camillo Golgi in 1898. It isassociated with the endoplasmic reticulum101

graftGlyoxylate cyclebut lacks ribosomes. It consists of stacks offlattened membrane-bounded sacs (cisternae)that bud off vesicles (Golgi vesicles)containing the products of the secretory activityin the Golgi apparatus. The Golgiapparatus is rich in secretory cells. The cisternaeare spread randomly (as dictyosomes)in plant cells but form a singlenetwork in most animal cells. In the cisternae,materials (e.g. enzymes and polysaccharides)are processed and leave in Golgivesicles for transport, often to the plasmamembrane for secretion. If the vesicles releasetheir contents on the cell surface, theirmembranes become part of the plasmamembrane; thus the Golgi apparatus contributesto cell membrane synthesis.graft The transplantation of an organ ortissue in plants and animals. In plants,grafting is an important horticultural techniquein which an above-ground part (thescion) of one individual is united with theroot of another (the stock) of the same or adifferent species. (Where more than twoparts are grafted together, the middle pieceis called the interstock.) Usually the shootor bud of the scion is grafted onto thelower part of the stock. The cambia ofscion and stock merge, allowing the vasculartissues of the two plants to combine.Production of callus around the graft areahelps to secure the union. Grafting is usuallysuccessful only between closely relatedspecies. It is often used to propagate varietiesderived from mutations or hybridizationthat cannot be grown from seed,especially woody species such as apples,pears, and roses.graft hybridgrainSee caryopsis.See chimera.Gram, Hans Christian Joachim (1853–1938) Danish bacteriologist. He devisedthe important method of staining bacteriathat bears his name (see Gram’s stain) inBerlin in 1884, building on the method ofPaul Erlich. The discovery that some bacteriaretain the stain (Gram positive), whileothers do not (Gram negative), has beenimportant in identifying and classifying102

ground layerbacteria and also in deciding upon thetreatment of bacterial diseases.GramineaeSee Poaceae.Gram’s stain A stain containing thebasic dye crystal violet or gentian violetthat forms the basis for the division intoGram positive and Gram negative bacteria.The bacteria are first killed and stainedwith the violet dye, then treated with an organicsolvent such as acetone or ethanol.Gram positive bacteria (e.g. Actinobacteria)retain the deep purple color of crystalviolet; Gram negative bacteria (e.g.Cyanobacteria) are decolorized, and maybe counterstained with other dyes, such assafranin, carbol fuchsin, or neutral red. Anumber of physiological differences are associatedwith the difference in staining, includingnutrition, and susceptibility toantibiotics.granum (pl. grana) A stack of membranes(resembling a pile of coins) in aCHLOROPLAST. With the light microscopethese stacks are just visible as grains(grana). The photosynthetic pigments andaccessory pigments are bound to thesemembranes, which also contain the componentsof the electron-transport chainand enzymes needed for the light-dependentreactions of photosynthesis.grape sugarSee glucose.grassland A major world plant community(biome) dominated by grasses(Poaceae). Grasslands occur in many temperateregions where annual rainfall is250–500 mm and in tropical regions with750–1500 mm. They are widespread in theinteriors of continents, where there is a distinctrainy season in spring and summer.Many grasslands, especially in the wetterareas, are naturally maintained by grazingmammals and, to some extent, fire.graticuleSee micrometer.gravireceptor A receptor that can sensegravity. This is usually an object thatmoves in response to a gravitational field (aSTATOLITH). Starch-filled plastids (AMYLO-PLASTS) are thought to act as gravireceptors,moving in response to gravity andpressing on the endoplasmic reticulumnear the outer edge of specialized cellscalled STATOCYTES located in the root capor young root, and in a layer (termed theSTARCH SHEATH) adjacent to the vasculartissues. Other possible gravireceptors arestretch-activated ion channels in membranes,which may sense movement of theprotoplasm in response to gravity. Suchchannels may be involved in the responseof the endoplasmic reticulum to movingamyloplasts. See gravitropism. See alsophotoreceptor.gravitropism (geotropism) A directionalgrowth movement of part of a plant in responseto gravity. Primary roots (tap roots)grow vertically toward gravity (positivegravitropism) whereas primary shootsgrow vertically away from gravity (negativegravitropism), though the direction ofshoot growth may also be modified bylight. Dicotyledon leaves and some stemstructures (e.g. rhizomes and stolons) growhorizontally (diatropism). Secondary (lateral)roots and stem branches may grow atan intermediate angle with respect to gravity(plagiotropism). Gravitropic responsesare believed to involve hormones, maybeauxins or gibberellins. See also gravireceptor;klinostat; tropism.See Chlorophyta; Gamo-green algaephyta.greenhouse effect The rise in temperatureof the atmosphere, analogous to thatin a greenhouse. Solar (short-wave) radiationpasses easily through the atmosphere(or glass in a greenhouse) and is absorbedby the earth’s surface. It is re-emitted in theform of infrared (long-wave) radiation,which is absorbed by water vapor, carbondioxide, methane, chlorofluorocarbons(CFCs), and other so-called greenhousegases in the atmosphere with a consequentincrease in the atmospheric temperature.Many scientists believe that increasing at-103

ground meristemmospheric pollution by carbon dioxide,mainly due to the burning of fossil fuels,and many other gases is leading to a rise inglobal temperatures (global warming),which will eventually affect other aspectsof climate and have profoundly damagingeffects on natural ecosystems and agriculture,as well as raising sea level.Grew, Nehemiah (1641–1712) Britishphysician and botanist. Grew carried outimportant work in plant anatomy and wasa contemporary of MALPIGHI. His work culminatedin his publication of The Anatomyof Plants, in which he was among the firstto observe chloroplasts and stomata, describingthe latter as ‘breathing pores’.ground layer The lowermost layer ofa plant community, usually comprisingmosses, lichens, fungi, and low-growingherbs, especially rosette plants and creeping,prostrate forms. Compare field layer.ground meristem The central region ofthe apical meristem from which the groundtissues of pith, cortex, medullary rays, andmesophyll differentiate. In root meristem itis also called periblem.ground tissues The unspecialized tissuesthat are found in any region of theplant not occupied by the specialized tissueof vascular bundles, cambium, epidermis,etc. The pith and cortex of the root andstem are ground tissue, as are the mesophylllayers of the leaf.growth An irreversible increase in sizeand/or dry weight. It excludes certain developmentalprocesses that involve no sizechange, such as cleavage and uptake ofwater by seeds (IMBIBITION). Growth involvescell division and cell expansionthrough synthesis of new materials, and isclosely related to subsequent developmentalprocesses. If some measure of growth ofan organism, such as height or weight, isplotted throughout its life, a characteristicS-shaped (sigmoid) growth curve is obtainedfor most organisms. In some organisms,including many plants growth neverstops entirely, though it may become extremelyslow (indeterminate or indefinitegrowth). This allows plants to continuegrowing and many plants grow laterallyshowing clonal growth. The result is thatsome can be extremely long-lived, withclones, such as those of the creosote bush(Larrea tridentata) known to have lived forover 10 000 years.growth factor Any substance that affectsthe growth of a plant. This includesGROWTH SUBSTANCES (HORMONES) andGROWTH INHIBITORS, as well as food reservesand minerals.growth inhibitor A substance thatslows the growth of a plant, for examplethe plant hormones ABSCISIC ACID and ETH-YLENE (ethene), which act at very low concentrations.growth ringgrowth substanceSee annual ring.See hormone.GSC Gas–solid chromatography. Seegas chromatography.guanine A nitrogenous base found inDNA and RNA. Guanine has a purine ringstructure. The nucleoside guanosine triphosphate(GTP) is derived from guanine,and supplies energy for certain enzymecatalyzedmetabolic reactions.guanosine (guanine nucleoside) A nucleosidepresent in DNA and RNA andconsisting of guanine linked to D-ribose viaa b-glycosidic bond.guard cell A specialized kidney-shapedepidermal cell, located to the side of aSTOMA. Two guard cells together encircleeach stoma and control the opening andclosing of the stomatal aperture. The controlis effected through changes in turgidity.The wall of the guard cell bordering thepore is heavily thickened while the oppositewall is comparatively thin. Thus whenthe guard cell is turgid the thin wall becomesdistended, bulging out away from104

gynomonoeciousthe pore, and causes the thickened wall,which cannot distend, to be drawn outwardswith it. This results in an aperturebeing formed between adjacent guard cells.When osmotic pressure of the guard celldrops the pore closes. Most plants exhibita stomatal rhythm whereby the stomataopen in light and close in darkness.gum Any substance that swells in waterto form a gel or a sticky solution. Similarcompounds that produce slimy solutionsare called mucilages. Gums and mucilagesare not distinguishable chemically. Mostare heterosaccharides, being large, complex,flexible, and often highly branchedmolecules.guttation Loss of water as liquid fromthe surface of a plant. Water is normallylost as vapor during transpiration but, ifthe atmosphere is very humid, water mayalso be forced from the leaves through hydathodesas a result of root pressure. Dropsof guttated water are often seen at the tipsof grasses in the early morning, when thelow temperature reduces transpiration. It isalso common in tropical plants living in environmentswith high humidity that reducestranspiration rates.gymnosperm A general term for anyseed plant except the angiosperms, characterizedby bearing naked seeds. There arefour widely different living groups: the cycads,ginkgo, conifers, gnetophytes, andseveral fossil groups. Gymnosperms lackendosperm and the precise double fertilizationcharacteristic of the angiosperms, althoughsome show a primitive form of it.Cycads and ginkgo have motile sperm, unlikegnetophytes and conifers, which producepollen. The female gametophyte isreduced, but larger than in angiosperms; itis not autotrophic. Most gymnosperms retainarchegonia except for the most ad-105

Hhabitat The place where a particular organismlives, described in terms of its climatic,vegetative, topographic, and otherrelevant factors.hair (trichome) An outgrowth from aplant epidermal cell. Plant hairs may take awide variety of forms, often characteristicof particular species or genera. They maybe unicellular or multicellular, simple,forked, stellate (starlike), downy, silky,long, or short. Some, such as the hairs onthe leaves of geraniums (Pelargonium) areglandular. See also root hair.halophyte A plant that grows in soilswith a high concentration of salt, as foundin salt marshes or other littoral habitats.Examples are species of Spartina. Halophytesare adapted to obtain water fromsoil water with a higher osmotic pressurethan normal soil water, so they need to beable to accumulate a high concentration ofsalts in their root cells. Many have a succulentgrowth form with swollen stems orleaves.halosere A plant community in a SUC-CESSION that starts on land periodically inundatedby the sea. Haloseres include saltandflooding-tolerant pioneer communitieson mudflats and saltmarshes.haplobiontic Describing life cycles inwhich only one type of somatic body isformed, which may be either haploid ordiploid, i.e. either the sporophyte or the gametophytegeneration is missing. For example,in the unicellular Chlamydomonasthe vegetative cell is haploid, and the onlydiploid cells are the zygospores, which germinateto produce haploid zoospores. Indiatoms the vegetative cells are diploid,and are formed directly by the fusion of gametes.Haplobiontic is thus a collectiveterm for haplontic and diplontic. Comparediplobiontic. See alternation of generations.haploid (monoploid) Describing an organism,cell, or nucleus containing onlyone representative from each of the pairs ofhomologous chromosomes found in thenormal DIPLOID cell. Haploid chromosomesare thus unpaired and the haploid chromosomenumber (n) is half the diploid number(2n). Meiosis, which usually precedesspore or, sometimes, gamete formation,halves the chromosome number to producehaploid gametes. The diploid condition isrestored when the nuclei of two gametesfuse to give the zygote. Gametes may developwithout fertilization, or meiosis maysubstantially precede gamete formation,leading to the formation of haploid organisms,or haploid phases in the life cycles oforganisms.haplont A haploid organism that representsthe vegetative stage in life cycles inwhich diploidy is restricted to the zygote.Haplontic life cycles are typical of the filamentousgreen algae. Compare diplont.haplostele A type of protostele having asolid strand of stele with the xylem on theinside encircled by phloem, pericycle, andendodermis. This type of stele was presentin the earliest known vascular plants, suchas Rhynia.hapteron (pl. haptera) 1. See holdfast.2. In species of Equisetum (horsetails), theouter wall of the spore, which forms an X-shaped structure, the arms of which arewrapped around the spore. On drying out,106

heaththe arms, or haptera, act in similar mannerto the elaters of liverworts, and are thustermed elaters. See elater.Haptomonada (Haptophyta) A phylumof the PROTOCTISTA that containsmostly unicellular algae, most of whichhave a threadlike structure called a haptonemabetween their two undulipodia.The haptonema is thought to be used fortemporary anchorage rather than for locomotionand may be coiled while the cell ismotile. Haptomonads are common in marineplankton and freshwater.haptonasty (haptonastic movements) ANASTIC MOVEMENT in response to contact.An example is the sensitive plant (Mimosapudica), whose leaflets progressively collapse,sometimes throughout the wholeplant, once one leaflet is touched. See pulvinus.haptophytaSee thigmotropism; tro-haptotropismpism.See Haptomonada.hardpan 1. A hardened soil horizon,usually in the middle or lower part of a soilprofile, A hardpan is typical of PODSOLS,where iron compounds leached from theupper layers accumulate in the B horizon,staining it red-brown and forming an impermeablelayer (iron pan) that can preventdrainage, resulting the in waterlogging ofhigher horizons.2. Also called calcrete, a hardened layer inor on a soil formed on calcareous substrata,formed in arid and semiarid regionsas a result of fluctuating climatic conditionsby calcite that precipitates out of solutionas water evaporates at the surface.Hardy–Weinberg equilibrium The situationin a large, randomly mating, stablepopulation under specific conditions, inwhich the proportion of dominant to recessivegenes remains constant from onegeneration to the next. If a pair of alleles,A and a, have the frequencies p and q ina population, then the Hardy-Weinbergequilibrium is described by the equationp 2 + 2pq + q 2 = 1, where p 2 and q 2 are thefrequencies of the double dominant anddouble recessive respectively, and 2pq isthe frequency of the heterozygote. TheHardy–Weinberg equation p 2 + 2pq + q 2 =1 is obtained from the expansion of (p +q) 2 , the total of the frequencies making upthe gene pool being unity. The law was formulatedin 1908 and disproved the thencurrent theory that dominant genes alwaystend to increase in a population at the expenseof their equivalent recessive alleles.The equilibrium holds only when the populationis sufficiently large to avoid chancefluctuations of allele frequencies in thegene pool (GENETIC DRIFT) and providingthere is no mutation, natural selection, ormigration into or out of the population.The fact that allele frequencies may be seento change fairly rapidly in large populationsthat show minimal mutation and migrationemphasizes the important rolenatural selection must play. Until theHardy–Weinberg law was formulated, theextent of natural selection was not fully appreciated.Hartig netSee ectomycorrhiza.Hatch–Slack pathwaySee C 4 plant.haustorium (pl. haustoria) A specializedoutgrowth of many parasitic fungi andcertain parasitic plants such as dodder(Cuscuta) and broomrape (Orobanche),that penetrates into and withdraws foodmaterial from the cells of the host plant.heartwood (duramen) The hard centralregion of a tree trunk made up of xylemvessels that are no longer involved in watertransport. Such vessels are often blockedby substances (e.g. resins and tannins) thatgive the wood a darker color. Comparesapwood.heath An area dominated by dwarfshrubs of the heath family (Ericaceae).Legumes (family Fabaceae) may also becommon components of heath floras.Heaths normally develop on poor, sandy,107

The proportion of phenoheavymetal tolerancewell-drained soils in temperate climates,and are usually subclimax communitiesmaintained by grazing, burning, and sometimes,cutting.heavy metal tolerance Biochemicaland physiological adaptation to concentrationsof heavy metals, such as copper, lead,and zinc, that would prevent the growth ofmost plant species or genotypes. Suchadaptation usually involves converting themetal to a harmless form; some species excludethe metal from the plant altogether,while others confine it to the roots, thusprotecting the more sensitive shoots. Certainspecies or strains have evolved suchtolerance, and some, for example certainstrains of bent grass (Agrostis capillaris),benefit from the reduced competition insoil containing heavy metal ions.See spiral thicken-helical thickeninging.helicase Any of a group of enzymes thatcatalyze the unwinding of double-strandednucleic acids such as DNA, using energyfrom ATP.heliophytehabitats.A plant typical of sunnyheliotropism A form of PHOTOTROPISMin which flowers move to point toward thesun, tracking it as it moves across the sky.It is a daily movement, mainly in springflowers that are pale and cup-shaped likewood anemones and buttercups.helophyte A perennial marsh plant thathas its overwintering buds under water. Anexample is bulrush (Typha).hematoxylin A blue dye derived fromthe logwood tree (Haemotoxylon campechianum),which when oxidized tohematin stains nuclei and cellulose cellwalls blue. See staining.heme (haem) An iron-containing porphyrinthat is the prosthetic group in cytochromes.See porphyrins.hemicellulose One of a group of substancesthat make up the amorphous matrixof plant cell walls together with pecticsubstances (and occasionally, in maturecells, with lignin, gums, and mucilages).They are heteropolysaccharides, i.e. POLY-SACCHARIDES built from more than one typeof sugar, mainly the hexoses (mannose andgalactose) and the pentoses (xylose andarabinose). Galacturonic and glucuronicacids are also constituents. They varygreatly in composition between species.The hemicelluloses and pectic substancesinteract with the cellulose microfibrils ofthe cell wall to give extra strength and atthe same time retain some plasticity. Insome seeds (e.g. the endosperm of dates)hemicelluloses are a food reserve. See cellwall.hemicryptophyte A perennial plant,usually nonwoody, with its over-winteringbuds at or just below soil level. The budsare protected by the soil or surface litter.See also Raunkiaer’s plant classification.hemiparasite A parasitic plant that haschlorophyll and can photosynthesize, butdepends on another plant for its water andmineral salts, e.g. mistletoe.hemizygous Describing genetic materialthat has no homologous counterpart and isthus unpaired in the diploid state. Both singlegenes and chromosome segments maybe hemizygous; for example, the X chromosomein the heterogametic sex, andwhole chromosomes in aneuploids.Hepatophyta (Marchantiopsida; liverworts)A phylum containing leafy andprostrate thallose forms, commonly knownas liverworts. Thallose liverworts are prostrateand dichotomously branched withthalli several cells thick. There are five ordersof liverworts, including the Metzgeriales,e.g. Pellia, and the Marchantiales, e.g.Marchantia.herbaceous perennialheritabilitySee perennial.108

heterosistypic variation due to genetic factors. It canbe estimated from measurements of individualsfrom different generations, and isused in plant and animal breeding to predicthow successful genetic selection will bein improving a particular trait.hermaphrodite (bisexual; monoclinous)A plant bearing stamens and carpels in thesame flower. In many such plants, selffertilizationand inbreeding are preventedby specific self-incompatibility systems.Compare monoecious; dioecious.hesperidium (pl. hesperidia) A type ofberry with a leathery epicarp, as in orangesand lemons. Each carpel forms a segmentof the fruit, filled with fluid-filled trichomes.See berry.heteroecious Denoting rust fungi thatrequire two host species to complete theirlife cycle. An example is Puccinia graminis,the stem rust of cereals and grasses, whichoverwinters on the barberry. Compare autoecious.heterogametic sex The sex with dissimilarSEX CHROMOSOMES, one (in mammalsthe Y chromosome) being shorter than theother (the X chromosome).heterogamySee anisogamy.heterokaryosis The presence of two ormore nuclei with differing genotypeswithin a single cell (termed a heterokaryon).Fusion of fungal hyphae of differinggenetic complements leads to heterokaryosis.Heterokaryons often grow betterthan homokaryons, since any genetic deficiencies,for example in the ability to synthesizecertain compounds, in one type ofnucleus may be overcome by alleles presentin the other nuclei.heteromorphism The existence of morethan one form, used especially with referenceto life cycles in which the alternatinggenerations are markedly different morphologically,as in ferns, bryophytes, andHeterophylly: examples of heterophylly in twodifferent aquatic plantsalgae. Compare isomorphism; polymorphism.See alternation of generations.heterophylly Denoting plant speciesthat have more than one form of foliageleaf on the same individual. Examples includecertain species of Lycopodium andSelaginella, which have two rows of lateralleaves and one or two rows of smallerabaxial or adaxial leaves.heterosis (hybrid vigor) The conditionin which the expression of a characteristicis greater in the heterozygous offspringthan in either of the homozygous parents.The effect arises from an accumulation ofdominant genes in the F 1 . Thus, if height iscontrolled by two genes, A and B, and talland short forms are determined by domi-109

heterosporynant and recessive alleles respectively, thenthe cross AAbb ´ aaBB would give an F 1AaBb, containing both dominant genes fortallness. Usually the more unlike the parentsare the more hybrid vigor is released,but the effect diminishes in subsequentgenerations as more recessive homozygotesreappear. For crop plants, hybrid vigor isparticularly important for characteristicssuch as growth rate, yield, and disease resistance.heterospory The production of twodifferent sizes of spore: MICROSPORES andMEGASPORES. The larger megaspores usuallycontain more food reserves. Comparehomospory.heterostyly A DIMORPHISM in which thestyles of flowers of the same species are ofdifferent lengths, thus dividing the speciesinto groups. An example is the primrose,which is divided into pin-eyed and thrumeyedtypes. Pin-eyed plants have long stylesand short stamens and thrum-eyed plantsconversely have short styles and long stamens.Such differences promote pollinationbetween groups.heterothallism A condition found inalgae and fungi in which sexual reproductionoccurs only between genetically differentself-incompatible mating types (strains)of the same species. The strains may varymorphologically or in the size of the gametesthey produce. When heterothallismis purely physiological, morphologicallyidentical strains exist (often designatedplus and minus strains). Compare homothallism.heterotrophism A type of nutrition inwhich the principal source of carbon is organic,i.e. the organism cannot make all itsown organic requirements from inorganicstarting materials. Most heterotrophicorganisms are chemotrophic (i.e. showchemoheterotrophism); these comprise allanimals and fungi, most bacteria, andparasitic plants. A few heterotrophic organismsare phototrophic (i.e. show photoheterotrophism).The nonsulfur purplebacteria, for instance, require organic moleculessuch as ethanol and acetate. Compareautotrophism. See chemotrophism;phototrophism.heterozygous Having two different allelesat a given locus. Usually only one ofthese, the dominant allele, is expressedin the phenotype. On selfing or crossingheterozygotes some double recessives mayappear, giving viable offspring. Selfing heterozygoteshalves the heterozygosity, andthus outbreeding maintains heterozygosityand produces a more adaptable population.Compare homozygous.Heterostyly: example in Primula110

Holocenehexose A SUGAR that has six carbonatoms in its molecules.hexose monophosphate shunt See pentosephosphate pathway.high-energy phosphate bond A phosphatelinkage that hydrolyzes spontaneously,releasing energy, for example thelast two phosphate bonds of ATP. ATP isthe main form in which chemical energy istransported from one part of the cell to another.Hill reaction The reaction, first demonstratedby Robert Hill in 1937, by whichisolated illuminated chloroplasts bringabout the reduction of certain nonphysiologicalelectron acceptors with accompanyingevolution of oxygen. For example, theblue dye dichlorophenol indophenol(DCPIP) may be reduced to a colorless substance.The reaction involves part of thenormal light reaction of photosynthesis.Electrons from water involved in noncyclicphotophosphorylation are used to reducethe added substance. It provided supportfor the idea that a light reaction precededreduction of carbon dioxide in photosynthesis.See cyclic photophosphorylation.hilum (pl. hila) 1. A scar on the testa ofa seed marking the point at which it was attachedto the ovary wall by the funicle. It isa feature that distinguishes seeds fromfruits.2. The center of a starch grain aroundwhich the layers of starch are deposited.3. A small projection at the base of a basidiosporenear its attachment to thesterigma.hip A type of PSEUDOCARP (false fruit)consisting of a cup-shaped receptacle containingachenes, each of which bears smallhooklike hairs. Hips are typical of thegenus Rosa.histidine A basic AMINO ACID found onlyrarely in proteins. It is formed from ATPand 5-phosphoribosyl pyrophosphate in acomplex biosynthetic pathway. Glutamateis an intermediate in its breakdown.histochemistry The location of particularchemical compounds within tissues bythe use of specific staining techniques, forexample phloroglucinol to stain lignin.histogen theory A theory, proposed byHanstein, in which the apical meristem isconsidered to consist of three main zones,the dermatogen, periblem, and plerome,which differentiate into the epidermis, cortex,and stele respectively. This concept hasnow been replaced, for stem apices, by thetunica–corpus theory. In roots, however,the concept is still applied, and in some angiospermroots a fourth histogen zone isrecognized, the calpytrogen, which givesrise to the root cap. Compare tunica–corpus theory.histology The study of tissues and cellsat microscopic level.histone One of a group of relativelysmall proteins found in chromosomes ofeukaryotic cells, where they organize andpackage the DNA. When hydrolyzed, theyyield a large proportion of basic aminoacids, as they are rich in the basic aminoacids arginine and lysine. They dissolvereadily in water, dilute acids, and alkalisbut do not coagulate readily on heating.hnRNA (heterogeneous nuclear RNA)See messenger RNA.Hofmeister, Wilhelm Friedrich Benedict(1824–77) German botanist whofirst demonstrated the ALTERNATION OFGENERATIONS in lower plants. Hofmeisteralso showed that the gametophytes andsporophytes of seed-bearing plants werehomologous with those of ferns andmosses and that the gymnosperms formeda plant group between the latter and theangiosperms.holdfast (hapteron) In algae, the cell ororgan that attaches the plant to the substrate.It is often disklike, sometimes di-111

holoenzymevided into many fingerlike processes, and isparticularly prominent in the brown algae(e.g. Fucus).Holocene (Recent) The present epoch inthe geological time scale, being the secondepoch of the Quaternary period, datingfrom the end of the last glaciation, about10 000 years ago, to the present day.holoenzyme A catalytically active complexmade up of an APOENZYME and a CO-FACTOR (nonprotein prosthetic group). Theformer is responsible for the specificity ofthe holoenzyme whilst the latter determinesthe nature of the reaction. Someholoenzymes, such as pyruvate dehydrogenase,are highly complex, with several cofactors.See coenzyme.holophytic The type of nutrition inwhich complex organic molecules are synthesizedfrom inorganic molecules usinglight energy. It is another term for phototrophic.holozoic (heterotrophic) Designatingorganisms that feed on other organisms orsolid organic matter, i.e. insectivorous andparasitic plants, fungi, many bacteria andmost animals. Compare holophytic.homeobox A segment of DNA found inmany so-called homeotic genes concernedwith controlling the development of organisms.It consists of 180 base pairs, and thesequence of bases is remarkably similaracross a wide range of eukaryotic species,from yeasts to human beings. This suggestsit arose early in evolutionary time and hasbeen little changed since. The sequence encodesthe amino acids of a peptide sequence(the homeodomain) that enablesthe parent protein to bind to DNA. This isconsistent with the suggested role ofhomeotic proteins as genetic switches,binding to genes to control their expression.See differentiation.homeotic gene (homeogene) Any of aclass of genes that are crucial in determiningthe DIFFERENTIATION of tissues in differentparts of the body during development.They encode proteins that regulate the expressionof other genes by binding to DNA.This binding capability can be pinpointedto a characteristic base sequence known asa HOMEOBOX. Such genes often occur incomplexes in which the physical order ofthe genes corresponds to the order in whichthey are expressed from anterior to posteriorin the developing embryo.homogametic sex The sex with homologousSEX CHROMOSOMES, in mammals,designated XX.homogamy The condition in flowers inwhich the anthers and stigmas ripen at thesame time, so encouraging self-pollination.Homogamy occurs in the closed cleistogamousflowers that appear late in the seasonin certain plants. Compare dichogamy.homologous 1. Describing structuresthat, though in different species, are believedto have the same origin in a commonancestor. Thus the stamens of angiospermsand the MICROSPOROPHYLLS of lower vascularplants are said to be homologous, beingderived from a leaflike structure bearingthe MICROSPORANGIA (anthers). Compareanalogous.2. See isomorphism.homologous chromosomes Chromosomesthat pair at meiosis. Each carries thesame genes as the other member of the pairbut not necessarily the same alleles for agiven gene. One member of each pair ofmaternal origin, the other of paternal origin.With the exception of the sex chromosomes(X and Y) they are morphologicallysimilar, being of similar size and shapewith centromeres at the same location.During the formation of the germ cells onlyone member of each pair of homologs ispassed on to the gametes. At fertilizationeach parent contributes one homolog ofeach pair, thus restoring the diploid chromosomenumber in the zygote.homospory The production of only onekind of asexual spore, which then develops112

humusinto a hermaphrodite gametophyte. In thevascular plants the condition is seen inmost ferns, psilophytes, and Lycopodiumspecies. Compare heterospory.homostyly The usual condition foundin plants in which the styles of all flowersof the same species are about the samelength. Compare heterostyly.homothallism A condition found insome algae and fungi in which each thallusis self-compatible. Homothallic speciesmay produce distinctly different sizes ofgametes that can fuse with each other andare thus effectively hermaphrodite. Compareheterothallism.homozygous Having identical allelesfor any specified gene or genes. A homozygotebreeds true for the character in questionif it is selfed or crossed with a similarhomozygote. An organism homozygous atevery locus produces offspring identical toitself on selfing or when crossed with agenetically identical organism. Homozygosityis obtained by inbreeding, andhomozygous populations may be welladapted to a certain environment, but slowto adapt to changing environments. Compareheterozygous. See pure line.honey guides Lines or spots on petalsthat direct a pollinating insect to the nectaries.Hooke, Robert (1635–1703) Englishphysicist. In 1665 he published his Micrographia,a beautifully illustrated book thatdescribed both his theories of the nature oflight and the investigations he had madewith his improved microscope. While observingcork, he described it as resemblinga honeycomb and was the first to refer tothe cavities as ‘cells’.Hooker, Sir Joseph Dalton (1817–1911) Plant taxonomist and explorer.Son of the British botanist William JacksonHooker (1785–1865), the first director ofKew Gardens, Hooker studied medicine atGlasgow University. His famous voyage tothe Antarctic (1839–43) on HMS Erebusresulted in a six-volume flora of Antarctica,New Zealand, and Tasmania that attractedthe attention of Charles DARWIN.Hooker was one of those in whom Darwinconfided his theory and was instrumentalin encouraging Darwin to present his jointpaper with Wallace on the theory of evolutionby natural selection.In 1865 he succeeded his father as directorof Kew Gardens, which became aninternational center for botanical research.With George BENTHAM, he produced theworld flora Genera Plantarum.hormone (growth substance; plant hormone;phytohormone) One of a group ofessential organic substances produced inplants. They are effective in very low concentrationsand control growth, development,and responses to external stimuli bytheir interactions. They affect physiologicalprocesses, but are not actually part ofmajor metabolic pathways. The same hormonemay have many different effects, dependingupon the balance between theconcentrations of different hormones, theparticular tissues concerned, and the age ofthe plant. See auxin; gibberellin; cytokinin;abscisic acid; ethylene.horsetailsSee Sphenophyta.host An organism used as a source ofnourishment by another organism, the parasite,which lives in or on the body of thehost. See parasitism.humidity The moisture content of theatmosphere. The absolute humidity is theamount of water vapor present in a unitvolume of air (usually 1 m 3 of air). The relativehumidity is the ratio of the amount ofwater vapor actually present in the air tothe greatest amount that the air could holdat the same temperature, and is usually expressedas a percentage.humus The nonliving finely divided organicmatter in soil derived from thedecomposition of animal and plant substancesby soil bacteria. Humus consists of113

hyaline cells60% carbon, 6% nitrogen, and smallamounts of phosphorus and sulfur, and isvalued by horticulturalists and farmers asit improves the fertility, water-holding capacity,and workability of the soil. Humushas colloidal properties that enable it to retainwater, so it can improve the moisturecontent of sandy soils. It aids the formationof crumbs in the soil, and is often added toclay soil to increase the particle size, promotingdrainage and aeration.hyaline cells Any leaf cell with nochlorophyll that is normally used for storageof water or solutes.hyaloplasm (cell matrix; ground substance)See cytoplasm.hybrid An organism derived from crossinggenetically dissimilar parents. Thusmost individuals in an outbreeding populationcould be called hybrids, and geneticistsapply the term to the F 1 generationresulting from a monohybrid cross, wherethe offspring differ in the alleles of a singlegene. However, the term is usually reservedfor the product of a cross between individualsthat are markedly different. If two differentspecies are crossed, the offspring isoften sterile, owing to the nonpairing of thechromosomes necessary for gamete formation.In plants this is sometimes overcomeby the doubling of the chromosome number,giving an allopolyploid. By contrast,hybrids derived from different varieties ofthe same species are often more vigorousthan their parents (hybrid vigor), and areselected and propagated by vegetativemeans by agriculturists and horticulturists.See chimera; heterosis.hybrid sterility The reduced ability ofsome HYBRIDS to produce viable gametes. Itis due to the lack of homologous pairs ofchromosomes, so chromosomes cannotpair in meiosis, and the resulting gametesmay have extra or missing chromosomes.Hybrid sterility is sometimes overcome bypolyploidy. Compare heterosis.hybrid vigorSee heterosis.hydathode A specialized secretory structureinvolved in the removal of excesswater from plants. It may be a modifiedstoma with the guard cells permanentlyopen so that water is lost passively byhydrostatic pressure, as in sea lavenders(Limonium), or a glandular hair fromwhich water is actively secreted, as in runnerbean (Phaseolus). Hydathodes arefound at the leaf tips or along the leaf margins.hydrocarbon An organic compoundthat contains carbon and hydrogen only,e.g. acetylene, butane. Many of the componentsof natural gas, petroleum, and coalare hydrocarbons.hydrogen An essential element in livingtissues. It enters plants, with oxygen, aswater and is used in building up complexreduced compounds such as carbohydratesand fats. Water itself is an importantmedium, making up 70–80% of the weightof organisms, in which chemical reactionsof the cell can take place. Hydrogenatedcompounds, particularly fats, are rich inenergy and on breakdown release energyfor driving living processes.hydrogen bond A type of bond occurringbetween molecules. Hydrogen bondingtakes place between oxygen, nitrogen,or fluorine atoms on one molecule, and hydrogenatoms joined to oxygen, nitrogen,or fluorine on the other molecule. The attractionis due to electrostatic forces. Hydrogenbonding is responsible for theproperties of water. It is important in manybiological systems for holding together thestructure of large molecules, such as proteinsand DNA.hydrolase An enzyme that catalyzes aHYDROLYSIS reaction. Glycosidases, peptidases,and phosphatases are examples.Hydrolases play an important part in renderinginsoluble food material into a solubleform, which can then be transported insolution. They are important in the mobilizationof stored food, and in the uptake of114

hypocotylnutrients by fungi and other heterotrophicorganisms.hydrolysis The breakdown of a compounddue to a reaction with water (H + orOH – ions).hydrophilic Describing a molecule orsurface that has an affinity for water. Suchmolecules are usually polar, for exampleproteins. Compare hydrophobic.hydrophily Pollination in which watercarries the pollen from anther to stigma.The pollen may be transported on thewater surface or through the water. Pollenthat floats is light and water repellent, andmay have a coating of oil that alters thesurface tension of the water.hydrophobic Describing a molecule orsurface that has no affinity for water. Suchmolecules are nonpolar, e.g. benzene.Compare hydrophilic.hydrophyte A plant found growing inwater or in extremely wet areas. Hydrophytesshow certain adaptations tosuch habitats, notably development ofaerenchyma, reduction of cuticle, root system,and mechanical and vascular tissues,and divided leaves. Large intercellular airspaces in leaves, stems, and roots allowoxygen to diffuse through the plant. Hydrophyteswith floating leaves can exchangegases with the atmopshere; manysubmerged hydrophytes, such as spikedwater milfoil (Myriophyllum spicata) haveno stomata, absorbing water and gasesover their entire surface. Compare mesophyte;xerophyte.hydroponics (water culture) The growthof plants in liquid culture solutions ratherthan soil. The solutions must contain thecorrect balance of all the essential mineralrequirements. The method is used commercially,especially for glasshouse crops, andalso in experimental work in determiningthe effects of mineral deficiencies.hydrosere Any plant community in aSUCCESSION that starts in fresh water.hydrotropism (hydrotropic movement)A TROPISM in which the stimulus is water. Itis a special kind of chemotropism. Rootsare positively hydrotropic and hypocotylsnegatively hydrotropic. If water is in shortsupply, the stimulus of water is strongerthan the stimulus of gravity in determiningresponse.hygroscopic Able to absorb water fromthe surroundings, including moisture fromthe atmosphere.hymenium (pl. hymenia) A layer of thefruiting body of certain ascomycete andbasidiomycete fungi in which the asci orbasidia are borne. The hymenium may bedirectly exposed to the air, as in the gills ofthe mushroom (a basidiomycete) or mayopen into a flask-shaped cavity, as in theperithecia of certain ascomycetes.hyperplasia Enlargement of a tissue dueto an increase in the number of its cells. Inplants this usually occurs in response to adisease-causing organism.hypertonic Designating a solution withan osmotic pressure greater than that of aspecified other solution, the latter beinghypotonic. When separated by a selectivelypermeable membrane (e.g. a cell membrane)water moves by osmosis into the hypertonicsolution from the hypotonicsolution. Compare hypotonic; isotonic.hypertrophy Enlargement of a tissue ororgan due to an increase in the size of itscells. An example is the swelling of rootsof plants from the Brassicaceae familyinfected with club root (Plasmodiophorabrassicae). Compare hyperplasia.hypha (pl. hyphae) In fungi, a finenonphotosynthetic tubular filament thatspreads to form a loose network termed amycelium or aggregates into fruiting bodies(e.g. toadstools). Hyphae may bebranched or unbranched and may or may115

hypodermisnot have cross walls (septa) dividing theminto cells. Many fungi have incompletesepta. Hyphae are parasitic or saprophyticand the tips secrete enzymes to digest andpenetrate the food supply. The hyphalwalls of most species of fungi differ fromthose of plants in being composed of microfibrilsof a nitrogenous compoundcalled chitin or a form of fungal cellulose.Hyphae also differ in lacking plastids, anypigment being contained in the walls, cytoplasm,or oil globules.hypocotyl The stem below the cotyledons,occupying the region between thecotyledon stalks and the point where lateralroots arise. Rapid elongation of thehypocotyl after germination pushes thecotyledons above ground in plants showingepigeal germination. Compare epicotyl.hypodermis One or more layers of cellsthat may be found immediately below theepidermis of plants. It may be composed ofthin-walled colorless cells and functions aswater-storing tissue as in certain succulentleaves and the aerial roots of epiphytes.hypogeal germination Germination inseed plants during which the cotyledons remainunderground, as in broad bean (Viciafaba). The cotyledons thus act only as foodstorage organs and not as photosyntheticorgans and the hypocotyl does not elongate.Compare epigeal germination.hypogyny The simple arrangement offlower parts in which the receptacle is expandedat the top of the pedicel in such away that the androecium and the periantharise from beneath the gynoecium giving asuperior ovary. Compare epigyny; perigyny.hypolimnion The lower noncirculatingwater in a thermally stratified lake.hyponasty (hyponastic movements) Thecurving of a plant organ upward and towardthe axis, caused by greater growth on116

IA naturally oc-IAA (indole acetic acid)curring auxin. See auxin.immunoelectrophoresis See electrophoresis.ICBN See International Code of BotanicalNomenclature.Ice Age A period in the latter part of thePleistocene characterized by successivecoolings and warmings of the earth. In atleast four major glaciations (cold periods),ice caps spread south from the Arctic andnorth from the Antarctic.idioblast Any specialized plant cell thatis dispersed among cells of a very differentkind. Idioblasts may contain a variety ofmaterials, e.g. tannins, oils, crystals, andwaste products. Isolated sclereids can becalled idioblasts.idiogramSee karyogram.imbibition The phenomenon in which asubstance absorbs a liquid and swells, butdoes not necessarily dissolve in the liquid.The process is reversible, the substancecontracting on drying. Water is imbibed bymany biological substances: cellulose,hemicelluloses, pectic substances, lignin(all plant cell wall constituents); starch;certain proteins, especially in seeds; etc.Dry seeds absorb water by imbibition, initiallyvia the testa, and as seed volumeincreases great imbibitional pressures develop.immunity The ability of plants and animalsto withstand harmful infective agentsand toxins. In most plants this is achievedby physical barriers preventing entry ofpathogens and by physiological reactionsto isolate the pathogen and its effects.inbreeding Breeding between closely relatedindividuals. The most extreme formof inbreeding is self-fertilization, which occursin some plants and algae (e.g. Spirogyra).Inbreeding increases homozygosityso that deleterious recessive genes are expressedmore often in the phenotype, anddecreases heterozygosity and hence the potentialgenetic variability of the population.There is also a general lowering ofvigor in inbred stock (inbreeding depression),which is especially pronouncedamongst normally outbreeding populations.Compare outbreeding. See cleistogamy.incipient plasmolysis The condition ofa cell that results when it is surrounded bya solution having the same osmotic pressureas the cell contents. See plasmolysis.incompatibility 1. The rejection ofgrafts, transfusions, or transplants betweenplants or animals of different genetic composition.2. A mechanism in flowering plants thatprevents fertilization and development ofan embryo following pollination by thesame or a genetically identical individual.It results in self-sterility, thus preventinginbreeding. It is due to interaction betweengenes in the pollen grain and those in thestigma, in such a way that the pollen is eitherunable to grow or grows more slowlyon the stigma.3. A genetically determined mechanism insome fungi that prevents sexual fusion betweenindividuals of the same race or strain(e.g. Mucor). See also heterothallism.117

incomplete dominanceincomplete dominance (co-dominance)The situation in which two different allelesare equally dominant. If they occur togetherthe resulting phenotype is intermediatebetween the two respectivehomozygotes. For example, if white antirrhinums(AA) are crossed with red antirrhinums(A¢A¢) the progeny (AA¢) willbe pink. Sometimes one allele may beslightly more dominant than the other(partial or incomplete dominance) inwhich case the offspring, though still intermediate,will resemble one parent morethan the other.indefinite growth (indeterminategrowth) A form of GROWTH that has nofinite limit: the plant or organ continues togrow indefinitely. All plants are somewhatindeterminate, as they have meristems thatcan differentiate into almost any part giventhe right conditions.indehiscent Describing a fruit or fruitingbody that does not open at maturity torelease the seeds or spores. The fruit walleither decays releasing the seeds in situ, orthe fruit is eaten by an animal and the seedspass out intact in its droppings. Comparedehiscent.independent assortment The law, formulatedby MENDEL (Mendel’s SecondLaw), that genes segregate independentlyat meiosis so that any one combination ofalleles is as likely to appear in the offspringas any other combination. The work ofT. H. Morgan later showed that genes arelinked together on chromosomes and sotend to be inherited in groups. The law ofindependent assortment therefore appliesonly to genes on different chromosomes.The term can also be applied to wholechromosomes. See linkage; Mendel’s laws.indeterminate growthgrowth.See indefiniteindicator A substance used to test foracidity or alkalinity of a solution by a colorchange. Examples are litmus and phenolphthalein.A universal indicator showsa range of color changes over a wide rangefrom acid to alkaline, and can be used toestimate the pH.indicator species An organism that canbe used to measure the environmental conditionsthat exist in a locality. For example,lichen species are indicators of levels ofpollution, as different species are sensitiveto different levels and types of pollutants.indigenous Describing an organism thatis native to an area, rather than introduced.A naturally oc-indole acetic acid (IAA)curring auxin. See auxin.inducer A substance that activates astructural gene or block of genes by combiningwith a REPRESSOR. See operon.inducible enzyme (adaptive enzyme) Anenzyme that is produced by a cell only inthe presence of high concentrations of itssubstrate or a structurally similar substance.indusium The flap of tissue, derivedfrom the placenta, that encloses the developingsporangia in the sorus of a fern.inferior Below. In botany, the term isgenerally applied to the position of theOVARY of a flower in which the sepals,petals, and stamens arise above it. Theovary appears to have sunk into and fusedwith the cup-shaped receptacle. This conditionoccurs only in epigynous flowers. Ina floral formula, an inferior ovary is denotedby a line above the gynoecium symboland number. Compare superior. Seealso epigyny.inflorescence A collection of flowerssharing a common stalk, the peduncle andusually subtended by a bract. Each flowerusually arises in the axil of a small bract, orbracteole, and may or may not be borne onits own individual stalk, called a pedicel.The development of an inflorescence mayrepresent the end of vegetative growth ofthat apex (definite growth), or allow it to118

inflorescencecontinue (indefinite growth). There aremany types of inflorescence, determinedmainly by the method of branching. If thestem ends in a flower and subsequentgrowth is then from lateral buds below theapex, which themselves form flowers andmore lateral shoots, the inflorescence istermed a cymose inflorescence or cyme,e.g. lime (Tilia). If one shoot develops behindeach axis a monochasial cyme, ormonochasium, is formed, as in avens(Geum). Variations in the inflorescencearrangement occur, according to the directionof the lateral branches, for example inforget-me-not (Myosotis), all the branchesarise on the same side of the parent stemwhile in buttercup (Ranunculus), thebranches arise on alternate sides of the parentstem. If two shoots develop below eachaxis this gives a dichasial cyme, or dichasium,as in the campions and catchflies (Silene).Cymose inflorescences typically openfrom the apex downwards; in flat-toppedforms the oldest flowers are in the center.In a raceme (racemose inflorescence) theapex continues growing, and subsequentflowers develop in sequence up the stem,e.g. foxglove (Digitalis). In a compoundraceme each branch of the inflorescencebears a smaller raceme of flowers, e.g. fescues(Festuca). If the lateral branches of araceme are themselves branched, as inmany grasses, the inflorescence is called apanicle. This term is often applied to anysort of branched racemose inflorescence,for example the horse chestnut, in whicheach branch is actually a cyme (Aesculus).A spike is a type of racemose inflorescencehaving sessile flowers (they have nopedicels) borne on an elongated axis, as inwheat (Triticum). The catkin and spadixare modifications of the spike. A catkin is ashort, densely packed raceme bearing unisexualflowers with highly reduced or absentperianth, e.g. oak (Quercus). A spadixis a type of inflorescence found in the familyAraceae, e.g. cuckoopint (Arum maculatum).It is a modified spike with a largefleshy axis on which are borne small hermaphroditeor, more usually, unisexualflowers. The inflorescence is enclosed by alarge bract, the spathe, which may be folioseor petalloid and has been shown to attractinsects in certain species. In aInflorescence: types of inflorescence119

initialcapitulum (pl. capitula), as in many Asteraceae,e.g. daisy (Bellis), the inflorescencecomprises many unstalked florets insertedon the flattened disklike end of the peduncleand surrounded by a ring of sterilebracts, the involucre. The internodes arenot recognizable. Each floret may or maynot be borne in the axil of a bract on thedisk. Many species of composite have twodistinct types of floret in the capitulum:disk florets in the center are tubular floretsending in five short teeth, while ray floretshave a straplike extension to the tube andoccur around the edge of the capitulum,rather like petals. Some species, such asthistles, have only ray florets, others haveonly disk florets, e.g. chicory (Cichorium),and many have both disk and ray florets,e.g. sunflower (Helianthus). In racemoseinflorescences the flowers typically openfrom below upwards. An umbel is a type ofinflorescence in which the stem axis is notelongated and individually stalked flowersappear to arise from same point on thestem. These flowers are massed on oneplane, giving the appearance of an umbrella,with the oldest flowers on the outsideand the youngest in the middle. Theumbel is typical of the carrot family (Apiaceae).Umbels may be grouped into acompound umbel, composed of umbels ofsmaller umbels, as in onion (Allium). Acorymb is an inflorescence with flowerstalks of different lengths, the lowest beingthe longest. This gives a flat-topped clusterof flowers at the same level that is characteristicof many brassicas, e.g. candytuft(Iberis). Umbels, corymbs, and capitulacan in turn be grouped into cymose inflorescences,e.g. onion, Viburnum, andscabious (Scabiosa) respectively.initial A cell permanently in a MERISTEM,actively dividing and adding new cells tothe plant body. An initial never becomesdifferentiated. One of its daughters remainsin the meristem as an initial, whilethe other differentiates to form a nonmeristematiccell. Two basic groups exist:apical initials at root and shoot apices, andlateral meristem initials whose positiondepends on the location of the meristem,e.g. intrafascicular CAMBIUM between xylemand phloem. Apical initials are rarely singlecells except in certain lower plants, and initiatemore than one structure, e.g. lateralshoots, flowers, and leaves, as well as newstem tissues. Vascular cambium possessestwo distinct types of initials, ray initialsforming medullary rays, and fusiform initialsproducing xylem and phloem elements.initiation factorSee translation.inositol An optically active cyclic sugaralcohol, synthesized from glucose 6-phosphate.A derivative of iniositol, phytic acid,is an important storage compound inseeds, which is believed to be convertedduring germination to inositol, and thenceto glucuronic and uronic acids for the synthesisof cell-wall components.insectivorous plantplant.See carnivorousintegument A layer surrounding the nucellusin the ovules of gymnosperms andangiosperms. Most angiosperm ovules possesstwo integuments, while gymnospermsusually have only one. Enclosure of the nucellusby the integuments is incomplete, themicropyle remaining to allow access to theembryo sac or, in gymnosperms, to thearchegonium.intercalary meristem A region of activelygrowing primary tissue clearly separatefrom the apical meristem. Intercalarymeristems occur at the internode and leafsheathbases (joints) of many monocotyledonsincluding grasses, and at theinternodes of horsetails, where they serveto increase longitudinal growth.intercellular Describing materials foundand processes occurring between cells.Compare intracellular.interfascicular cambium A single layerof actively dividing cells between the vascularbundles in stems. It is formed whenparenchyma cells resume meristematic120

inulinactivity. The interfascicular cambium andINTRAFASCICULAR CAMBIUM (within the vascularbundles) link into a complete cambiumcylinder that cuts off secondaryxylem tissue to the inside, secondaryphloem tissue to the outside, and parenchymacells both sides (forming medullaryrays). See vascular cambium.International Code of Botanical Nomenclature(ICBN) A set of rules forthe scientific naming of wild plants, algae,fungi, and slime molds, which allocates asingle unambiguous name to each taxon.Naming of families and lower taxa is basedon the first valid name published, the starting-pointbeing taken as the publication ofSpecies Plantarum, by LINNAEUS, in 1753.But if a plant is later assigned to differenttaxa or rank, a different name may begiven. Thus a plant may legitimately havetwo names, according to which genus differentauthors place it in. If several taxa arecombined into one, that TAXON takes onthe name of the former taxon with the oldestname. Different starting dates apply tofossil plants and bacteria. There is a separatecode for cultivated plants, the InternationalCode of Nomenclature of CultivatedPlants (ICNCP). See classification.International Union for the Conservationof Nature and Natural Resources(IUCN) See World Conservation Union.internode The region of the stem betweentwo NODES. See intercalary meristem.interphase The stage in the CELL CYCLEwhen the nucleus is not in a state of division.Interphase is divisible into variousstages each characterized by a differingphysiological activity.intine (endosporium)See pollen.intracellular Describing the materialenclosed and processes occurring interiorto the cell membrane. Compare intercellular.intrafascicular cambium (fascicularcambium) The part of the VASCULARCAMBIUM within the vascular between thexylem and phloem.intraspecific selection Natural selectionacting on individuals of the samespecies.introgression (introgressive hybridization)The introduction of genetic materialfrom one gene pool to another byhybridization and subsequent back-crossingto one or other of the parents. It formsthe basis of the evolution of most cropplants, e.g. sunflower (Helianthus annuus).Where the ranges of two species overlap,and fertile hybrids can be produced, thesehybrids will tend to backcross with themost abundant species, so the offspringdown many generations come to resemblethe most abundant parents.See intro-introgressive hybridizationgression.intron A noncoding DNA sequence thatoccurs between coding sequences (exons)in many eukaryote genes. Messenger RNA(mRNA) does not contain introns, thesebeing removed during the transcriptionprocess. Intron removal is now thought tobe an autocatalytic process in which theRNA acts as its own enzyme (see ribozyme).However, in the case of mRNAin the nucleus the process is regulated by acomplex of proteins called a spliceosome.Compare exon.introrse Denoting anthers in which dehiscencelines are toward the center of theflower. Pollen is consequently shed towardthe carpels, which favors self-pollination.Compare extrorse.intussusception The incorporation ofcellulose molecules into the existing cellwall, giving an increase in wall area. Theprocess is usually preceded by water uptake,increasing the pressure on the cellwall, which stretches. The new wall is thenthickened by APPOSITION.121

inversioninulin A polysaccharide food reserve ofsome higher plants, particularly the Asteraceae,e.g. Dahlia root tubers. It is a polymerof fructose.inversioninvertaseinvert sugarSee chromosome mutation.See sucrase.See sucrose.in vitro Literally ‘in glass’; describingexperiments or techniques performed inlaboratory apparatus rather than in the livingorganism. Cell tissue culture is an example.Compare in vivo.in vivo Literally ‘in life’; describingprocesses that occur within the living organism.Compare in vitro.involucre 1. A protective structure consistingof a ring of bracts arising below theINFLORESCENCE in angiosperms with condensedinflorescences (e.g. the capitulumand umbel).2. A sheathlike outgrowth of tissue inbryophytes protecting the archegonia orantheridia in certain liverworts.3. A tubular extension of the thallus ofhornworts that rises up and surrounds thebase of the sporophyte.iodineSee staining.ion-exchange chromatography A typeof CHROMATOGRAPHY in which the compoundsare separated according to theiracidity.ionic bond A bond formed when anelectron passes from one atom to another.The atom that loses the electron becomes apositively charged ion, and the receivingatom becomes negatively charged, so thetwo ions become bonded by a strong electrostaticforce. Compare covalent bond;hydrogen bond.iron A MICRONUTRIENT essential forplant growth. It is found in porphyrins,and is essential for chlorophyll synthesis.Lack of iron leads to CHLOROSIS, especiallyin young leaves. It is also found in cytochromes,which are important componentsof the electron-transport chains ofrespiration and photosynthesis.isoelectric focusing A technique usedin ELECTROPHORESIS to separate amphotericmolecules (able to combine with eitheracids or bases).isoelectric point The value of pH in amedium at which a molecule has no electriccharge. Repulsion between moleculesceases, so they precipitate. This principle isused to separate mixtures of amino acids orproteins. See isoelectric focusing.isoenzyme (isozyme) An enzyme thatoccurs in different structural forms withina single species. The isomeric forms allhave the same molecular weight but differingstructural configurations and properties,usually due to different combinationsof their component subunits. They areformed by different alleles, and are a valuabletool for detecting genetic variation.Isoetales (quillworts) An order of theLYCOPHYTA containing two genera – Isoetes,found in most parts of the world, andStylites, found only in the Peruvian Andes.Quillworts are heterosporous, mainlyaquatic perennials, with a dense rosette oftubular quill-like leaves arising from ashort, stout rootstock.isogamy The sexual fusion of gametesof similar size and form. It occurs in fungiand some protoctists. Compare anisogamy.isolating mechanism Structural, physiological,behavioral, genetic, geographical,or other factors that restrict the interbreedingof one population with another, hencerestricting gene flow. The development ofisolating mechanisms promotes the formationof new varieties and species.isoleucine A nonpolar AMINO ACID, synthesizedfrom threonine.122

Ivanovsky, Dmitri Iosifovichisomerase An enzyme that catalyzes theconversion of a molecule from one isomericform to another. For example, in glycolysis,glucose phosphate isomerasecatalyzes the conversion of glucose 6-phosphateto fructose 6-phosphate.isomorphism A condition seen in certainalgae (e.g. Ulva, the sea lettuce) inwhich the alternating generations of thelife cycle are morphologically identical(HOMOLOGOUS). Compare heteromorphism.isotonic Designating a solution with anosmotic pressure or concentration equal tothat of a specified other solution, usuallytaken to be within a cell. It therefore neithergains nor loses water by osmosis.Compare hypertonic; hypotonic.isotopic datingisozymeIUCNSee isoenzyme.See radiometric dating.See World Conservation Union.Ivanovsky, Dmitri Iosifovich (1864–1920) Russian botanist. In 1862, whileworking on tobacco mosaic disease atthe Technical Institute in St Petersburg,Ivanovsky noticed that healthy plantscould be infected with the sap of diseasedtobacco plants. He also noticed that thesap filtrate contained tiny crystalline particlesthat he believed were causative123

JJacob–Monod model A model of regulationof gene activity in bacteria proposedby F. Jacob and I. Monod in 1960. Usingthe concept of the OPERON, it proposed thatmetabolism is regulated and coordinatedby controlling messenger RNA synthesis(i.e. TRANSCRIPTION) and hence the productionof enzymes. A similar but more complexsystem is known to operate ineukaryotes. See inducible enzyme.Johannsen, Wilhelm Ludwig (1857–1927) Danish botanist and geneticist. Johannsenbecame interested in the Theoryof Heredity, a work by the British anthropologistSir Francis Galton, which statedthat natural selection does not work on theoffspring of self-fertilizing plants. Johannsenexperimented with the Princessbean and found that if the self-fertilizingparent plants were from a mixed population,then selection did indeed work. Hedescribed the offspring of a single parent asa ‘pure line’ and stated that they were geneticallyidentical. In 1905 he coined theterms ‘phenotype’ to describe the outwardappearance of an individual and ‘genotype’to describe the genetic makeup of an individual.Following the rediscovery ofMENDEL’S LAWS, he published his Elementsof Heredity in 1905 and used the word‘gene’ for Mendel’s factors of inheritance.His application of statistical methods tocharacteristics of populations proved to bea cornerstone of modern genetics.See exponen-J-shaped growth curvetial growth.Juncaceae (rushes)A family of monocotyledonousplants, mostly herbs, withlong, narrow grasslike or cylindrical leavesspirally arranged, but usually arising closeto the ground. They are found in temperateand cold regions, and on tropical mountains,forming an important part of thevegetation in poorly drained areas.junk DNA The noncoding part ofDNA, i.e. the part that does not code forproteins. Its function is not yet understood,but it appears that part of it contains intructionsthat help control where and whenthe different proteins are expressed. Comparerepetitive DNA; selfish DNA.Jurassic The middle period of the Mesozoicera, 215–145 million years ago. Theclimate was warm and humid, and therewas widespread expansion of forests,which gave rise to thick deposits of coal.The Jurassic has been called the ‘Age of Cycads’.The seed-ferns declined, and therewas a great increase in the number and diversityof cycads, palmlike gymnosperms,and conifers. Fossils of monocotyledonouspollen and stems of palms suggest that theangiosperms evolved during this period.See also geological time scale.Jussieu, Antoine-Laurent de (1748–1836) French plant taxonomist. Workingat the Jardin du Roi in Paris, Jussieubased his classification system on manyplant characteristics rather than the smallnumber used by LINNAEUS. Using the collectionsof Banks, Linnaeus, and PhilibertCommesson, he established 15 classes and100 families of plants in his classification.124

Kkaryogamy The fusion of two nucleithat exist within a common cytoplasm, asoccurs in the formation of the zygote fromtwo gametes. In the more advanced fungi,especially the Basidiomycota, nuclei mayfuse some considerable time after fusion ofhyphae of different mating strains, forminga dikaryon.karyogram (idiogram) The formalizedlayout of the KARYOTYPE of a species, oftenwith the chromosomes arranged in a certainnumerical sequence. This usually takesthe form of drawings or photographs of theindividual chromosomes, often arranged inhomologous pairs in numerical sequence.of NAD. In the Krebs cycle it undergoes decarboxylationto form succinyl CoA. a-ketoglutarate is also involved in aminoacid metabolism, especially TRANSAMINA-TION.ketohexose A ketose SUGAR with sixcarbon atoms, e.g. fructose.ketopentose A ketose SUGAR with fivecarbon atoms, e.g. ribulose.ketose A monosaccharide SUGAR con-karyokinesis During cell division, theprocess of nuclear division that precedescytoplasmic division. There are two maintypes of nuclear division: MITOSIS, whichresults in daughter cells identical to theirparents; and MEIOSIS (reduction division),which produces daughter cells which havehalf the number of chromosomes of theirparent cells, and in which the genetic materialhas been recombined. Compare cytokinesis.karyotype The physical appearance ofthe chromosome complement of a givenspecies. A species can be characterized byits karyotype since the number, size, andshape of chromosomes vary greatly betweenspecies but are fairly constant withinspecies. See karyogram.kelpSee Laminariales.a-ketoglutaric acid A five-carbon dicarboxylicacid that is an intermediate inthe KREBS CYCLE. It is synthesized by oxidationof isocitrate, coupled to the reductionKetose: typical ketose sugars125

kinasetaining a ketone (=CO) or potential ketonegroup, e.g. dihydroxyacetone.kinase (phosphotransferase; phosphorylase)1. Any enzyme that transfers aphosphate group, usually from ATP.2. An enzyme that activates the inactiveform of other enzymes. For instance, whentrypsinogen, the inactive form of trypsin,comes in contact with enterokinase, activetrypsin is released.kinetinSee cytokinin.kinetosome (basal body) A barrelshapedbody found at the base of all eukaryoteUNDULIPODIA (cilic and flagella)and identical in structure to the CENTRIOLE.It is essential for formation of undulipodia.kingdom Formerly, the highest rankingcategory in most classification systems.Most taxonomists today recognize therank of domain as being higher than that ofkingdom. See Five Kingdoms classification.klinostat (clinostat) An apparatus usedin tropism experiments to remove the unidirectionalinfluence of a stimulus on aplant organ.Kramer, Paul Jackson (1904– )American plant physiologist. Working atDuke University, Kramer made a significantcontribution to the study of wateruptakemechanisms in plants. Using radioactivelylabeled elements he demonstratedthat the region of maximum absorption ofwater is a few centimeters behind the roottip and that quantities of minerals are absorbedpassively in the transpiration stream.Kranz anatomy The specific arrangementof the photosynthetic tissues in theleaves of C 4 PLANTS. The cells of the bundlesheath that surround the vascular tissuesare large and contain specialized elongatedKrebs cycle126

Krebs cyclechloroplasts that do not contain grana, butform starch grains. The chloroplasts of themesophyll, by contrast, appear normal andcontain grana, but do not form starchgrains. In these chloroplasts carbon dioxidecombines with phosphoenolpyruvate toform oxaloacetic acid, which is transportedto the bundle sheath cells, where thecarbon dioxide is released, then fixed bythe enzyme ribulose bisphosphate carboxylaseto form glycerate 3-phosphate, thefirst step in the Calvin cycle. See photosynthesis.Krebs, Sir Hans Adolf (1900–81) German-bornBritish biochemist best knownfor his elucidation of the KREBS CYCLE. Heshared the 1953 Nobel Prize for Physiologyor Medicine with Fritz Lipmann(1899–1986).Krebs cycle (citric acid cycle; TCA cycle;tricarboxylic acid cycle) A complex cycleof reactions in which pyruvate, producedby glycolysis, is oxidized to carbon dioxideand water, with the production of largeamounts of energy. It is the second stage ofAEROBIC RESPIRATION, requires oxygen, andoccurs in the matrix of mitochondria. 2-carbon acetate (acetyl coenzyme A), derivedfrom pyruvate by decarboxylation,reacts with 4-carbon oxaloacetate to form6-carbon citrate, which is then decarboxylatedin a series of steps to reconstitute oxaloacetate.Some ATP is produced bydirect coupling with Krebs cycle reactions,but most production is coupled to the ELEC-TRON-TRANSPORT CHAIN. The operation ofthis depends on the generation of reducedcoenzymes, NADH and FADH 2 , by theKrebs cycle. These reduced cofactors providethe energy for ATP formation in therespiratory electron-transport chain andOXIDATIVE PHOSPHORYLATION. Consideringthe Krebs cycle and electron-transportchain together, each pyruvate moleculeyields 15 ATP molecules. Since two pyruvatemolecules enter the cycle from glycolysis,30 ATP are produced in all. Theintermediates in the Krebs cycle may beused directly or indirectly in the synthesisof amino acids.127

Llabeling The technique of using isotopes(usually radioactive isotopes) or other recognizablechemical groups to investigatebiochemical reactions. For instance, a compoundcan be synthesized with one of theatoms replaced by a radioactive isotope ofthe element. The radioactivity can then beused to follow the course of reactions involvingthis compound.LabiataeSee Lamiaceae.lactic acid A 3-carbon hydroxy-acidformed as a result of fermentation by certainbacteria, e.g. Lactobacillus.lactic acid bacteria A group of bacteriathat ferment carbohydrates in the absenceof oxygen, with lactic acid always a majorend product, e.g. Lactobacillus spp., Streptococcuslactis. They have a high toleranceof acid conditions. Lactic acid bacteria areinvolved in the formation of yoghurt,cheese, sauerkraut, and silage. They canoccur as spoilage organisms and some arepathogenic causing infections.lacuna (pl. lacunae) 1. An intercellularcavity in plant tissues resulting from cellbreakdown, tissue splitting, or organizedformation. In hydrophytes a well-arrangedsystem of these air-filled cavities may existto provide buoyancy. A large lacuna occupiesthe center of many stems.2. See leaf gap.3. A depression in a lichen thallus.lag phaseSee exponential growth.lamella (pl. lamellae) 1. A layer withinthe cytoplasm or inside an organelle, whichis formed from a flattened membraneboundedvesicle or tube. The layer thusconsists of two membranes lying close togetherwith a cavity between them. An exampleis the internal system of lamellae(thylakoids) in CHLOROPLASTS.2. A general term applied to any thin platelikestructure, e.g. the blade of a leaf.Lamiaceae (Labiatae) A dicotyledonousfamily of herbs and low-growing shrubs,most of which have stems of square crosssectionand simple leaves with stipulesarranged in opposite pairs. There are about6700 species, distributed worldwide butespecially numerous in the Mediterraneanregion. The family includes many herbs,such as basil (Hyssopus officinalis), marjoram(Origanum), mint (Mentha), sage(Salvia officinalis), savory (Satureja), andthyme (Thymus vulgaris).lamina (pl. laminae) 1. (of angiosperms)A thin usually flat foliage leaf blade, commonlyattached to the stem by a petiole. Inmost plants the leaf laminae are the mainphotosynthetic organs. A lamina is termedsimple if complete, and compound if dividedinto leaflets. Internal organizationshows a wide photosynthetic mesophylllayer permeated by veins and bounded byan epidermis. Leaf variation is mainly dueto the structural diversity of the lamina.2. (of algae) The bladelike part of the thallusof certain algae, notably the Phaeophyta(brown algae), e.g. kelps (Laminaria,Macrocystis, Nereocystis) and bladderwrack(Fucus vesiculosus).Laminariales (kelps) An order of thePHAEOPHYTA (brown algae) that containsthe kelps (e.g. Macrocystis and Nereocystis)and is distinguished by its heteromorphicALTERNATION OF GENERATIONS inwhich a large sporophyte alternates with a128

leaf curlmicroscopic gametophyte, and in whichgrowth takes place in a special meristem region(see meristoderm) on the surface ofthe thallus.laminarin The chief carbohydrate foodreserve of the Phaeophyta (brown algae).lateral meristem A MERISTEM that liesparallel to the sides of the organ in which itoccurs and gives rise to secondary tissuesthat increase the girth of the organ. Seecambium; secondary growth. Compareapical meristem.lateral root Any ROOT that arises fromthe PERICYCLE of another root.laterite A hard crust that may form onthe surface of the SOIL in tropical regionswith alternating wet and dry seasons.latex A liquid found in some floweringplants contained in special cells or vesselscalled laticifers (or laticiferous vessels). It isa complex variable substance that maycontain terpenes (e.g. rubber), resins, tannins,waxes, alkaloids, sugar, starch,enzymes, crystals, etc. in solution or suspension.It is often milky in appearance,as in dandelion (Taraxacum spp.) and lettuce(Lactuca sativa) but may be colorless,orange, or brown. Commercial rubbercomes from the latex of the rubber plantsFicus elastica and Hevea brasiliensis.Opium comes from alkaloids found in thelatex of the opium poppy (Papaver somniferum).laticifersSee latex.Lauraceae A family of trees and shrubs(bays and laurels) usually with simple,leathery, evergreen alternate or oppositeleaves without stipules. There are about2500 species, mainly in the tropics andsubtropics, especially in Amazonia andSoutheast Asia.Law of Independent AssortmentMendel’s laws.SeeLaw of SegregationSee Mendel’s laws.layering A method of vegetative propagationin which RUNNERS or STOLONS arepegged down to the soil surface. Where anode touches the soil, adventitious rootsdevelop and a shoot arises from the LAT-ERAL MERISTEM. If the intervening runner orstolon rots or is cut away, the daughterplants becomes independent.leaching The removal of humus and soilnutrients in solution by water movingdown the soil profile. It makes soils moreacid since cations (e.g. potassium and magnesium)are replaced by hydrogen ions.Leaching leads to the formation of podsolizedand lateritic soils.leaf A flattened appendage of the stemthat arises as a superficial outgrowth fromthe apical meristem. Leaves are arranged ina definite pattern, have buds in their axils,and show limited growth. Most foliageleaves are photosynthetic, bilaterally symmetrical,and externally differentiated intoLAMINA, MIDRIB, and PETIOLE. Many variationsoccur. Not all foliage leaves are photosyntheticas they may be modified whollyto bud scales and spines or partly to formtendrils. See illustration overleaf.leaf buttress A leaf primordium, appearingas a protuberance on the side of thestem apex, produced by periclinal divisionof the tunica and corpus, and associatedwith a procambium. It is the earliest stagein the development of a leaf, and forms theleaf base as the leaf grows out from it. Theleaf axis arises from the leaf buttress andmaintains procambial continuity in thecentral region. This differentiates into thevascular tissue of the developing leaf.leaf curl Any disease of plants thatcauses an increase in numbers of cells thatresults in curling and puckering of theleaves. It may be caused by a fungus (e.g.Taphrinia deformans causes peach leafcurl), or by viruses (e.g. tobacco and cottonleaf curl) transmitted by insects such aswhiteflies (Bemisia).129

Leaf shapesLeaf margins130

lichensleaf fall The shedding of leaves, usuallyassociated with a fall in AUXIN concentration.In both evergreen and deciduous treesold leaves are continually shed throughoutthe growing season, but the remainingleaves are also shed at the onset of winteror a dry season in deciduous species. Seeabscission.leaf gap (lacuna) A region of parenchymadifferentiated in the stem vascularcylinder immediately above a divergingLEAF TRACE. Lateral connections insure nobreak in the vascular system.leaf mosaic 1. The nonoverlapping patternof leaves on a plant that results mainlyfrom the genetically programmed arrangementof leaves and branches, and in somespecies also by the twisting of the petiolesdue to EPINASTY. It ensures that the maximumamount of light is intercepted by theleaves. See phyllotaxis.2. A virus disease of plants characterizedby yellow and green or light- and darkgreenmottling of the leaves.leaf trace A vascular bundle or group ofvascular bundles connecting the vascularsystems of leaf and stem. It stretches fromthe leaf base to the vascular ring in the stemaxis.lecithin (phosphatidyl choline) One of agroup of phospholipids that contain glycerol,fatty acid, phosphoric acid, andcholine that are found widely in higherplants and animals, particularly as a componentof cell membranes.legume 1. (pod) A dry dehiscent singleormany-seeded fruit formed from a singlecarpel, that dehisces mechanically by splitting,often explosively, along both sides.The opposite tensions set up by obliquefibers in the drying pericarp cause dehiscence.The valves may continue twistingand remove any remaining seeds. It is thetypical fruit of the FABACEAE family, and isfound in certain other families. Comparelomentum.2. Any plant of the family Fabaceae.lemmalenticelSee bract.See bark.leptosporangiate Describing the condition,found in certain ferns (e.g. Filicales),in which the sporangium develops from asingle initial cell. The wall of a leptosporangiumis usually only one cell thick, andas it dries out, it flicks the spores away explosively.Compare eusporangiate.leptoteneSee meiosis; prophase.leucine An amino acid synthesized frompyruvate. It can be broken down to yieldacetoacetate. See amino acids.leukoplast A colorless PLASTID, i.e. onenot containing chlorophyll or any otherpigment. Leukoplasts are common in thecells of roots and underground stems andstorage organs.liana (liane) A long-stemmed woodyclimbing plant that grows from groundlevel to the canopy of trees, and oftenhangs down freely from branches, e.g. curare(Strychnos toxifera). Lianas may havestems up to 100 meters (330 feet) long, andmay climb over the canopy to gain maximumsunlight. Many flower only whenthey reach the top of the canopy. Up to aquarter of all the woody growth in somerainforests may be lianas.lichens Symbiotic associations betweenan alga or cyanobacterium (the photobiont)and a fungus (the mycobiont). Theyare slow-growing but can colonize areastoo inhospitable for other plants. Usuallythe fungus is an ascomycete but occasionallyit is a basidiomycete. Reproduction inlichens may be asexual by soredia (algalcells enclosed by fungal hyphae) or by sexualfungal spores, which can survive only ifsome algal cells are also present. Fragmentsof the thallus containing both phycobiontand mycobiont cells may also grow intonew lichens, a form of asexual reproduction.Examples include Peltigesa and Xanthoria.131

life cycleLife cycle132

limiting factorlife cycle The sequence of developmentalchanges making up the span of an organism’slife from the fertilization ofgametes to the same stage in the subsequentgeneration. See diplobiontic; haplobiontic.ligase An enzyme that catalyzes thebond formation between two substrates atthe expense of the breakdown of ATP orsome other nucleotide triphosphate. Thedegree of bond formation by ligases is proportionalto the amount of ATP availablein the cell at a particular instant.light green A stain used in light microscopyto show up cytoplasm and cellulose.It is commonly used as a counterstainwith safranin to highlight cytoplasm. Seestaining.light microscopeSee microscope.light reactions The light-dependent reactionsof PHOTOSYNTHESIS that convertlight energy into the chemical energy ofNADPH and ATP, which are used in thedark reactions for carbon dioxide fixation.Light absorbed by two groups of pigmentsin the chloroplast – Photosystems I and II.The absorbed light energy excites chlorophylla molecules, causing them to emithigh-energy electrons that pass down anELECTRON-TRANSPORT CHAIN, releasing energyto synthesize ATP (photophosphorylation)and NADPH. The light reactionstake place on the thylakoid membranes ofchloroplasts.See compensa-light saturation pointtion point.lignin One of the main structural materialsof vascular plants. Together with celluloseit is one of the main constituents ofwood, where it imparts high tensile andcompressive strengths, making it ideal forsupport and protection. Lignified tissuesinclude SCLERENCHYMA and XYLEM. Ligninis deposited during secondary thickeningof cell walls. The degree of lignificationvaries from slight in protoxylem to heavyin sclerenchyma and some xylem vessels,but values of 25–30% lignin and 50% celluloseare average. It is a complex variablepolymer, derived from sugars via aromaticalcohols.ligule 1. A scalelike outgrowth, varyingin shape and size, of certain angiospermleaves. In grasses the ligule occurs at thejunction of the leaf sheath and lamina. Althoughmembranous in most species, itmay be only a fringe of hairs. Ligules areimportant taxonomic features in grasses.2. A toothed, strap-shaped structureformed by the extension of one side of thecorolla tube in the florets of certain speciesof the ASTERACEAE. The teeth indicate thenumber of fused petals. In some species,e.g. dandelion (Taraxacum spp.), all floretsof a capitulum are ligulate but in others e.g.daisy (Bellis perennis), only the ray floretshave ligules.3. A very small tongue-shaped flap of tissueinserted on the upper surface of the leaves(microsporophylls) and sporophylls of certainclubmosses (e.g. Selaginella) and quillworts(Isoetes).Liliaceae (lilies) A large family of monocotyledonousplants. Their taxonomy ismuch in dispute, with groups that were formerlysections now regarded with newevidence as being separate families. Theyhave long, narrow parallel-veined leaves,whorled or alternate, arising at the base ofthe stem or along it. Most are herbs, e.g.lilies (Lilium) and tulips (Tulipa); some aresucculents or dwarf shrubs, e.g. butcher’sbroom (Ruscus aculeatus), and a few areclimbers (e.g. Smilax). Many species haveperennating organs such as bulbs, corms,or rhizomes.liming The addition of lime to a soil toreduce its acidity, improve crumb structure,or increase its calcium content. Seeflocculation.limiting factor Any factor in the environmentthat governs the behavior ormetabolic activity of an organism or systemby being above or below a certain133

lipid A collective term used to describe agroup of substances in cells characterizedby their solubility in organic solvents suchas ether and benzene, and their absence ofsolubility in water.The group is rather heterogeneous interms of both function and structure. Theyencompass the following broad bands ofbiological roles: (1) basic structural unitsof cellular membranes and cytologicallydistinct subcellular bodies such as chlorolimitinglayerlevel. In general, a number of different factors(e.g. light intensity, temperature, carbondioxide concentration) may belimiting, but at any one time the factor thatis closest to its critical minimum is the limitingfactor. For instance, the rate of photosynthesisrises with increasing lightintensity as long as there is sufficient carbondioxide available, but at high light intensitiescarbon dioxide may become thelimiting factor instead.limiting layerSee meristoderm.limnology The scientific study of freshwaterand its flora and fauna. Limnologyalso includes studying the chemical andphysical aspects of inland water. It may bedivided into the study of standing waterhabitats, e.g. lakes and ponds, and runningwater habitats, e.g. rivers and streams.linear Describing leaves that are long,narrow, and parallel-sided for most oftheir length.linkage The occurrence of genes togetheron the same chromosome so thatthey tend to be inherited together and notindependently. Groups of linked genes aretermed linkage groups and the number oflinkage groups of a particular organism isequal to its haploid chromosome number.Linkage groups can be broken up byCROSSING OVER at meiosis to give new combinationsof genes. Two genes close togetheron a chromosome are more stronglylinked, i.e. there is less chance of a crossover between them, than two genes furtherapart on the chromosome. Linked genesare symbolized Ab...Y/aB...y, indicatingthat Ab...Y are on one homolog whileaB...y are on the other homolog. Linkage isindicated when the associated inheritanceof two or more nonallelic genes is greaterthan would be expected from independentassortment. The genes on a single chromosomeform one linkage map. See chromosomemap; independent assortment.Linnaeus, Carolus (1707–76) Swedishbotanist. While working at Uppsala Universityhe began to investigate the sexualityof plants and by 1730 he had begun to setout a classification system based on thenumber and arrangement of stamens andpistils. After traveling through Laplandand discovering many new species, he settledin Holland and published his SystemaNaturae, in which he set out a classificationof animals, plants, and minerals. Heplaced flowering plants into classes basedon the number of stamens and subdividedthe classes into orders based on the numberof their pistils.Linnaeus’s most famous contributionto taxonomy was his system of BINOMIALNOMENCLATURE, in which he gave plantstwo Latin names. The first name is thegeneric name and the second is the specific(or species) name.linoleic acid A common unsaturated18-carbon fatty acid occurring in glyceridesin linseed oil, cottonseed oil, andother vegetable oils. It is an essential nutrientin the diet of mammals.linolenic acid An unsaturated 18-carbonfatty acid occurring commonly inplants as the glyceryl ester, for example inlinseed oil and poppy-seed oil. Linoleicacid is the predominant fatty acid in thechloroplast. It is an essential nutrient in thediet of mammals.lipase Any of various enzymes that catalyzethe hydrolysis of fats to fatty acidsand glycerol. Lipases are important in germinatingseeds, e.g. castor oil (Ricinuscommunis) where they help to break downstorage fats.134

loessplasts and mitochondria; (2) compartmentalizingunits for metabolically active proteinslocalized in membranes; (3) a store ofchemical energy and carbon skeletons; and(4) primary transport systems of nonpolarmaterial through biological fluids. Plantssynthesize a wide range of lipids. Longchainfatty acids (often 16–18 carbonatoms) are esterified to glycerol for use inmembranes, and PHOSPHOGLYCERIDES areparticularly important components of membranes;esters of long-chain monohydric alcoholsand fatty acids form the WAXES ofthe cuticle and the SUBERIN of the endodermis;and triacylglycerols are importantstorage lipids in seeds, especially in suchspecies as the castor oil seed (Ricinis communis),where they are stored in oil bodies,as small lipid droplets coated with lipidsand protein. The fatty acid components ofplant lipids come mainly from acetyl CoA,produced in plastids (including chloroplasts)and mitochondria. GLYCOLIPIDS aresynthesized in the endoplasmic reticulum,mitochondria, and plastids.lipopolysaccharide A conjugated polysaccharidein which the noncarbohydratepart is a lipid. Lipopolysaccharides are amajor component of the cell walls ofGram-negative bacteria.lipoprotein Any conjugated proteinformed by the combination of a proteinwith a lipid. Most membranes, includingthe plasma membrane, are composedmostly of lipoprotein. The lipid componentsare mainly phosphoglycerides andglycolipids.litmus paper Red or blue acid–alkali indicatorpapers. In acids blue litmus paperturns red and in alkalis red litmus paperturns blue. Litmus solution has a pH rangefrom 4.5–8.3.littoral 1. The zone of the seashore betweenthe high and low tide mark. Theterm is also applied to organisms living inthis zone. Since tidal ranges vary continually,the zone is often defined in terms ofthe upper and lower limits of certainspecies of organism. Compare benthic;sublittoral.2. The zone between the water’s edge anda depth of about six meters in a pond orlake, where light reaches the bottom sediments.Rooted hydrophytes, both emergentand submergent, are found in thiszone. Compare profundal; sublittoral.liverwortsSee Hepatophyta.living fossil Modern organisms withanatomical or physiological features foundelsewhere only in extinct species. An exampleis the maidenhair tree (Ginkgo biloba),discovered in Japan in the 17th century,and later in China, but found only in cultivation.Fossil members of the Ginkgophytaare common from rocks of Mesozoic age.loam A medium-textured soil containinga mixture of large and small mineralparticles. Loams are easy soils to work andcombine the good properties of sandy andclay soils.locule (loculus) An air-filled compartmentin an ovary in which the ovules develop.The term locule can also be appliedto any other cavity in an organ in whichother structures develop, for example thelocules of anthers in which pollen isformed. When all the carpel edges meet atthe middle of a syncarpous ovary there areas many locules as carpels, and the ovary isdesignated bi-, tri-, quadri-, or multilocularaccording to number. Adjoining carpelwalls form septa that separate locules. Aunilocular condition occurs in monocarpellaryovaries and syncarpous ovarieslacking septa.loculus (pl. loculi)See locule.locus (pl. loci) The position of a gene ona chromosome. Alleles of the same gene occupythe equivalent locus on homologouschromosomes.loess A fine-textured, fertile, often calcareousuniform soil comprising mainlyquartz particles about 0.015–0.005 mm in135

log phasediameter. Widespread in central Europe,southern Russia, China, the central USA,and Argentina, it is derived from windblownclay and silt particles originally depositedat the edge of the ice sheets at theend of the last ICE AGE. See chernozem.See expo-log phase (logarithmic phase)nential growth.lomasome An infolding of the plasmamembrane found particularly in fungal hyphaeand spores, and also in some algaeand higher plants.lomentum (pl. lomenta) A dry dehiscentfruit formed from a single carpel and bearingmore than one seed, as in sainfoin(Onobrychis viciifolia). It resembles alegume or siliqua but is divided by falsesepta into single-seeded compartments thatrupture at maturity.long-day plant (LDP) A plant thatflowers in response to a dark periodshorter than a critical maximum, e.g.spinach (Spinacea oleracea), which flowersin summer, when days are longer andnights shorter. See photoperiodism; criticalday length.lumen The central space that remains,surrounded by cell walls, in a cell that haslost its living contents (e.g. in xylem elements).lutein The commonest of the xanthophylls,an orange pigment found in greenleaves and certain algae, e.g. the Rhodophyta.See photosynthetic pigments.lyase An enzyme that catalyzes the separationof two parts of a molecule with theformation of a double bond in one of them.For example, fumarase catalyzes the interconversionof malic acid and fumaric acid.The reaction does not involve hydrolysis.Lycophyta (Lycopodophyta; Lycopsida)A phylum of spore-bearing vascular plantscontaining about 1000 living species and 6genera, mostly found in tropical regions.Most lycopods belong to the genera Lycopodiumand Selaginella. There are fiveorders, three of which – the Lycopodiales,Selaginellales, and Isoetales – contain bothliving and fossil representatives. The remainingorders – Lepidodendrales andPleuromeiales – are represented only byfossils. The extinct trees of the genus Lepidodendronwere once distributed widelyand contributed largely to the coal seamsof the Carboniferous.Lycopodiales An order of the LYCO-PHYTA containing the homosporous clubmossesor ground pines. They are mainlytropical, some of them epiphytic, but alsooccur in temperate regions, especially inmontane habitats. Lycopodium and relatedgenera comprise the common clubmossesor (North America) ground pines.LycopodophytaLycopsidaSee Lycophyta.See Lycophyta.Lysenko, Trofim Denisovich (1898–1976) Ukrainian agriculturalist. Whileworking at the Kiev Agricultural Institute,Lysenko claimed to have invented VERNAL-IZATION, the cold treatment of grain to promotenext-season flowering in species thatwould normally take two years to flower.However, this had long been a commonpractice in agriculture. Lysenko went furtherby stating that vernalized plants couldpass this ‘acquired characteristic’ to theiroffspring, so that no further treatment wasnecessary – a return to the discredited ideasof Lamarck. In the 1930s he achieved greatinfluence in the Soviet Union under Stalinand used this to enforce his unorthodoxideas. In particular, he rejected the chromosometheory of inheritance, acting todiscredit those scientists who followed thisview and even changing school textbooksand courses to suit his own beliefs. He finallyfell from power in 1964.lysine A basic amino acid synthesizedfrom aspartic and pyruvic acids, and brokendown through acetyl CoA and the136

lysosomeKrebs cycle. Lysine is a precursor of certainalkaloids. See amino acids.lysis (degeneration) The death and subsequentbreakdown of a cell. See also autolysis;lysogeny.lysogeny 1. The formation of an intercellularspace in plants by dissolution ofcells. Compare schizogeny.2. A stable phage–bacteria relationship inwhich lysing of the bacteria does not occur.The phage (known as a temperate phage)penetrates the host cell and its nucleic acidbecomes integrated into the bacterialDNA. In this state the phage is termed aprophage; most of the viral genes arerepressed and both bacteria and phagereproduce together, producing infecteddaughter cells. In a process called induction,certain environmental factors cancause the phage to leave the host DNA andresume the lytic cycle.lysosome An organelle of plant and animalcells that contains a range of digestiveenzymes whose destructive potential necessitatestheir separation from the rest of thecytoplasm. Lysosomes are bounded by asingle membrane and have homogeneouscontents that often appear uniformly graywith the electron microscope. They areusually spherical and about 0.5 m m in diameter,although lysosomal compartmentsmay range from small Golgi vesicles tolarge plant vacuoles. Lysosomes may beformed directly from endoplasmic reticulumor by budding off of Golgi vesicles137

Mmacromolecule A very large molecule,usually a polymer, having a very high molecularweight (10 000 or more atoms).Proteins and nucleic acids are examples.macronutrient A nutrient required inmore than trace amounts by an organism.It may be an organic or inorganic compound.See essential element. Compare micronutrient.macrosclereids Elongated rod-shapedSCLEREIDS that form a close outer protectivelayer in the seed testas and fruit walls ofsome plants.magnesium An element essential forplant and animal growth. It is contained inthe chlorophyll molecule and is thus essentialfor photosynthesis. It is an importantcomponent of membranes, and an essentialcofactor for certain phosphate-transferringenzymes, e.g. phosphohydrolase and phosphotransferase.High concentrations ofmagnesium ions, Mg 2+ , are needed to maintainribosome structure. Deficiency ofmagnesium may result in CHLOROSIS,, NE-CROSIS, stunted growth, and in somespecies, whitening and puckering of theleaf edges. It may be remedied by addingmagnesium sulfate or magnesium oxide tothe soils.Magnoliaceae A family of primitive dicotyledonsthat includes Magnolia and thetulip trees (Liriodendron spp.). They aremostly trees and shrubs of warm temperateto tropical parts of east Asia and tropicalNorth America.maidenhair fernsmale sterilitySee Adiantaceae.A condition in whichpollen production is prevented by the mutationof one or more of the genes involved.Male sterility is used by plant breeders toensure cross-pollination takes place. It isalso used in genetic engineering to ensurethat genetically modified varieties do notrelease pollen containing modified genes.malic acid (2-hydroxybutanedioic acid)A colorless crystalline 4-carbon carboxylicacid, which occurs in acid fruits such asgrapes and gooseberries. In biologicalprocesses malate ion is an important intermediatein the KREBS CYCLE.Malpighi, Marcello (1628–1694) Italianhistologist. Often described as thefather of microscopy, Malpighi, a philosopherand professor of medicine, initiatedthe detailed study of plant and animal tissues.He is best known for his investigationsinto mammalian blood and alsostudied many body organs, including theskin. He described the stomata on theabaxial surfaces of leaves, the annual ringsof woody plants, and also made a distinctionbetween monocotyledonous and dicotyledonousplants.maltose A sugar found in germinatingcereal seeds. It is a disaccharide composedof two glucose units linked by an a-1,4–GLYCOSIDIC BOND. Maltose is an importantintermediate in the enzymehydrolysis of starch. It is further hydrolyzedto glucose.manganese A metallic MICRONUTRIENTneeded by plants for growth. Manganeseions are cofactors in various enzymes, e.g.kinases, IAA oxidase, and certain enzymesinvolved in the biosynthetic pathway forchlorophyll. Manganese is required for the138

mass flowlight reactions of Photosystem II (see photosynthesis).Deficiency of manganese maycause dwarfing, mottling of the upperleaves, CHLOROSIS between the leaf veins,‘blight’ of sugar cane, and ‘gray speck’ ofoats.mangrove Any of a range of trees andshrubs that form forests and dense thicketsalong muddy coasts and estuaries and insalt marshes, some of which have proproots. Adventitious roots arch downwardto the mud and send up new trunks. Theprop roots trap sediment, building upthe land and aiding SUCCESSION. Manymangrove species have PNEUMATOPHORES,or ‘breathing knees’, that rise up out of themud. They have abundant lenticels,through which air diffuses to supply thewaterlogged roots below the mud.mannitol A soluble sugar alcohol (carbohydrate)found widely in plants andforming a characteristic food reserve of thePhaeophyta (brown algae). It is also themain soluble sugar in fungi and lichens.Mannitol is a hexahydric alcohol, i.e. eachof the six carbon atoms has an alcohol (hydroxyl)group attached. It may be derivedfrom mannose or fructose.mannose A simple sugar found in manypolysaccharides. It is an aldohexose, isomericwith glucose. In some plants, such asmembers of the family Fabaceae (legumes).It is the main unit of storage polysaccharides,forming polymers called mannans. Itis also a component of some hemicelluloses.Reduction of mannose yields mannitol,a major sugar in fungi, lichens, andsome algae.mantleSee mycorrhiza.maquis A kind of Mediterranean scrubfound on poor soils in regions with a pronounceddry season. It is made up of thicketsof evergreen sclerophyllous (havingthick leathery leaves) or spiny bushes andshrubs up to about 3 meters (9 ft) tall, e.g.gorse (Ulex), broom (Cytisus, Genista),laurels (Laurus), and members of the Ericaceae(heath family), interspersed witharomatic herbs and smaller shrubs, such asthyme (Thymus), Cistus, and myrtle (Myrtus),and scattered small trees such as olive(Olea europaea) and figs (Ficus). Similarvegetation in California is called CHAPAR-RAL.Marattiaceae (giant ferns) A family oflarge tropical EUSPORANGIATE ferns, sometimescalled the giant ferns, that dates backto the Carboniferous period. They havestout erect stems and large compoundfronds, in some species, e.g. Angiopterisevecta, up to 4.5 m (15 ft) long.Marchantiales An order of thallose liverwortswith flat, ribbonlike, dichotomouslybranching thalli that lie close to theground. Marchantia, a common species ofgardens, riverbanks, and wet moorland,has umbrellalike, separate male and femalereproductive structures on long uprightstalks.marker gene A gene of known locationand function that can is used to establishthe relative positions and functions ofother genes. During gene transfer, amarker gene may be linked to the transferredgene to determine whether or not thetransfer has been successful. See chromosomemap; genetic engineering.marshSee swamp.Marsileaceae A family of highly specializedheterosporous water ferns. Theleaves are borne on creeping rhizomes, andmay be cylindrical and pointed, as in Pilularia,or divided into two or four leaflets, asin Marsilea and Regnelidium.mass flow A hypothesis put forward byMünch (1930) to explain the mechanismof phloem transport. The movement ofsubstances is believed to be the result ofchanges in osmotic pressure. Thus in an activelyphotosynthesizing region (source)where sugars are being produced the osmoticpressure is high and water is takenin. Conversely, in regions (sinks) where139

meiosis The process of cell divisionleading to the production of daughter nucleiwith half the genetic complement of theparent cell. Cells formed by meiosis giverise to gametes and fertilization restoresthe correct chromosome complement.Meiosis consists of two divisions duringwhich the chromosomes replicate onlyonce. Like mitosis the stages PROPHASE,METAPHASE, and ANAPHASE can be recognized.However during prophase homologouschromosomes attract each other andbecome paired forming bivalents. At theend of prophase genetic material may beexchanged between the chromatids of homologouschromosomes. Meiosis also differsfrom mitosis in that after anaphase,instead of nuclear membranes forming,there is a second division, which may be dividedinto metaphase II and anaphase II.The second division ends with the formamedullaphotosynthetic products are being used upor converted to storage compounds there isa lowering of osmotic pressure and water islost. A system is then set up in which thereis mass flow of water and dissolved sugarsfrom source to sink.medulla 1. See pith.2. In fungal fruiting bodies, the layer oflongitudinal hyphae.3. In lichens, the layer of loosely organizedhyphae lying below the cortex and algallayer.4. In multicellular algal thalli, the inner layersof the thallus, comprising nonpigmentedcells that often form a storagetissue.medullary ray A linear group of livingparenchyma cells in the secondary xylemof a woody plant. They extend from thevascular cambium for various distancesinto the wood.megaphyll (macrophyll) A foliage LEAFwith a branched system of veins in theblade and with the leaf trace leaving a leafgap in the stele. It is typical of ferns andseed-bearing plants. The large often pinnatelydivided megaphyll of ferns, oftentermed a frond, contrasts with the generallymuch smaller leaves (MICROPHYLLS) ofclubmosses and horsetails. The scale- orneedlelike leaves of conifers have characteristicsof both types since they have a singlevein like a microphyll but leave a leaftrace like a megaphyll.megasporangium (pl. megasporangia) Asporangium that produces megaspores. InSelaginella the megasporangium is borne inthe axis of a sporophyll located in a strobilus.Usually all the spore mother cells degenerateexcept one, which forms a tetradof cells. One or more of these may developinto MEGASPORES, which, when shed, developinto the female gametophyte. Theovule of seed plants may be consideredequivalent to the megasporangium ofthe pteridophytes. Compare microsporangium.See heterospory.megaspore The larger of the two typesof spores in heterosporous pteridophytesand seed plants that produces the femalegametophyte. In some pteridophytes themegaspores are released from the sporophyte,in others they are retained and fertilizedon the sporophyte; in the seed plantsthey are retained. The megaspores of pteridophytesand gymnosperms produce a femalegametophyte called a PROTHALLUSthat forms two or more archegonia eachcontaining a haploid female gamete. In angiospermsthe megaspore becomes the embryosac, lacking an obvious prothallus butcontaining, most commonly, eight nuclei,one of which is organized as the female gamete.Other angiosperms have four or sixteenor more nuclei in the embryo sac.Compare microspore.megasporophyll A leaf, modified leaf,or leaflike structure that bears the megasporangium.Simple megasporophylls includethe fertile ligulate leaves of heterosporouslycopods (e.g. Selaginella). Theyare usually grouped in a STROBILUS thatmay also contain MICROSPOROPHYLLS orvegetative leaves. The carpel of angiospermsand the ovuliferous scale of gymnospermsare modified megasporophylls.140

Mendel, Gregor Johanntion of four haploid nuclei, which developinto gametes. Compare mitosis. See alternationof generations; telophase.membrane A structure consisting mainlyof lipid and protein (lipoprotein) surroundingall living cells as the plasma membrane,or plasmalemma, and also found surroundingorganelles within cells. Membranesfunction as selectively permeablebarriers, controlling passage of substancesbetween the cell and its organelles, andtheir internal environment, either activelyor passively. Membranes are typically7.5–10 nm in thickness with two regularlayers of lipid molecules (a bilayer) containingvarious types of protein molecules.Some proteins penetrate through the membrane,while others are associated with oneside; some float freely over the surfacewhile others remain stationary. Some areenzyme controlling, for instance, in the activetransport of molecules or ions throughthe membrane. Larger molecules or particlescan pass from one side of a membraneby ENDOCYTOSIS or EXOCYTOSIS.The lipids are mostly phospholipids.These are polar molecules: one end (thephosphate end) is hydrophilic (waterloving)and faces outward, while the otherend (consisting of two fatty acid tails) ishydrophobic (water-hating) and faces inward.Short chains of sugars may be associatedwith the proteins or lipids formingglycoproteins and glycolipids. The particulartypes of carbohydrates, lipids, and proteinsdetermine the characteristics of themembrane, affecting, for example, cell–cellrecognition (as in embryonic developmentand immune mechanisms), permeability,and hormone recognition. Membranesmay contain efficient arrangements of moleculesinvolved in certain metabolicprocesses, e.g. electron transport and phosphorylation(ATP production) in mitochondriaand chloroplasts. See osmosis;freeze fracturing; plasma membrane.membrane potential The potential differencethat exists across a membrane as aresult of the action of PROTON PUMPS, whichmaintain an imbalance of positive and negativeions on either side of the membrane.Such pumps are usually coupled to ATP:energy is required to maintain the potentialdifference.Mendel, Gregor Johann (1822–84)Austrian plant geneticist. Having developedan interest in horticulture as a childgrowing up on a peasant farm, Mendel wasable to carry out many plant breeding experimentsafter entering an Augustinianmonastery at Brünn (now Brno in theCzech Republic). He used the pea plant inhis experiments and selected seven characteristicsof the plant to observe. First, heisolated his selected pure-bred parentplants and carried out his crosses by transferringpollen from one to another. He thencollected and counted the seeds and grewthem on, patiently observing and carefullyrecording the characteristics and their frequencyin the next and often in successiveMeiosis: the differences in chromosome behavior in meiosis and mitosis141

Mendelian ratiogenerations. Mendel concluded from hisresults that each character is controlled bytwo ‘factors’ in the somatic cells, but eachgamete can pass on only one of these factorsto the next generation. He also discoveredthat the factors were distributed atrandom in the gametes and that each pairof factors segregates independently of oneanother (see Mendelism; Mendel’s laws).Mendel’s ‘factors’ of inheritance are, infact, genes and he is recognized as being thefirst person to provide genetics with amathematical basis.Mendel published his results in a localscientific journal and his paper was widelycirculated, but unfortunately ignored.When Mendel became abbot of themonastery, he had to abandon his scientificstudies. It was not until 1900, some 16years after his death, that his work was rediscoveredby DE VRIES and others and itsaccuracy and ingenuity appreciated.Mendelian ratio The ratio of contrastingalleles present in the offspring of a crossinvolving genes that behave according toMENDEL’S LAWS.Mendelism The theory of inheritanceaccording to which characteristics are determinedby particulate ‘factors’, or genes,that are transmitted by the germ cells. It isthe basis of classical genetics, and isfounded on the work of MENDEL in the1860s. See Mendel’s laws.Mendel’s laws Two laws formulated byMENDEL to explain the pattern of inheritancehe observed in plant crosses. The firstlaw, the Law of Segregation, states that anycharacter exists as two factors, both ofwhich are found in the somatic cells butonly one of which is passed on to any onegamete. The second law, the Law of IndependentAssortment, states that the distributionof such factors to the gametes israndom; if a number of pairs of factors isconsidered, each pair segregates independently.Today Mendel’s ‘characters’ are termedgenes and their different forms (factors) arecalled alleles. It is known that a diploid cellcontains two alleles of a given gene, each ofwhich is located on one of a pair of homologouschromosomes. Only one homologof each pair is passed on to agamete. Thus the Law of Segregation stillholds true. Mendel envisaged his factors asdiscrete particles but it is now known thatthey are grouped together on chromosomes.The Law of Independent Assortmenttherefore applies only to unlinkedgenes. See linkage.meristem A distinct region of actively orpotentially actively dividing cells primarilyconcerned with growth. Numerous meristemsoccur in plants. In active meristemsseparation occurs between the cell that remainsmeristematic (initial) and the cell ultimatelybeing differentiated. Two basicmeristematic groups are the primary apicalmeristems at root and shoot apices, and thesecondary lateral meristems, which includevascular and cork cambia. See also intercalarymeristem; secondary growth.meristoderm (limiting layer) The outermostcellular layer of the thallus of certainPhaeophyta (brown algae). It consists ofsmall densely packed rectangular cells containingbrown pigmented plastids and coveredby a mucilaginous layer to preventdesiccation. The meristoderm maintains itsmeristematic activity and assists the outercortical layers in adding to the thickness ofthe thallus and replacing tissues wornaway by tidal action.mesarch Denoting a stele or part of astele in which the protoxylem is surroundedby metaxylem. Compare centrarch;endarch; exarch.mesocarpSee pericarp.mesophilic Designating microorganismswith an optimum temperature for growthbetween 25–45° C. Compare psychrophilic;thermophilic.mesophyll Specialized tissue located betweenthe epidermal layers of the leaf andcomposed mainly of photosynthetic cells142

metaxylem(CHLORENCHYMA). Veins, supported bysclerenchyma and collenchyma, are embeddedin the mesophyll. Palisade mesophyllconsists of cylindrical cells, at rightangles to the upper epidermis, with manychloroplasts and small intercellular spaces.It is the main photosynthesizing layer in theplant. Spongy mesophyll, adjacent to thelower epidermis, comprises interconnectingirregularly shaped cells with fewchloroplasts and large intercellular spacesthat communicate with the atmospherethrough stomata allowing gas exchangebetween the cells and the atmosphere. Thedistribution of mesophyll tissue varies indifferent leaves depending on the environmentin which the plant lives.mesophyte A plant that is adapted togrow under adequate conditions of watersupply and has no particular adaptationsto withstand environmental extremes. Indrought conditions wilting is soon apparentas the plants have no specialmechanisms to conserve water. Most angiospermsare mesophytes. Compare hydrophyte;xerophyte.Mesozoic The middle era in the most recent(Phanerozoic) eon of the geologicaltime scale, dating from about 230–66 millionyears ago. It is divided into three mainperiods: the Triassic, Jurassic, and Cretaceous.During the Triassic period seedfernspredominated, but as the climatewarmed the cycads came to dominate, andthe Mesozoic is sometimes called ‘The Ageof Cycads’. In the Jurassic period, tropicaland temperate regions were dominated byforests of palmlike cycads and conifers,and the dawn redwoods made their firstappearance. Ginkgoes were also widespread.The first angiosperms arose in theCretaceous period, and rapidly diversified,becoming the dominant vegetation by theend of the Mesozoic as the climate cooledand dried. See also geological time scalemessenger RNA (mRNA) The form ofRNA that transfers the information necessaryfor protein synthesis from the DNA inthe nucleus to the ribosomes in the cytoplasm.One strand of the double helixof DNA acts as a template along whichcomplementary RNA nucleotides becomealigned and joined together by the enzymemRNA polymerase. These form a polynucleotideidentical to the other DNA strand,except that the thymine bases are replacedby uracil. This polynucleotide is called heterogeneousnuclear RNA (hnRNA) and itcontains both coding and noncoding sequences(see exon; intron). In the nucleolusPOST-TRANSCRIPTIONAL PROCESSING occurs:adenine polynucleotide (ply A) is added toone end and a guanine derivative to theother, and (in eukaryotic cells) the intronsare then removed to produce mRNA(sometimes called mature mRNA). Thewhole process is termed transcription. Thenew mRNA molecule thus has a copy ofthe genetic code, which directs the formationof proteins in the ribosomes. Comparetransfer RNA.metabolism The chemical reactionsthat take place in cells. The molecules takingpart in these reactions are termedmetabolites. It is metabolic reactions, particularlythose producing energy, that keepcells alive. Metabolic reactions characteristicallyoccur in small steps, comprising ametabolic pathway. Metabolic reactionsinvolve the breaking down of moleculesto provide energy (CATABOLISM) and thebuilding up of more complex moleculesand structures from simpler molecules (AN-ABOLISM).metaboliteSee metabolism.metaphase The stage in mitosis andmeiosis when the chromosomes becomealigned along the equator of the nuclearspindle. Metaphase follows prophase andprecedes anaphase.metaphloem The primary PHLOEM formedfrom the PROCAMBIUM after the PRO-TOPHLOEM. It is found behind the zone ofelongation below the meristem, and ismore durable than the protophloem. Inplants showing no secondary thickeningthe metaphloem is responsible for the143

methioninetransport of most of the organic materialsin the plant, but this function is taken overby the secondary phloem in regions wheresecondary tissues have differentiated.metaxylem The primary XYLEM elementsthat are differentiated from thePROCAMBIUM after the PROTOXYLEM. Theyare found some distance behind the apicalmeristem beyond the zone of elongation.The secondary cell walls show reticulateand scalariform thickening and are thus inextensible.methionine A nonpolar sulfur-containingAMINO ACID synthesized from aspartate.Methionine can be broken down to succinylCoA, which is further oxidized in theKrebs cycle. The ATP-activated derivativeof methionine, S-adenosyl methionine, donatesits methyl group to many differentmolecules in various metabolic pathways.methylene blue A blue dye used in lightmicroscopy to stain bacterial protoplasm.It is also used to stain nuclei blue. See staining.microbiology The study of microscopicorganisms (e.g. bacteria and viruses), includingtheir interactions with other organismsand with the environment.microbody A common organelle ofplant and animal cells, bounded by a singlemembrane, spherical, and usually about0.0–1.0 m m in diameter. Microbodiesoriginate from the ENDOPLASMIC RETICULUMand contain oxidative enzymes such ascatalase, which decomposes hydrogen peroxide,a toxic waste product of the activitiesof other enzymes in the microbody.GLYOXYSOMES contain enzymes of theglyoxylate cycle, transaminases, and enzymesassociated with b-oxidation of fattyacids. They play a major role in conversionof lipids to sucrose in fatty or oily seedlingtissues, e.g. the endosperm of castor-oilseeds. PEROXISOMES occur in certain animaltissues, and are numerous in photosyntheticcells of plant leaves, where they areconcerned with glycolate metabolism inphotorespiration and contain high levels ofglycolate oxidase and other associated enzymes.Glycolate comes from chloroplastsand products such as glycine are passed tomitochondria. Hence these three organellesoften appear close together. See photorespiration.microevolution EVOLUTION on a scalewithin a species, which results from the actionof natural selection on the genetic variationbetween individuals of a givenpopulation.microfibril A ribbonlike structure atleast 60 nm long and about 45 nm in diameter,made of parallel chains of cellulosemolecules linked by hydrogen bonds. Microfibrilsare found in the CELL WALLS ofplants and some algae. In secondary cellwalls they are packed more closely, leavingless room for water between them.microfilament A minute filament, about0.4–0.7 nm wide, found in eukaryotic cellsand having roles in cell motion and shape.Microfilaments are made of two helicallytwisted strands of globular subunits of theprotein ACTIN, almost identical to the actinof muscle. They can undergo rapid extensionor shortening by subunit assembly ordisassembly, or form complex threedimensionalnetworks. Their occasionalassociation with myosinlike protein (as inmuscle) suggests they may also be contractile.They often occur in sheets or bundlesjust below the plasma membrane and at theinterface of moving and stationary cytoplasm.They are involved in CYTOKINESIS,CYCLOSIS, cell movements, e.g. ameboidmovement, and movement of subcellularcomponents, e.g. pinocytotic vesicles.micrograph A photograph taken withthe aid of a microscope. Photomicrographsand electron micrographs are producedusing optical microscopes and electron microscopesrespectively.micrometer 1 In microscopy, a devicefor measuring the size of an object underthe microscope. An eyepiece micrometer144

microscope(graticule) of glass or transparent film,with a scale etched or printed on it, isplaced in the eyepiece so that both the objectto be measured and the scale are infocus. The scale of the graticule changes atdifferent magnifications and it must thereforebe calibrated against a stage micrometer,which is a glass slide with a scaleetched into it, and placed on the microscopestage.micrometer 2 (micron) Symbol: m m Aunit of length equal to 10 –6 meter (one millionthof a meter). It is often used in measurementsof cell diameter, sizes ofbacteria, etc. Formerly, it was called themicron.micronSee micrometer.micronutrient (trace element) A nutrientrequired in trace amounts by an organism.For example, a plant can obtainsufficient of the essential trace elementmanganese from a solution containing 0.5parts per million of manganese. Many actas enzyme cofactors or are components ofpigments. They include boron, cobalt, copper,manganese, molybdenum, and zinc.See deficiency disease.microphyll A foliage leaf that has a singleunbranched vein running from base toapex and no leaf gap associated with theleaf trace: the stele remains entire when aleaf trace branches off to a leaf. Microphyllsare the typical leaves of clubmossesand horsetails. Compare megaphyll.micropropagation The production oflarge numbers of plants under laboratoryconditions. It usually involves the use ofTISSUE CULTURE to produce large numbersof genetically identical plants (clones), butcan involve seeds, e.g. of orchids. It is usedto produce disease-free plants, and also tomultiply genetically engineered plants, andto increase rare plant populations for conservationpurposes.micropyle A pore leading to the nucellusformed by incomplete integumentgrowth around the apex of the ovule.Pollen tubes usually pass through it priorto fertilization. In most seeds the micropyleforms a small hole in the testa throughwhich water is absorbed, but in some seedsit is closed.microscope An instrument designed tomagnify objects and thus increase the resolutionwith which one can view them. Resolutionis the ability to distinguish betweentwo separate adjacent objects. Radiation(light or electrons) is focused eitherthrough the specimen by a condenser lensor onto it. The resulting image is magnifiedby further lenses. Since radiation must passthrough the specimen in transmission microscopy,it is usual to cut larger specimensinto thin slices of material (sections) with amicrotome. Biological material has littlecontrast and is therefore often stained. Ifvery thin sections are required the materialis preserved and embedded in a supportingmedium.The light microscope (optical microscope)uses light as a source of radiation.With a compound microscope the image ismagnified by two lenses, an objective lensnear the specimen, and an eyepiece, wherethe image is viewed, at the opposite end ofa tube. Its maximum magnification is limitedby the wavelength of light: it can distinguishbetween two points only 0.3 m mapart. Its main use is the observation ofthin sections of tissues and organs, and thestudy of fine detail of microorganisms. Forviewing relatively large specimens and fordissection, the lower resolution stereoscopicbinocular microscope is used: it usesincident light and has two eyepieces, givinga three-dimensional image. Various modificationsof the light source and backgroundillumination allow different aspectsof cell structure to be observed, e.g. brightfieldillumination, DARK-GROUND ILLUMI-NATION, PHASE CONTRAST MICROSCOPY,interference microscopy, and CONFOCALMICROSCOPY. Much greater resolution becamepossible with the introduction of theelectron microscope, which uses electronsas a source of radiation, because electronshave much shorter wavelengths than light.145

microsomesHowever, only dead material can be observedbecause the specimen must be in avacuum and electrons eventually heat anddestroy the material. The electron microscopeuses electromagnetic lenses to focusa parallel beam of electrons onto an objectand produce an image of the resulting electronscattering projected onto a photographicplate or fluorescent screen. Theprocess takes place in a vacuum to minimizeunwanted electron scattering. Sincethe wavelength of an electron beam is0.005 nm, an electron microscope hasabout 300 times greater resolution than alight microscope. In theory, this microscopecan distinguish between objects0.0025 nm apart, but in practice, resolutionsof less than about 1 nm are rarely attained.Electron microscopes are of twomain types, the transmission electron microscope(TEM) and the scanning electronmicroscope (SEM). The former producesan image by passing electrons through thespecimen. With the scanning microscopeelectrons scan the surfaces of specimensrather as a screen is scanned in a TV tube,allowing surfaces of objects to be seen withgreater depth of field and giving a threedimensionalappearance to the image. Thesurface of entire objects can be scanned,and material to be viewed is often given anelectron-reflecting coating. Scanning microscopescannot operate at such highmagnifications as transmission microscopes,but can still resolve to at least 2 nm.Of growing importance is the scanningprobe microscope (atomic force microscope),in which a highly sensitive probemounted on a cantilever is moved to andfro over a surface, and responds to theminute intermolecular distance betweenthe probe and the surface, which causes theprobe to be deflected from the surface. Thedeflection of the probe is measured with alaser beam, providing a topographical mapof the surface without penetrating it. Theprobe may be dragged over the surface ofthe object, tap the object at intervals, ortravel just above the surface. The scanningprobe microscope can achieve resolutionsof less than 1 nm in x-, y-, and z-directions,and can produce images of individual moleculessuch as myosin and DNA. In nontouchingmode it provides a tool for thestudy of real-time changes in the surfacewithout interfering with processes causingthe changes, e.g. the change in structure ofstarch as it is digested by an enzyme. Modificationsof the technique can give furtherinformation; it is used, for example, tomeasure the volume of chromosomes.microsomes Fragments of endoplasmicreticulum and Golgi apparatus in the formof vesicles formed during homogenizationof cells and isolated by high-speed centrifugation.Microsomes from rough endoplasmicreticulum are coated with ribosomesand can carry out protein synthesis in thetest tube.microsporangium (pl. microsporangia)In heterosporous plants, the sporangiumthat produces the MICROSPORES, locatedon the microsporophyll. The microsporangiumwall splits to disperse the maturemicrospores. Microsporangia are found insome pteridophytes (e.g. Selaginella, waterferns) and are represented by the pollensacs in gymnosperms and angiosperms.Compare megasporangium.microspore The smaller of the twotypes of spores produced in large numbersby seed plants and heterosporous pteridophytes.In pteridophytes, microsporesdevelop into the male gametophyte generation,but in gymnosperms and angiosperms,the microspores (POLLEN grains)develop into a very reduced gametophyterepresented by the pollen tube and the vegetativenucleus (pollen-tube nucleus) andthe generative nuclei. The latter are themale gametic nuclei. Compare megaspore.microsporophyll A leaf or modifiedleaf on which the microsporangium isborne. Simple microsporophylls includethe scales found in the male cones of gymnospermsand the fertile photosyntheticleaves of clubmosses, usually grouped in astrobilus. In gymnosperms the male scale isthe microsporophyll, while in angiosperms146

mitochondrionthe stamen is a highly modified microsporophyll.Compare megasporophyll.microtome An instrument for cuttingthin sections (slices a few micrometersthick) of biological material for microscopicexamination. The specimen is usuallyembedded in wax for support and cutby a steel knife. Alternatively it is frozenand a freezing microtome, which keeps thespecimen frozen while cutting, is used(cryo-microtomy). For electron microscopy,extremely thin (20–100 nm) sectionscan be cut by an ultramicrotome. Here thespecimen is embedded in resin or plastic forsupport and mounted in an arm that advancesslowly, moving up and down, towarda glass or diamond knife. As sectionsare cut they float off on to the surface ofwater contained in a trough behind theknife.microtubule A thin cylindrical unbranchedtube of variable length found ineukaryotic cells, either singly or in groups.Its walls are made of the globular proteintubulin. Microtubules have a skeletal role,helping cells to maintain their shape. Theyoccur freely in the cell, and also form partof the structure of the SPINDLE during celldivision, bringing about chromosomemovement. Microtubules also help to orientatematerials and structures in the cell,e.g. cellulose fibrils during the formation ofplant cell walls, and may be involved in thetransport of materials with and betweencells, e.g. phloem transport. They are alsopart of the centrioles found in many fungaland algal cells, and the basal bodies andundulipodia of many motile unicells.middle lamella In mature plant tissues,the material cementing the walls of adjacentcells. It is derived from the firstformedlayer of the primary cell wall,which is laid down by the PHRAGMOPLAST(cell plate), and is composed mainly of calciumand magnesium pectates.midrib 1. The thick vein running downthe middle of the leaf from the petiole tothe leaf tip of a plant.2. Any thickened linear structure runningdown the center of an algal thallus, such asa seaweed, e.g. Fucus spp.mildew A fungal disease of plants inwhich the hyphae appear on the surface ofthe plant.Mississippian In North America, theLower Carboniferous period, dating from345 to 310 million years ago. See Carboniferous;Pennsylvanian.mitochondrial DNA (mt-DNA) TheDNA found in mitochondria. It is a circularmolecule not associated with histonesor other proteins and independent of nuclearDNA. It codes for specific RNA componentsof ribosomes, and also for certainrespiratory enzymes that occur in the mitochondriaand are synthesized on mitochondrialribosomes. Plant mt-DNA evolvesvery slowly (unlike animal mt-DNA), andcan break apart and recombine. It is usuallytransmitted from females to their offspringin the cytoplasm of the ova, butmitochondria do not usually pass into malegametes.mitochondrion (pl. mitochondria) Anorganelle of all plant and animal cellschiefly associated with aerobic respiration.It is surrounded by two membranes separatedby an intermembrane space; the innermembrane forms fingerlike processescalled cristae, which project into the gellikematrix. Mitochondria are typicallysausage-shaped, but may assume a varietyof forms, including irregular branchingshapes. The diameter is always about0.5–1.0 m m, and the length averages 2m m. Mitochondria contain the enzymesand cofactors of AEROBIC RESPIRATION andtherefore are most numerous in active cells(up to several thousand per cell). The reactionsof the KREBS CYCLE take place in thematrix and those of electron transport coupledto oxidative phosphorylation (i.e. therespiratory chain) on the inner membrane.Within the membrane the components ofthe respiratory chain are highly organized.The matrix is also involved in amino acid147

mitosismetabolism via Krebs cycle acids andtransaminase enzymes, and in fatty acidoxidation. Like chloroplasts, mitochondriacontain their own ribosomes and DNA,which is circular in form, like that of bacteria,and reproduce by binary fission. It isbelieved that mitochondria and chloroplastsmaybe the descendants of once independentorganisms that early in evolutioninvaded eukaryotic cells, leading to an extremeform of symbiosis. See endosymbionttheory. See also electron-transportchain.mitosis (karyokinesis) The ordered processby which the cell nucleus and cytoplasmdivide in two during the division ofbody (i.e. nongermline) cells. The chromosomesreplicate prior to mitosis to formtwo sister chromatids, which are then separatedduring mitosis in such a way thateach daughter cell inherits a genetic complementidentical to that of the parent cell.Although mitosis is a continuous process itis divided into four phases; prophase,metaphase, anaphase, and telophase. Telophasemay be followed by cytokinesis.Compare meiosis; amitosis; endomitosis.See Fungi Anamor-mitosporic fungiphici.molecular systematics A branch of biologythat compares functionally equivalentmacromolecules from differentorganisms as a basis for classification. Sequencesof amino acids in proteins (e.g. enzymes)or of nucleotides in nucleic acids(e.g. ribosomal RNA) are determined usingautomated techniques, and compared statisticallyusing sophisticated computerprograms. Essentially, how closely two organismsare related in evolutionary terms isreflected in the degree of similarity of theirmacromolecules.molybdenum A MICRONUTRIENT (traceelement) needed for plant growth. It formspart of the enzymes nitrate reductase,which is involved in nitrate reduction, andnitrogenase, the key enzyme in nitrogenfixation. Deficiency of molybdenum maylead to CHLOROSIS between the leaf veins.Monera An alternative name for thekingdom BACTERIA (Prokaryotae).monochasial cyme (monochasium)inflorescence.SeeMonocotyledonae A class of the floweringplants (phylum Anthophyta) derivingtheir name from single cotyledon in the embryo(though some dicotyledons have this,too). They are usually herbaceous plantsand most do not show secondary growth.Examples of monocotyledons are thegrasses (Poaceae) and lilies (Liliaceae).monoculture The growing of a singlespecies or variety of crop over a large area.The economic reasons for this are the abilityto grow crops of uniform height for harvesting,and economies of scale in theharvesting process. The major disadvantageis increased susceptibility to pests andpathogens, especially if a single variety isgrown.monoecious Denoting plants in whichthe male and female reproductive organsare borne on separate structures on thesame individual plant. Monoecious floweringplants bear separate unisexual maleand female flowers, e.g. maize and manytemperate trees such as birch, hazel, andoak. Many monoecious plants are windpollinated.Compare dioecious; hermaphrodite.monohybrid An indiviudal heterozygousat one locus (i.e. for a single gene), obtainedby crossing homozygous parentswith different alleles at a given locus; forexample, Mendel’s cross between tall (TT)and dwarf (tt) garden peas to give a tallmonohybrid (Tt). When a monohybrid isselfed, dominant and recessive phenotypesappear in the offspring in the ratio of 3:1(the monohybrid ratio). Compare dihybrid.monoploidSee haploid.148

mutagenmonopodial Describing the system ofbranching in plants in which the main axisof the stem (the monopodium) continues togrow indefinitely by the terminal bud.Monopodial growth is also called indefiniteor racemose branching and is typicalof the formation of a racemose inflorescence.Compare sympodial.monosaccharide A SUGAR that cannotbe hydrolyzed to simpler carbohydrates ofsmaller carbon content. Glucose and fructoseare examples. Monosaccharides maybe classified according to the number ofcarbon atoms they contain: 5-carbon pentoses(e.g. ribose); 6-carbon hexoses (e.g.glucose, fructose). The general formula ofa monosaccharide is (CH 2 O) n .monosomySee aneuploidy.monotrichous Describing BACTERIA thatpossess one flagellum, e.g. Vibrio.Moraceae A family of trees, shrubs,lianas, and herbs, found mainly in tropicaland subtropical regions. There are about12 000 species. The Moraceae containsmany important food plants, such as ediblefigs (Ficus carica), breadfruit (Artocarpusartilis), jackfruit (A. heterophyllus), andmulberry (Morus).mosquito fernsSee Azollaceae.mosses In older classifications, a class ofbryophytes containing leafy plants withmulticellular rhizoids. In the FIVE KING-DOMS CLASSIFICATION, mosses comprise thephylum BRYOPHYTA. They differ from liverwortsin their greater differentiation of thegametophyte and also their complex mechanismsof capsule dehiscence, with no formationof elaters in the capsule. Ordersinclude the Bryales (e.g. Funaria, Polytrichum,Mnium) and the Sphagnales (e.g.Sphagnum).mucilage Any of various gumlike carbohydratesthat swell when wet, becomingslimy and jellylike. See gum.See polysaccha-mucopolysaccharideride.Mucorales (pin molds) An order of theZYGOMYCOTA, containing fungi that form adense mycelium bearing sporangia or conidia.The tips of the spore-bearing hyphaeoften become pigmented, hence the namepin molds (e.g. Mucor mucedo). Most aresaprobes in soil, dung, and other organicdebris. Many cause spoilage of storedfood. A few are parasites on other fungi,plants, or animals.morelSee Pezizales.multicellularConsisting of many cells.morphogenesis The development ofform and structure.See contin-multifactorial inheritanceuous variation.morphology The study of the form oforganisms. The term may be used synonymouslywith ‘anatomy’ although generallythe study of external form is termed ‘morphology’while the study of internal structuresis termed ‘anatomy’.mosaic 1. A hybrid organism whosecells differ genetically, although they haveall arisen from a single zygote.2. A pattern of light and dark green patcheson the leaves of a plant, usually due to viralinfection.3. See leaf mosaic.multiple allelism The existence of a seriesof alleles (three or more) for one gene.Only two alleles of the series can be presentin a diploid cell. Dominance relationshipswithin an allelic series are often complicated.Multiple alleles are common in theincompatibility systems of plants, such asSinapis, poppy (Papaver), and clover (Trifolium).The numbers of such alleles areoften considerable, e.g. Brussels sprouts(Brassica oleracea var. bullata) has up to40 incompatibility alleles, or S alleles.multiple fruit (composite fruit)A type149

mutationof pseudocarpic (‘false’) fruit that incorporatesparts of the inflorescence. Examplesare figs (Ficus) and pineapples (Ananus comosus).mutagen Any physical or chemicalagent that induces MUTATION or increasesthe rate of spontaneous mutation. Chemicalmutagens include compounds that reactwith nucleotides, e.g. nitrous acid, proflavin;base analogues, which are incorporatedin place of normal bases during DNAreplication; and acridines, which causebase-pair deletions or additions. Physicalmutagens include short-wave radiationsuch as ultraviolet light, x-rays, and cosmicrays; and ionizing radiation, such as a andb particles.mutation Mutations are the ultimatesource of all genetic variation. They are inheritedonly if they occur in the cells thatgive rise to the gametes; somatic mutationsmay give rise to chimaeras and cancers.GENE MUTATIONS alter only a single gene,resulting in new allelic forms of the geneand hence new variations upon which naturalselection can act. Most mutations aredeleterious but are often retained in thepopulation because they also tend to be recessiveand can thus be carried in the genotypewithout affecting the viability of theorganism. The natural rate of mutation islow, and varies with different gene loci.The mutation frequency can be increasedby mutagens. See also chromosome mutation;mutagen; polyploid.mutualism The close relationship betweentwo or more species of organisms inwhich all benefit from the association.There are two types of mutualism: obligatorymutualism, in which one cannot survivewithout the other, for example thealgal/fungal partnership found in lichens;and facultative or nonobligatory mutualism,in which one or both species can surviveindependently, for example, in theassociation between the bacterium Rhizobiumand members of the family Fabaceae,which leads to the formation of root nodulesin which nitrogen fixation occurs, thebacterium can live independently of theplant; in lichens often the alga can live independentlyof the fungus. In many mutualisticassociations, the relationship canchange from a parasitism to a mutualismor vice versa. Compare symbiosis; commensalism;amensalism.mycelium (pl. mycelia) A filamentousmass comprising the vegetative body of afungus, each filament being called a hypha.The mycelium often forms a loose mesh asin Mucor, or hyphae may anastomose toform a netlike structure, as in Ascomycotaand Basidiomycota. See hypha.mycoplasmas (PPLO; pleuropneumonialikeorganisms) A group of extremelysmall bacteria that naturally lack a rigidcell wall. They often measure less than 200nm in diameter and their cells are delicateand plastic. Mycoplasmas are implicated incertain plant diseases, such as witchesbroom of alfalfa, maize stunt, and yellowsdiseases. They can kill tissue cultures andcause serious diseases in humans and animals,e.g. pleuropneumonia.mycorrhiza (pl. mycorrhizae) The associationbetween the hyphae of a fungus andthe roots of a higher plant. Two main typesof mycorrhiza exist, ectomycorrhizae (ectotrophicmycorrhizae), in which the fungusforms a mantle around the smallerroots, as in many temperate and borealtrees, and endomycorrhizae (endotrophicmycorrhizae), in which the fungus growsaround and within the cortex cells of theroots, as in many herbaceous plants. Orchidsand heathers have further specializedtypes of mycorrhizae, in which there isoften a mantle and cell penetration. In ectomycorrhizaethe fungus, which is usuallya member of the Agaricales, benefits by obtainingcarbohydrates and possibly B-group vitamins from the roots. The treesbenefit in that mycorrhizal roots absorbnutrients more efficiently than uninfectedroots, and it is common forestry practice toinsure the appropriate fungus is appliedwhen planting seedling trees. In endomycorrhizaethe fungus is generally a species of150

MyxomycotaRhizoctonia and again both partners benefitnutritionally from the relationship. Ina special form of endomycorrhiza, thevesiculo-arbuscular mycorrhiza (arbuscularmycorrhiza), the fungus lives betweenthe cells of the cortex and projections ofits hyphae actually penetrate the corticalcells. Sometimes the projections are finelybranched hyphae called arbuscules.myosin A contractile protein found incells, which is involved in the movement ofcytoplasm, for example in the cytoplasmicstreaming of plant cells, the ameboidmovement of some protoctists and fungusgametes, and the movement of slime moldplasmodia. Myosin is thought to interactwith the protein ACTIN to bring aboutmovement.Myrtaceae A family of trees, shrubs andcreepers found in most parts of the tropicsand warm temperate regions, especiallyAustralia. The family includes many importantforest trees such as Eucalyptusspecies, especially in Australia, sources oftimber, tannin and oils. Some species produceedible fruits, e.g. guava (Psidium guajava),or spices, e.g. allspice (Pimentadioica) and cloves (Eugenia caryophyllus).151

NNAD (nicotinamide adenine dinucleotide)A pyridine-based nucleotide that acts as acoenzyme to oxido-reductase enzymes inelectron-transfer reactions, such as the oxidationof citrate to a-ketoglutarate in theKREBS CYCLE. The reduced form is writtenas NADH, and may act as a reducing agentor be reoxidized in the respiratory chaincoupled to ATP formation. Its main importanceis as an electron acceptor in oxidationreactions.NADP (nicotinamide adenine dinucleotidephosphate) A pyridine-based nucleotidethat acts as a COENZYME to oxidoreductaseenzymes. Unlike NAD, it usuallyacts as an electron donor in enzymecatalyzedreduction reactions.Naegeli, Karl Wilhelm von (1817–91)Naegeli was educated at Zurich, Geneva,and Berlin, where he studied philosophybefore resuming his botanical work. Heproduced an accurate account of cell division(1842) while studying pollen formationand even observed chromosomes individing nuclei, calling them ‘transitorycytoblasts’. His observations disprovedSCHLEIDEN’S theory that new cells wereformed by budding off the nuclei of existingcells. Working on apical growth inplants in 1845, he concluded that therewere distinct regions of cell division (meristems).Naegeli also laid the foundations ofcell ultrastructure when he studied starchgrains in cells and formulated his micellartheory. Sadly, his own entrenched views onheredity and evolution, which arose fromhis studies on the hawkweed (Hieracium),led him to reject MENDEL’s very importantwork on pea plants.thousand-millionth, or 10 –9 . For example,1 nanometer (nm) = 10 –9 meter. See SI units.nastic movements (nasties) Movementsof plant parts in which direction of movementis independent of the direction of thestimulus that induces them. With photonastythe stimulus is light. For instance,at constant temperature Crocus and tulip(Tulipa) flowers open in the light and closein the dark because of slight growth movements.Opening is caused by epinasty – i.e.greater growth of the upper surface of aplant organ – and closed by hyponasty –i.e. greater growth of the lower surface of aplant organ. Similarly, at constant light intensitycrocus and tulip flowers show thermonasty;i.e. they open in warm air andclose in cool air. Such day–night rhythmsare examples of nyctinasty and may alsooccur in leaves, as in Oxalis. Nongrowthnastic movements also occur and are morerapid. The ‘sensitive plant’ (Mimosa pudica)rapidly closes its leaflets upwards andthe petioles droop in response to touch,shock (seismonasty), or injury. This plant,and many other legumes, shows nongrowthnyctinastic movements. Movementis the result of osmotic changes in specialswollen groups of cells (pulvini) at thebases of the moving structures. When thestimulus is contact, the movement is haptonasticas in the closure movements of insectivorousplants. The closure of the twohalves of the Venus fly-trap leaf (Dionaeamuscipula) is a nongrowth haptonasticmovement caused by a loss of turgor in thecells along the midrib following stimulationof the sensitive hairs on the leaf. Seealso taxis; tropism.nano- Symbol: nA prefix denoting onenastiesSee nastic movements.152

neutral theorynatural selection The process, whichDARWIN called the ‘struggle for survival’, bywhich organisms less adapted to their environmenttend to perish, and better-adaptedorganisms tend to survive. There has beenmuch debate about the level at which naturalselection acts: traditionally it is regardedas the action of the environmenton individual organisms that determineswhich genotypes survive and reproduce,but there are strong arguments for naturalselection acting at the level of the gene (the‘selfish gene’ hypothesis) and some authorsargue that it happens in the long term athigher levels such as population or species.In the end the less successful types will dieout. According to DARWINISM, natural selectionacting on a varied population resultsin evolution.necrosis The death of a cell or group ofcells in a living plant, especially where thedead area is discolored. Necrosis is oftencaused by fungal infections.necrotrophic Describing a parasitic organismthat obtains its nutrients from deadcells and tissues of its host.nectar A sugar-containing fluid secretedby the nectaries in plants. It is mainly producedin flowers and attracts insects orother animals, who feed on it and may unintentionallytransfer pollen from oneflower to another, thus facilitating crosspollination.Floral nectar contains variableproportions of different monosaccharideand disaccharide sugars and, frequently,other substances such as amino acids andvolatile compounds as well. Some plants,particularly trees, produce nectar on stemsand leaves. This may be collected by insectssuch as ants that can protect the plant frominsect herbivores.nectary A patch of glandular epidermalcells usually on the receptacle or the petals,but sometimes in other parts of the flowersor in specialized organs. They produce asugary liquid (nectar) that attracts insects.negative feedbackSee feedback loop.negative staining A method of preparationof material for electron microscopyused for studying three-dimensional andsurface features, notably of viruses, macromolecules(e.g. enzyme complexes), andthe cristae of mitochondria. A stain is usedthat is not taken up by certain componentsof the specimen. Usually the stain coversthe background and penetrates surface featuresof the specimen, but leaves the specimenitself unstained. For example, nigrosinor Indian ink is used to make bacteria visible.In electron microscopy a similar techniquemixes the specimen material withelectron-dense negative stains, such asphosphotungstic acid (PTA) or potassiummolybdate, which is not taken up by proteinaceousmaterial in the specimen. Theelectron-transparent proteinaceous materialshows up as dark areas against a lightbackground on the resulting electron micrograph.See staining.neo-Darwinism DARWIN’s theory ofevolution through NATURAL SELECTION,modified and expanded by genetic studiesarising from the work of MENDEL and hissuccessors. This fusion of Darwin’s theoryof natural selection with Mendel’s geneticswas called the neo-Darwinian synthesis inthe 1930s. The inclusion of genetics in evolutionarystudies answered many questionsthat Darwin’s theory raised but could notadequately explain because of lack ofknowledge at the time it was formulated.Notably, genetics has revealed the sourceof variation on which natural selection operates,namely mutations of genes andchromosomes, and provided mathematicalmodels of how alleles fluctuate in naturalpopulations, thereby quantifying the processof evolution. More recent discoveriesin molecular biology have added to our understandingof the causes of variation andthe nature of evolution at the molecularlevel.neriticSee sublittoral.neutral theory The theory that mostevolutionary changes are caused by randomGENETIC DRIFT (random changes in thefrequencies of alleles in a population)153

niacinrather than by NATURAL SELECTION. GeneticMUTATIONS arise at random. Many are neutral– they do not affect the fitness or survivalof the carrier. They therefore survivein the genome and changes in their frequenciesare due to the breeding systemand chance rather than to natural selection.See gene frequency; variation.niacinSee nicotinic acid.nitrification The oxidation of ammoniato nitrite, and/or nitrite to nitrate, carriedout by certain nitrifying bacteria in the soil.The chemosynthetic bacteria Nitrosomonasand Nitrobacter carry out the first andsecond stages respectively of this conversion.The process is important in the NI-TROGEN CYCLE since many plants assimilatenitrate as their source of nitrogen. Comparedenitrification.nicheSee ecological niche.nitrifying bacteria See nitrification.nicotinic acid (niacin) A carboxylicacid that is one of the water-soluble B-group of VITAMINS. It also forms part of thealkaloid nicotine, hence its name. It issythesized from various precursors, but especiallyfrom tryptophan. As nicotinamide,it is a constituent of two coenzymes, NADand NADP, which operate as hydrogenand electron-transfer agents and play avital role in metabolism. In humans, deficiencyof nicotinic acid causes pellagra.Diets based on maize, which is particularlypoor in nicotinic acid, tend to lead to an increasein pellagra in the population.Nidulariales The bird’s nest fungi – anorder of the BASIDIOMYCOTA in which thebasidioma is nestlike, usually cup-shaped(e.g. splash cups, Cyathus striatus) or funnel-shaped(e.g. white-egg bird’s nest, Crucibulumlaeve), containing several separatespore cases (the ‘eggs’) that are exposedwhen the covering membrane dies back.The spores are dispersed by rain splash.Bird’s nest fungi may be found on theground, or on wood or dung.ninhydrin A reagent used to test for thepresence of proteins and amino acids. Acolorless aqueous solution turns blue in thepresence of a-amino acids in solution.When dissolved in an organic solvent itis used as a developer to color amino acidson chromatograms. If a chromatogramtreated with ninhydrin is heated stronglythe amino acids appear as purple spots thatcan be identified by measuring the Rfvalue. Ninhydrin is carcinogenic. See alsopaper chromatography.nitrogen An essential element found inall amino acids and therefore in all proteins,and in various other importantorganic compounds, e.g. nucleic acids.Gaseous nitrogen forms about 80% of theatmosphere but is unavailable in this formexcept to a few nitrogen-fixing bacteria.Nitrogen is incorporated into plants as thenitrate ion, NO – 3 or, especially in acidsoils, the ammonium anion NH + 4 , absorbedin solution from the soil by roots.Plants suffering from nitrogen deficiencytend to develop CHLOROSIS and become etiolated,the effects being seen first in theoldest parts. See also nitrogen cycle.nitrogenaseSee dinitrogenase.nitrogen cycle The circulation of nitrogenbetween organisms and the environment.Atmospheric gaseous nitrogen canbe used directly only by certain nitrogenfixingbacteria (e.g. Clostridium, Nostoc,Rhizobium). They convert nitrogen to ammonia,nitrites, and nitrates, which are releasedinto the soil by excretion and decay.Some are free-living, while others formsymbiotic associations with plants (see nitrogenfixation). Another method by whichatmospheric nitrogen is fixed is by lightning,which causes nitrogen and oxygen tocombine. The oxides so produced dissolvein rain to form nitrous and nitric acids; inthe soil these acids combine with mineralsalts to form nitrites and nitrates. Whenplants and animals die, the organic nitrogenthey contain is converted back into nitratein the process termed nitrification.Apart from uptake by plants, nitrate may154

nucleic acidsalso be lost from the soil by denitrificationand by leaching. The use of nitrogen fertilizersin agriculture and the emission of nitrousoxides in car exhaust fumes haveinfluenced the nitrogen cycle and contributedto urban air pollution and acidprecipitation. See acid rain.nitrogen fixation The formation of nitrogenouscompounds from atmosphericnitrogen. In nature this may be achieved byelectric discharge in the atmosphere or bythe activities of certain microorganisms.For example, symbiotic bacteria of thegenus Rhizobium are associated with leguminousplants (FABACEAE), causing the rootcortex to form root nodules that house thebacteria. These bacteria contain the nitrogenaseenzyme that catalyzes the fixationof molecular nitrogen to ammonium ions,which the plant can assimilate. In returnthe legume supplies the bacteria with carbohydrate.node The point of leaf insertion on astem. At the apex of the stem the nodes arevery close together but become separatedin older regions of the stem by intercalarygrowth, which forms the internodes. Incertain monocotyledons such as thoseforming bulbs, the nodes are very closelyspaced on a condensed stem.noncompetitive inhibition A form ofenzyme inhibition in which the inhibitorbinds to the enzyme at a site other than theactive site, altering the conformation of theenzyme and affecting its ability to bindwith the substrate.noncyclic photophosphorylationphotosynthesis.Seenondisjunction The failure of homologouschromosomes to move to separatepoles during anaphase I of meiosis, bothhomologs going to a single pole. This resultsin two of the four gametes formed attelophase missing a chromosome (i.e. beingn – 1). If these fuse with normal haploid (n)gametes then the resulting zygote is monosomic(i.e. 2n – 1). The other two gametesformed at telophase have an extra chromosome(i.e. are n + 1) and give a trisomic zygote(i.e. 2n + 1) on fusion with a normalgamete. If two gametes deficient for thesame chromosome fuse then nullisomy (2n– 2) will result, which is almost alwayslethal, and if two gametes with the sameextra chromosomes fuse, tetrasomy (2n +2) results. All these abnormal chromosomeconditions are collectively referred to asaneuploidy.nonsense triplet A triplet of bases(CODON) in DNA or RNA that does notcode for an amino acid. Some nonsensecodons (stop or termination codons) specifythe termination of polypeptide synthesisduring TRANSLATION. See genetic code.nucellus (pl. nucelli) The parenchymatoustissue core of an ovule, enclosing themegaspore or egg cell (or in angiospermsthe EMBRYO SAC). The pollen tube gainsentry to the nucellus through a gap in thesurrounding integuments called the MI-CROPYLE. In some angiosperm species thenucellus persists as the perisperm, providingnourishment for the developing embryo.nuclear membraneSee nucleus.See endonuclease; exonucle-nucleasease.(nuclear envelope)nucleic acid hybridization (DNA hybridization)The pairing of a singlestrandedDNA or RNA molecule withanother such strand, forming a DNA–DNA or RNA–DNA hybrid. In order toachieve hybridization the base sequences ofthe strands must be complementary. Thisphenomenon is exploited in many techniques,notably in GENE PROBES, which aredesigned to bind to particular complementarybase sequences among a mass of DNAfragments.nucleic acids Organic acids whose moleculesconsist of chains of alternating sugarand phosphate units, with nitrogenous155

nucleolar organizerbases attached to the sugar units. Essentiallythey are polymers of NUCLEOTIDESformed by condensation reactions betweenthe component nucleotides, which arelinked by phosphodiester bonds. There aretwo forms of nucleic acids, DNA (deoxyribonucleicacid) and RNA (ribonucleic acid).In DNA the sugar is deoxyribose; in RNAit is ribose. Nucleic acids occur in the cellsof all organisms, and may be single- ordouble-stranded.nucleolar organizerSee nucleolus.nucleolus (pl. nucleoli) A more or lessspherical structure found in nuclei of eukaryotecells, and easily visible with a lightmicroscope, staining densely with basicdyes. There may be one to several per nucleus.The nucleolus is the site of manufactureof ribosomal subunits and is thus mostconspicuous in cells making large quantitiesof protein. Nucleoli disappear duringcell division. The nucleolus synthesizes ribosomalRNA (rRNA) and is made ofRNA (about 10%) and protein. It formsaround particular loci of one or more chromosomescalled nucleolar organizers.These loci contain numerous tandem repeatsof the genes coding for ribosomalRNA. TRANSCRIPTION of this code requiresa specific RNA polymerase found only inthe nucleolus.nucleoplasm The material of the NU-CLEUS, especially the material inside the nuclearenvelope.nucleoprotein A compound consistingof a protein associated with a nucleic acid.Examples of nucleoproteins are the chromosomes,made up of DNA, some RNA,and histones (proteins); and the ribosomes(ribonucleoproteins), consisting of ribosomalRNA and proteins.nucleoside A molecule consisting of aPURINE or PYRIMIDINE base linked to asugar, either ribose or deoxyribose. Adenosine,cytidine, guanosine, thymidine, anduridine are common nucleosides.nucleosomeSee chromosome.nucleotide The compound formed bycondensation of a nitrogenous base (aPURINE or PYRIMIDINE) with a sugar (riboseor deoxyribose) and phosphoric acid. ATPis a mononucleotide (consisting of a singlenucleotide), the coenzymes NAD and FADare dinucleotides (consisting of two linkednucleotides), and the nucleic acids arepolynucleotides (consisting of chains ofmany linked nucleotides).nucleus (pl. nuclei) An organelle of eukaryotecells containing the genetic information(DNA) and hence controlling thecell’s activities. It is the largest organelle,typically spherical and bounded by a doublemembrane, the nuclear envelope or nuclearmembrane, which is perforated bymany pores (nuclear pores) that allow exchangeof materials with the cytoplasm.The outer nuclear membrane is an extensionof the endoplasmic reticulum. In thenondividing (interphase) nucleus the geneticmaterial is irregularly dispersed asCHROMATIN; during nuclear division (mitosisor meiosis) this condenses into denselystaining CHROMOSOMES, and the nuclearenvelope disappears, as do the nucleoli thatare normally present. See nucleolus.nullisomySee aneuploidy.nut A dry indehiscent fruit resemblingan achene but derived from more than onecarpel. It has a hard woody pericarp andcharacteristically a single seed. Cupules,supporting the nuts, may be distinctive ofthe species. Examples are the fruits of thebeech (Fagus), hazel (Corylus), oak (Quercus),and sweet chestnut (Castanea). Seeachene.nutationSee circumnutation.nyctinasty (nyctinastic movements)The156

Oobligate Describing an organism that requiresspecific environmental conditionsfor its survival and cannot adopt an alternativemode of living. For example, an obligateaerobe is an organism that can growonly under aerobic conditions and cannotsurvive in anaerobic conditions. Comparefacultative.octadecanoic acidSee stearic acid.offset A short runner arising from an axillarybud near the base of a stem andgrowing horizontally above ground awayfrom the parent, seen in daisies (Bellis) andhouseleeks (Sempervivum). It stores nofood but turns up at the end and producesa new plant from the apical bud. Likestolons and suckers, offsets are a means ofvegetative propagation in angiosperms.oil 1. A TRIACYLGLYCEROL that is liquidat room temperature. The commonest fattyacids in oils are oleic and linolenic acids,which are unsaturated. Storage oils, synthesizedin the endoplasmic reticulum,make up as much as 60% of the dry weightof certain seeds, such as the castor bean(Ricinus communis), corn (Zea mays), flax(Linum usitatissimum), soybean (Glycinemax), sunflower (Helianthus), safflower(Carthamus tinctorius), and mustards(family Brassicaceae). Oils can be stored inoil bodies (elaiosomes) – oil-secretingstructures that attract ants to disperse theseeds. Oil may also make up part of thefleshy part of a fruit, as in olive, the oilpalm, and coconut palm.2. Essential oils. Any of a number ofvolatile oils secreted by aromatic plantsthat are the source of characteristic odorsor tastes. They are stored as droplets inplant glands. Most essential oils are terpenoidsor derivatives of benzene. Someflowers, such as the vine Dalechampia (Euphorbiaceae)and some orchids, produceoils that are collected by pollinating bees.Some repel insects or grazing animals,while others (allelochemicals) deter encroachingneighboring plants. Resinousoils that appear at the site of wounds helpto prevent loss of sap and protect againstthe entry of pathogens and parasites.oil immersion A microscopic techniqueusing special high-powered objectivelenses. A drop of immersion oil (e.g. cedarwoodoil) is placed on the coverslip of a microscopeslide and the objective lenscarefully lowered into it. The oil has thesame refractive index as the lens glass andincreases the resolving power obtainableby letting a wider angle of rays enter theobjective lens.oleic acid An 18-carbon unsaturatedfatty acid occurring as the glyceride in oilsand fats. Oleic acid occurs naturally inlarger quantities than any other fatty acidin plant cells except in chloroplasts, wherelinolenic acid is the dominant fatty acid. Inmany organisms oleic acid can be synthesizeddirectly from stearic acid and furtherenzymatic paths exist for conversion tolinoleic acid and linolenic acid. This pathwaydoes not occur in humans and thehigher animals so plant sources are an essentialdietary element.Oligocene The epoch of the Tertiary period38–25 million years ago. The world’sclimate ranged from temperate to subtropical,and the epoch was marked by an expansionof grasslands at the expense offorests, a change that promoted the evolutionof large herbivores. Around the great157

oligosaccharideTethys Sea there were tropical swamps,which gave rise to extensive deposits of lignitein Germany and neighboring countries.The angiosperms were evolving anddiversifying rapidly, and overtook thegymnosperms in their abundance. Flowersand insects were evolving together, andcomplex coevolution was taking place aspollination mechanisms diversified. Seealso geological time scale.oogonium (pl. oogonia) The female reproductiveorgan of certain algae andfungi, often distinctly different in shapeand size from the male reproductive organ(the antheridium). This initially unicellulargametangium contains one or more largenonmotile haploid eggs called oospheres.These may be liberated prior to fertilization,e.g. Fucus, or remain within the oogonium,e.g. Pythium.oligosaccharideSee polysaccharide.OomyceteSee Oomycota.oligotrophic Describing lakes and pondsthat are low in nutrients and consequentlylow in productivity. With little decayingvegetation, the oxygen content of the wateris adequate for fish and other animal life.Compare eutrophic.one gene–one enzyme hypothesis Thetheory that each gene controls the synthesisof one enzyme, which was advanced followingstudies of nutritional mutants offungi. Thus by regulating the production ofenzymes, genes control the biosynthetic reactionscatalyzed by enzymes and ultimatelythe character of the organism.Genes also code for proteins, or polypeptidesthat form proteins, other than enzymes,so the idea is perhaps moreaccurately expressed as the one gene–onepolypeptide hypothesis.ontogeny The course of development ofan organism from fertilized egg to sexualmaturity and the production of gametesand the next generation. Occasionally ontogenyis used to describe the developmentof an individual structure.oogamy Sexual reproduction involvingthe fusion of two dissimilar gametes. Themale gamete is usually motile and smallerthan the female gamete, which is usuallynonmotile, contains a food store, and maybe retained by the parent. The term is generallyrestricted to descriptions of plants,particularly those that produce female gametesin oogonia. It is an extreme form ofanisogamy. Compare isogamy. See anisogamy.Oomycota (Oomycete) A phylum offunguslike protoctists that includes thewater molds, downy mildews, and whiterusts. Most oomycetes live in fresh wateror soil. They are mainly parasites or saprophytes,and feed by extending hyphae intothe tissues of their host, releasing digestiveenzymes and absorbing nutrients throughthe hyphal walls. Some are important croppests, such as potato blight (Phytophthorainfestans), damping off fungus (Pythium),mildew of grapes (Plasmopara viticola),and Saprolegnia parasitica, a parasite ofaquarium fish.oosphere A large nonmotile female gameteor egg cell. In lower plants oospheresmay be produced inside an oogonium; theymay be retained inside the oogonium andfertilized there, or released for fertilizationin the water. In angiosperms the oogoniumis enclosed in the EMBRYO SAC and protectedby the integuments of the ovule.oospore A diploid zygote of certain protoctists,produced by fertilization of the femalegamete (the oosphere), by the malegamete, the antherozoid. The zygote mayform a thick coat and go through a restingperiod before germination. The term‘oospore’ distinguishes a zygote producedoogamously from a zygote produced isogamouslyor anisogamously and called a zygospore.See zygote.OPSee osmotic pressure.operator gene A bacterial regulatorygene that contains the genetic code for158

Ordovicianstarting TRANSCRIPTION of one or morestructural genes. The structural genes togetherwith the relevant operator genes arelinked together in a functional unit calledan OPERON. The operator is under the controlof a regulator gene, which codes for asmall protein called a repressor molecule,which binds to the operator gene and preventsit from initiating transcription. Ifproduction of the repressor ceases or if anothermolecule, such as the substrate of anenzyme, binds with it, preventing it frombinding to the operator gene, TRANSCRIP-TION can proceed.operculum (pl. opercula) 1. The circularlid of the capsule of many mosses. Itcovers the peristome and may be forciblyblown off the capsule by pressure developingin the lower portion of the capsule, orby the swelling of cells of the annulus.2. In the pollen grains of many species, a lidcovering the aperture, which is displacedby the emerging pollen tube.3. A cap covering the ostiole of the peritheciumin certain ascomycetes.operon A genetic unit found in prokaryotesand comprising a group of closelylinked genes acting together and coding forthe various enzymes of a particular biochemicalpathway. They function as a unit,because either all are transcribed or none.At one end is an operator, which containsa site called the PROMOTER. For transcriptionof the structural genes (the genes thatactually code for the enzymes) to proceed,mRNA polymerase must first bind with thepromoter. Whether or not this happens iscontrolled by another gene outside theoperon, the regulator gene. The regulatorgene produces a regulator protein thatbinds with the operator, renders it inoperative,and so prevents enzyme production.The presence of a suitable substrate preventsthis binding, and so enzyme productioncan commence. An example is the lacoperon in the bacterium Escherichia coli,which is involved in the metabolism of lactose.In the absence of an inducer molecule(in this case, the substrate lactose or one ofits derivatives), no transcription occurs.But if lactose binds to the regulator protein,the regulator can no longer bind to theoperator, so transcription can proceed andenzyme synthesis is initiated. See Jacob–Monod model; repressor.Ophioglossales (moonworts andadder’s-tongue ferns) An order of eusporangiateferns (see Filicinophyta) characterizedby fronds that are divided into adistinct sterile green blade and a fertilespike bearing sporangia sunken into itssides. Unlike all other ferns, the fronds arenot rolled up as they push up through theground. They are found in temperate andtropical regions.opiate An ALKALOID derived from theopium poppy (Papaver somniferum) in alatex that oozes from cut seed capsules.Opiates are highly addictive drugs that resembleendorphins, chemicals naturallyproduced in the mammalian body that suppresspain and enhance mood by occupyingcertain receptor sites on nerve cells thataffect transmission of impulses. Opiatesoccupy the same receptor sites. They arethe source of codeine, morphine, heroin,and other drugs, many of which havemedicinal uses in the treatment of severepain.Orchidaceae The orchid family comprisingmonocotyledonous perennial herbs.The family includes terrestrial forms, epiphytes,and full saprophytes with no greenparts (e.g bird’s-nest orchid (Neottia nidusavis)and coralroot, (Corallorhiza spp.)). Itis the largest family of flowering plantscontaining over 25,000 species. See illustrationoverleaf.order A collection of similar families.The Latin names of plant orders generallyend in ales (e.g. Liliales). Orders may be dividedinto suborders. Similar orders constitutea class. See International Code ofBotanical Nomenclature.Ordovician The second oldest period ofthe Paleozoic era, some 510–440 millionyears ago. Algae were abundant in the159

organOrchidaceae: front view of an orchid floweroceans, especially mat-forming algae andalgae in symbiotic relationships in thecorals of the reefs dominating the manyshallow, warm seas of the time. Some algaemay also have been terrestrial, formingmats on wet ground. See also geologicaltime scale.organ A part of an organism that ismade up of a number of different tissuesspecialized to carry out a particular function.Examples include the stem, leaf,flower, and root.organ culture The maintenance orgrowth of living organs, usually embryonic,in vitro. Mature plant organs, notablyroots, may be cultured indefinitelyin suitable media. Leaves, embryos, meristems,and ovules have also been successfullycultured.organelle A discrete membraneenclosedsubcellular structure with a particularfunction. The largest organelle isthe nucleus; other examples are CHLORO-PLASTS, MITOCHONDRIA, VACUOLES, and RI-BOSOMES.Organelles allow division oflabor within the cell.organochlorines Organic compoundscontaining chlorides, used in pesticidessuch as DDT, and in other chemicals usedto control plant pests, which are not readilybroken down and persist in tissues ofanimals, affecting breeding success andcausing other problems. They are bannedfrom use in most countries, but are stillused in some, especially where insect-bornediseases such as malaria are prevalent.origin of life The development of livingorganisms from inorganic matter. Geologicalevidence strongly suggests that lifeoriginated on earth about 4600 millionyears ago. The earliest evidence of life isbacterialike fossils in rocks over 3500 millionyears old. The basic components of organicmatter – water, methane, ammonia,and related compounds – were abundant in160

osmotic pressurethe atmosphere, which had much greaterreducing properties than today’s atmosphere.Until recently, it was widely acceptedthat life evolved in warm lagoons orhot springs, where energy from the sun(cosmic rays) and lightning storms causedsimple molecules to recombine into increasinglycomplex organic molecules thateventually showed the characteristics ofliving organisms. However, recent discoveriessuggest that life may have evolveddeep underground in fissures in hot rocks –a habitat in which today vast numbers ofthermophilic bacteria live. Life probablyhad its origins in organic self-replicatingmolecules that consumed the chemicalsaround them to duplicate themselves.These molecules were probably a form ofRNA.ornithine A nonprotein AMINO ACID thatoccurs in the cell walls of certain bacteria.It is an important intermediate in the synthesisof the amino acid arginine, and thesynthesis of many alkaloids.orthogenesis An early theory of evolutionin which evolutionary change was envisagedas occurring in a definite directionand along a predetermined route, irrespectiveof natural selection.orthotropismSee tropism.orthotropous (atropous)See ovule.osmium tetroxide A stain used in electronmicroscopy because it contains theheavy metal osmium. It is fat-soluble, andunsatured fats will reduce it to black osmiumdioxide. It therefore stains lipids, includingmembranes, particularly intensely.It also acts as a fixative.osmometer An instrument that is usedto measure OSMOTIC PRESSURE (WATER PO-TENTIAL). In its simplest form, a porous potis made selectively peremeable by impregnatingits pores with copper ferrocyanide.The pot is filled with an osmotically activesolution, and a piston placed on the solution.The pot is then immersed in a containerof pure solvent. Solvent entering theporous pot by osmosis lifts the piston untilthe force of the water entering the potequals the hydrostatic pressure created bythe weight of the piston.osmosis The movement of solvent froma dilute solution to a more concentrated solutionthrough a semipermeable membrane(selectively permeable membrane) – onethat allows the passage of some kinds ofmolecule and not others. For example, if aconcentrated sugar solution (in water) isseparated from a dilute sugar solution by aselectively permeable membrane, watermolecules can pass through from the dilutesolution to the concentrated one by diffusion.Osmosis between two solutions willcontinue until they have the same concentration.If a certain solution is separatedfrom pure water by a membrane, osmosisalso occurs. The pressure necessary to stopthis osmosis is called the OSMOTIC PRESSURE(OP) of the solution. The more concentrateda solution, the higher its osmoticpressure. Osmosis is a very important featureof both plant and animal biology. Cellmembranes act as differentially permeablemembranes and osmosis can occur into orout of the cell. It is necessary for an animalto have a mechanism of osmoregulation tostop the cells bursting or shrinking. In thecase of plants, the cell walls are slightly‘elastic’ – the concentration in the cell canbe higher than that of the surroundings,and osmosis is prevented by the pressureexerted by the cell walls (wall pressure).Where the solvent is water, physiologistsnow describe the tendency for water tomove in and out of cells in terms of WATERPOTENTIAL.osmotic potentialSee water potential.osmotic pressure (OP) The force (pressure)that must be applied to a solution toprevent the passage of water or anotherpure solvent through a selectively permeablemembrane separating the solvent fromthe solution (i.e. to prevent OSMOSIS occurring).Osmotic pressure rises as the concen-161

Osmundalestration of the solution rises. The osmoticpressure is equal to the diffusion pressuredeficit plus the turgor pressure. This term isbecoming obsolete, as it ignores capillaryand imbibitional forces that are significantin certain situations. A better concept isthat of WATER POTENTIAL.Osmundales (royal ferns) An order offerns often large, retaining some primitivefeatures, whose whose sporangia combineeusporangiate and leptosporangiate characters(see Filicinophyta).ostiole A small pore through whichspores or gametes are released from the reproductivebodies of certain fungi andalgae. Examples include the CONCEPTACLESof wracks (Fucus spp.).outbreeding Breeding between individualsthat are not closely related. In plantsthe term is often used to mean crossfertilization,and various methods exist topromote it, e.g. stamens maturing beforepistils, SELF-INCOMPATIBILITY. The most extremeform – crossing between species –usually results in sterile offspring and thereare various mechanisms to discourage it.Outbreeding increases heterozygosity, givingmore adaptable and more vigorouspopulations. Compare inbreeding. See heterostyly;protandry; protogyny.ovary The swollen base of the CARPEL inthe gynoecium of plants, containing one ormore ovules, each attached by a funiculus.The gynoecium of angiosperms may consistof more than one carpel that fuses incertain species forming a complex ovary.Where the fused carpel walls remain intact,this is a multilocular ovary; where theybreak down it becomes a unilocular ovary.If the ovary lies below the other floral organsin a flower, it is said to be an inferiorovary, as in Rosa. In such cases, the ovaryusually appears sunk into and fused with acup-shaped receptacle. In a floral formula,an inferior ovary is denoted by a line abovethe carpel (gynoecium) symbol and number.If the petals, sepals, etc. are insertedbelow it, it is a superior ovary, as in buttercup(Ranunculus). In a floral formula asuperior ovary is denoted by a line belowthe carpel number. After fertilization, theovary wall becomes the pericarp of thefruit enclosing seeds in its central hollow.ovate Up to four times as long as broad,tapering at both ends, with the broadestpart below the middle.ovule Part of the female reproductiveorgans in seed plants. It consists of the nucellus,which contains the embryo sac, surroundedby the integuments. The ovule issurrounded by a cuticle, and is thought tobe derived from the megasporangium of itsnonseed-bearing ancestors. After fertilizationthe ovule develops into the seed. In angiospermsthe ovule is contained within anovary, and attached to the placental tissueof the ovary by a stalk, the funiculus. Itmay be orientated in different ways beingOvule: types of orientation162

ozone layerupright, inverted, or sometimes horizontal.Where the developing ovule has turnedthrough 180°, so that the micropylar endhas folded over and lies close to the base ofthe funiculus, it is termed anatropous. Thisis the most common position of the ovulein flowering plants. Where the ovule hasdeveloped vertically so that the micropylarend is directly over the funiculus, it istermed orthotropous, e.g. Polygonum.Where the funiculus appears to be attachedhalf way between the chalaza and the micropyle,and the micropylar end is turnedthrough 90° relative to the orthotropouscondition so that the ovule is horizontal, itis termed campylotropous, e.g. mallow(Malva sylvestris).In gymnosperms ovules are larger butare not contained within an ovary. Theyare borne on ovuliferous scales. Gymnospermseeds are thus naked while angiospermseeds are contained within afruit, which develops from the ovary wall.ovuliferous scale The megasporophyllfound in the axis of the bract scale in the femalestrobili of the Coniferophyta. It is alarge woody structure and bears ovulesand later seeds, on its upper surface. It isthus homologous with the carpel of the Anthophyta.ovum (pl. ova) (egg cell)egg cell.An unfertilizedoxalic acid (ethanedioic acid) A dicarboxylicacid, that occurs in various plants,e.g. in the leaves of rhubarb (Rheum),dumbcanes (Dieffenbachia), wood sorrel(Oxalis acetosella), and the garden oxalis(Oxalis spp.). It plays a role in the removalof excess cations, such as sodium, potassium,and calcium, which are incorporatedinto oxalates.oxaloacetic acid (OAA) A water-solublecarboxylic acid, structurally related tofumaric acid and maleic acid. Oxaloaceticacid is an intermediate of the KREBS CYCLE,and in the Hatch–Slack pathway of C 4PLANTS. It is produced from L-malate in anNAD-requiring reaction and itself is a steptoward the formation of citric acid in a reactioninvolving pyruvate and coenzyme A,part of a series of reactions that are used toreplace Krebs cycle intermediates removedfor use in biosynthesis. When these reactionsrun in reverse order, they form apathway for glucose synthesis.oxidase An enzyme that catalyzes a reactioninvolving the oxidation of a substrateusing molecular oxygen as electronacceptor, e.g. cytochrome c oxidase in therespiratory ELECTRON-TRANSPORTCHAIN.Many oxidases are flavin-linked dehydrogenasesin which the reduced COENZYMEcan be reoxidized by molecular oxygen toform water and hydrogen peroxide. See dehydrogenase;oxidative phosphorylation.b-oxidation The main pathway for thebreakdown of fatty acids to acetyl CoA. b-oxidation involves the removal of carbonacids from fatty acid chains two at a time,forming one molecule of acetyl CoA eachtime. The acetyl CoA may enter the Krebscycle and be further oxidized, or it mayenter the glyoxylate cycle and be convertedto sugar. b-oxidation is a major energyliberatingprocess in germinating oil-richseeds, where the associated enzymes arelocated in the glyoxysome together withthose of the glyoxylate cycle.oxidative phosphorylation The productionof ATP from phosphate and ADPin aerobic respiration. Oxidative phosphorylationoccurs in mitochondria, the energybeing provided by steps in the electrontransportchain.oxygen An element essential to livingorganisms both as a constituent of carbohydrates,fats, proteins, and their derivatives,and in aerobic respiration. It entersplants in both carbon dioxide and water,the oxygen from water being released ingaseous form as a byproduct of photosynthesis.Plants are the main, if not the only,source of gaseous oxygen and as such areessential in maintaining oxygen levels inthe air for aerobic organisms.163

stabilizes the stratosphere, preventing itfrom mixing with other layers, so reducingwinds in the lower atmosphere.In the early 1980s it was discoveredthat the ozone layer was becoming thinner,with seasonal ‘holes’ appearing overAntarctica and elsewhere. This depletion isbelieved to be caused by increasing atmosphericconcentrations of chlorine compounds,especially chlorofluorocarbons(CFCs) – very stable chemicals used as refrigerantsand aerosol propellants – whichdisrupt the delicate balance between ozoneproduction and breakdown in the atmosozonelayerb-oxidationozone layer A layer consisting of ozone(O 3 ) molecules scattered through thestratosphere roughly 15–50 km above theearth’s surface. The ozone is formed by theaction of solar radiation on oxygen molecules.The ozone layer absorbs about99% of the harmful ultraviolet radiationentering the earth’s atmosphere, and henceprovides a shield for living organisms withoutwhich life on earth would be impossible.This absorption of ultravioletradiation has two consequences: it warmsthe stratosphere, helping to maintain thetemperature of the Earth’s surface; and it164

PP680P700See photosynthesis.See photosynthesis.pachytene In MEIOSIS, the stage in midprophaseI that is characterized by thecontraction of paired homologous chromosomes.At this point each chromosomeconsists of a pair of chromatids and thetwo associated chromosomes are termed atetrad.pairingpaleaSee synapsis.See bract.paleobotanySee paleontology.Paleocene The oldest epoch of the Tertiary,65–55 million years ago. It was a periodof warm humid climate with largeareas of temperate and subtropical forests,but only limited grasslands, as the grasseswere of very recent origin. During the Paleocenethere was a great deal of coevolutionof plants and animals, the plantsevolving adaptations to animal pollinationand fruit and seed dispersal. Bilateral flowersevolved, e.g. those of the pea family(Fabaceae) and orchids (Orchidaceae). Inthe latter part of the Paleocene, large angiospermfruits and seeds evolved as fruitandseed-eating mammals also evolved anddiversified. See also geological time scale.paleoecology The investigation of prehistoricecology as revealed by studyingfossils and ancient human artefacts, pollen(see palynology) samples, and the mineraldeposits in which such structures occur.paleontology The study of extinct organisms,including their fossil remains, andimpressions left by them. Sometimes thesubject is divided into paleobotany, thestudy of fossil plants, paleozoology, thestudy of fossil animals, and PALEOECOLOGY.Paleozoic The first and oldest era inwhich multicellular life became abundant,about 590–230 million years ago. It is dividedinto six main periods: the Cambrian,Ordovician, Silurian, Devonian, Carboniferous,and Permian. Beginning withaquatic invertebrates and algae, the eraended with the invasion of land by treeferns and reptiles. See also geological timescale.palisade mesophyllSee mesophyll.palmella (pl. palmellae) A stage formedunder certain conditions in various unicellularalgae in which, after division, thedaughter cells remain within the envelopeof the parent cell and are thus rendered immobile.The cells may continue dividinggiving a multicellular mass, which is containedin a gelatinous matrix. Palmelloidforms may develop undulipodia and revertto normal mobile cells at any time.palmitic acid (hexadecanoic acid) A 16-carbon saturated fatty acid occurringwidely in fats and oils of animal and vegetableorigin. It is the commonest saturatedfatty acid in plants, and an intermediate inthe synthesis of oleic and stearic acids. Seealso carboxylic acid; oleic acid.palynology The study of living and fossilpollen. It is mainly used as a means ofobtaining information on the compositionand extent of past floras, and may give reliablequantitative information on the veg-165

panicleetative cover many thousands of years ago.See also pollen analysis.panicleSee inflorescence.pantothenic acid (vitamin B 5 ) The precursorof coenzyme A, pantothenic acidis essential for several fundamental reactionsin metabolism. It is one of the watersolubleB group of VITAMINS, and is synthesizedby plants and bacteria but not by vertebrates,so is an essential vitamin in theirdiet.paper chromatography A chromatographicmethod by which minute amountsof material can be analyzed. An absorbentpaper strip with a drop of test material atthe bottom is dipped into the solvent(which rises up the paper by capillary action)and removed when the solvent frontalmost reaches the top of the strip.papilla (pl. papillae) A short sometimescone-shaped projection from a cell, usuallyan extension of the outer wall of an epidermalcell. Papillae are often found onpetals, giving them a velvety appearance.See hair.pappus (pl. pappi) The ring of hairs,scales, or teeth that makes up the calyx inflowers of the ASTERACEAE. It persists on thefruit and serves to aid the wind dispersal ofseeds. An example is the cypsela of the dandelion(Taraxacum) in which the pappusremains attached by a long thin stalk andacts as a parachute.parallel evolution (parallelism) The developmentof similar features in closelyrelated organisms as a result of strong selectionin the same direction. This mayoccur between species of the same genusthat are widely separated geographicallybut which live in similar environments.There are few examples of this phenomenonand some authorities deny its existence.Compare convergent evolution.paramylum A starchlike polysaccharidecomposed of b(1-3)-linked units of glucose,which is the assimilation product ofeuglenas (Eustigmatophyta).paraphysis (pl. paraphyses) A sterileunbranched (usually) multicellular hairfound in large numbers between the reproductiveorgans of certain algae andbryophytes. Club-shaped hyphae in the hymeniallayer of certain basidiomycete fungiare also termed paraphyses.parasitism An association between twoorganisms in which one, the parasite, benefitsat the expense of the other, the host.The tolerance of the host varies from beingalmost unaffected to serious illness andoften death. An obligate parasite such aspotato blight (Phytophthora infestans) canlive only in association with a host,whereas a facultative parasite such as thedamping-off fungus (Pythium) can exist inother ways, for example as a saprophyte.paratonic movements Movements ofplants or plant parts in response to externalstimuli. They may be divided into mechanicalmovements (e.g. the hygroscopicmovements of dead cells) and movementscaused by the stimulation of sensitive cells(e.g. tropisms and nastic movements).Taxes are also paratonic movements.Compare autonomic movements.parenchyma Tissue made up of livingthin-walled cells that are not differentiatedfor any specific function, but in which importantmetabolic processes are carriedout. The leaf mesophyll and the stemmedulla and cortex consist of parenchyma.The vascular tissue is also interspersedwith parenchyma; for instance themedullary rays of secondary vascular tissue.Parenchyma often forms a basicground tissue in which other tissues areembedded. It is generally considered to bethe tissue from which other types of tissuesevolved.parthenocarpy The development offruit in unfertilized flowers, resulting inseedless fruits. It may occur naturally, as inthe banana (Musa), which is triploid. It166

pepomay also be induced artificially by the applicationof auxin, as in commercialtomato growing.parthenogenesis Development of unfertilizedovules to form new individuals. Itoccurs regularly in certain plants, e.g. dandelion(Taraxacum), in which meiosis isdefective, resulting in a diploid egg. Inhawkweeds (Hieracium) a cell of the nucellustakes the place of the megaspore. Seealso apomixis.partial dominance See incompletedominance.passage cells Endodermal cells thathave CASPARIAN STRIPS but remain otherwiseunthickened after deposits of ligninand cellulose have been laid down elsewherein the endodermis. Passage cells arefound opposite the protoxylem and allowtransport of water and solutes from thecortex to the stele.pathogen Any organism that is capableof causing disease or a toxic response in anotherorganism.PCRSee polymerase chain reaction.peat Partially decomposed plant materialthat accumulates in waterlogged anaerobicconditions in temperate humidclimates, often forming a layer several metersdeep. Peat varies from a light spongymaterial (Sphagnum moss) to a densebrown humidified material in the lowerlayers.pectic substances Polysaccharides that,together with hemicelluloses, form the matrixof plant cell walls. They serve to cementthe cellulose fibers together. Fruitsare a rich source, especially apples (Malus),pears (Pyrus communis), and citrus fruits.See also middle lamella.pedicelpeduncleSee inflorescence.See inflorescence.pelagic Inhabiting the open upper watersrather than the bed of a sea or ocean.Compare benthic. See also photic zone;plankton.pellicle (periplast) A thin flexible transparentouter protective covering of someunicellular algae that lack a cellulose cellwall, e.g. Euglena. It is made of protein,and maintains the shape of the body.Pennsylvanian The US name for thelate Carboniferous period in the geologicaltime scale, 320 to 286 million years ago.Rocks that originated during this periodare widespread in the state of Pennsylvania,USA.pentose A SUGAR that has five carbonatoms in its molecules. Examples includeribose, deoxyribose, and arabinose. Pentosesare synthesized in the Calvin cycle,derived from hexoses in the PENTOSE PHOS-PHATE PATHWAY, and also made by decarboxylationof nucleoside diphosphatesugars. Polymers of pentoses are calledpentosans.pentose phosphate pathway (hexosemonophosphate shunt) A pathway ofglucose breakdown in which pentoses areproduced, in addition to reducing power(NADPH) for many synthetic reactions. Itis an alternative to GLYCOLYSIS, and producessugars containing 3 to 7 carbonatoms. Glucose 6-phosphate is decarboyxlatedand converted to a 5-carbonsugar, with the formation of NADPH. This5-C sugar than enters a sequence of reactions,until eventually glucose 6-phosphateis reformed.PEPSee phosphoenolpyruvate.pepo A succulent fruit resembling aberry. The hard outer ring orginates eitherfrom the epicarp or from noncarpellary tissuesof the plant. In members of the Cucurbitaceae,such as Cucurbita species (e.g.cucumber, squash, and pumpkin), the rindis formed from the receptacle, while the167

peptidasePeptide bondflesh enclosing the seeds is derived from theovary wall.peptidase An enzyme that is responsiblefor catalyzing the hydrolysis of certain peptidebonds. The peptidases help breakdown peptides into amino acids.peptide A molecule consisting of a numberof amino acid molecules linked together.Peptides can be regarded as formedby a CONDENSATION REACTION in which thecarbonyl group of one amino acid reactswith the amino group of another aminoacid with the elimination of water. Thislink between amino acids is called a peptidebond. According to the number ofamino acids linked together they are calleddi-, tri-, oligo-, or polypeptides. In general,peptides have an amino group at one endof the chain and a carboxyl group at theother. They can be produced by the partialhydrolysis of proteins.peptide bondSee peptide.peptidoglycan The principal componentof the cell walls of most BACTERIA (butnot of Archaea), consisting of polysaccharidemolecules cross-linked by short peptideschains to form a rigid framework.perennating organ In biennial or perennialplants, a storage organ that enables theplant to survive an adverse season, such aswinter or a dry season (perennation). Mostsuch organs are underground, e.g. rhizomes,corms, bulbs, swollen taproots, andtubers. Perennating organs are also used bymany plants in asexual reproduction (vegetativegrowth). Buds developing in theaxils of scale leaves on CORMS, BULBS, andstem TUBERS may become detached, putdown ADVENTITIOUS roots, and grow intoindependent plants. Similarly, new shootsarise from buds on RHIZOMES, and the interveningsegments of rhizome may breakor rot away.perennationSee perennating organ.perennial A plant that may live for severalyears. Perennials may reproduce intheir first growing season or may haveto attain a certain age before seed productioncommences. Herbaceous perennialsdie back each year and survive until thenext growing season as tubers (e.g. Dahlia),bulbs (e.g. daffodil, Narcissus), rhizomes168

peristome(e.g. Iris), etc. Woody perennials, such astrees and shrubs, persist above groundthroughout the year but may show adaptations(e.g. leaf fall) to survive unfavorableseasons.perforation plates The remains of thecross walls between the vessel elements inthe xylem vessels of angiosperms, the Gnetophyta,and some ferns, which leave anopening allowing water to move freelythrough the vessel. The cross wall mayhave disintegrated completely so that thevessel is effectively one long cylinder (e.g.oak) or parts of the cross wall may remainas bars across the cavity of the vessel, e.g.alder (Alnus). Compare sieve plate.perianth The part of the flower that encirclesthe stamens and carpels. It usuallyconsists of two whorls of leaflike structuresthat are differentiated into the sepals (calyx)and petals (corolla). In many monocotyledonsthere is no such distinction, and theindividual perianth units are called tepals,e.g. tulip (Tulipa). In some flowers, especiallythose that are wind pollinated, theperianth is reduced (e.g. grasses) or absent(e.g. willow).periblemtheory.See ground meristem; histogenpericarp The ovary wall that becomesthe wall of the FRUIT as the fruit develops.Depending on how the pericarp tissuesdifferentiate, the resulting fruit may bedry (papery or leathery) or succulent. Theouter layer is the exocarp, often a toughskin. The middle layer is the mesocarp,whose texture varies with different fruits,e.g. it is juicy in drupes like the plum(Prunus domestica), and hard in almonds(Prunus dulcis). The mesocarp may be protectiveor aid in dispersal. The endocarp isthe innermost layer and forms the stonycovering of the seed in a drupe, but in otherfruits (e.g. the berry), it is indistinguishablefrom the mesocarp. If the pericarp openswhen ripe, the fruit is said to be DEHISCENT;if it remains closed, it is INDEHISCENT.periclinal Describing a line of cell divisionparallel to the surface of the organ, resultingin the formation of periclinal wallsbetween daughter cells and an increase ingirth of the organ. Compare anticlinal.pericycle The parenchymatous layer ofcells that lies within the endodermis, formingthe outermost part of the stele. It remainsmeristematic in most roots and givesrise to the lateral roots. In most dicotyledonousroots showing secondary growth,the pericycle is also involved in the originationof the vascular cambium and the phellogen.periderm The secondary tissue thatarises from the activity of the cork cambium.It forms the outer layer of the stemand root in plants with secondary growth,having a protective role. It is made up ofthe PHELLOGEN (cork cambium), whichgives rise to the PHELLEM (cork) and PHEL-LODERM (secondary cortex). Periderm mayalso develop at the site of a wound, forminga barrier to pathogens.perigyny The arrangement in flowers inwhich the perianth and androecium are insertedon the receptacle around the gynoeciumrather than above or below it. Thereceptacle is extended to a flat or saucershapedorgan and carries the gynoecium inthe middle and the other floral partsaround the edge. The OVARY is technicallysuperior even though in extreme forms ofperigyny in which the receptacle is cupshaped,e.g. the wild rose (Rosa sp.), thefloral parts may be inserted at a level abovethe gynoecium. In perigynous flowers theovary is not fused with the receptacle.Compare epigyny; hypogyny.periplastSee pellicle.perisperm The nutritive tissue in theseeds of many Caryophyllaceae (the carnationfamily). It is a diploid tissue derivedfrom the nucellus rather than the embryosac.Such seeds therefore differ fromother angiosperm seeds that have either169

peritheciumcotyledons or endosperm tissue acting asthe food store.peristome A single or double ring ofteeth around the opening of the capsule inmosses that is involved in spore dispersal.It is revealed when the operculum falls offor is removed. The teeth absorb moisturefrom the atmosphere, and the coiled fibersof which they are made change length andtwist and bend when subjected to humiditychanges, either opening and closing thecapsule to release the spores or scatteringthem through jerky movements.See as-perithecium (pl. perithecia)coma.permanent stainSee staining.permanent wilting point The point atwhich soil has dried to the extent thatplants can no longer remove the remainingwater held on the soil particles, and beginto wilt and will not recover, even if movedto a cool, dark place, unless more water isadded to the soil. At permanent wiltingpoint the water potential of the soil is equalto or lower than the water potential of theplant.Permian The most recent period of thePaleozoic era, some 280–250 million yearsago. As the climate warmed and driedduring the Permian, the ferns, seed ferns,and lycophytes that dominated Permianmarshes and swamps were joined by theevolving coniferophytes and some othergymnosperms, whose evolution was probablydriven by the need to cope with drierenvironments. These groups graduallybegan to replace the older floras dominatedby nonseed-bearing plants. See alsogeological time scale.Peronosporales An order of the OOMY-COTA characterized by sexual reproduction(usually) by zoospores and sexual reproductionby zoospores produced fromoospores. The Peronosporales containssaprobes and many harmful plant parasites.Most are parasites found in damp terrestrialhabitats; they include downymildews (Peronospora spp.), late blightof potatoes (Phytophtherainfestans), anddamping-off fungus (Pythium spp.).peroxidase An enzyme that catalyzesthe oxidation of certain organic moleculesusing hydrogen peroxide as electron acceptor.peroxisomeSee microbody.petal One of the usually brightly coloredparts of the flower, which together makeup the corolla. The petals are pigmentedand often scented to attract insects. Theyare reduced or absent in most wind-pollinatedflowers. Petals are thought to bemodified leaves, with a much simplified internalstructure and vascular system, havingonly one vascular bundle. They mayderive from sterile stamens. Comparesepal.petiole The stalk that attaches the leafblade to the stem. It is similar to the stemexcept that it is asymmetrical in crosssectionwith the vascular and strengtheningtissues arranged in a V shape rather than acircle. Leaves that lack a petiole are termedsessile.Pezizales (cup fungi; morels) An orderof ascomycete fungi in which the asci (seeascus) are borne in a HYMENIUM that linesthe surface of a cup-shaped fruiting body(apothecium). The spores are dischargedforcibly into the air. Most are saprobes livingin soil, dung, or rotting wood. A fewspecies are involved in mycorrhizae.pH A measure of the acidity or alkalinityof a solution on a scale 0–14. A neutral solution(such as pure water) has a pH of 7.Acid solutions have a pH below 7; alkalinesolutions have a pH above 7. The lower thepH value, the higher the concentration ofH + . The pH is given by log 10 (1/[H + ]),where [H + ] is the hydrogen ion concentrationin moles per liter. Thus one integer onthe pH scale represents a tenfold differencein the concentration of H + .170

phenolphthaleinPhaeophyta (brown algae) A phylum ofprotoctists comprising mainly marinealgae, notably the macroscopic thalloseseaweeds that inhabit the intertidal zones.They contain the pigments chlorophyll aand c, b-carotene, and the xanthophylls(including fucoxanthin), which give thealgae their characteristic brown color.phageSee bacteriophage.phagocytosis The taking up of foodparticles by a cell using protrusions of theouter part of the cell, generated by cytoplasmicflow. These protrusions surroundthe particle, enclosing it in a membraneboundvesicle and so introducing it into thecell as a food vacuole. Lysosomes then congregatearound the food vacuole, fusingwith it and releasing hydrolytic enzymes todigest the particle. See endocytosis.Phallales (stinkhorns) An order of basidiomycetesin which the BASIDIOMA has atwo-layered wall enclosing a slimy, gelatinouslayer. See Basidiomycota.phanerogam In early classifications,any plant whose reproductive organs areflowers or cones. Compare cryptogam.phanerophyte A perennial plant withpersistent shoots and buds on uprightstems well above soil level. Phanerophytesinclude herbs, shrubs, trees, and climbingplants. See Raunkiaer’s plant classification.PhanerozoicSee Precambrian.phase contrast microscopy A form oflight microscopy that amplifies the differencein the phase of light after passingthrough a specimen to give greater contrastin the image. It is based on the fact thatlight is diffracted (changes phase) accordingto the refractive index or thickness ofthe material through which it passes. Thephase contrast microscope uses a specialcondenser, which produces a particularpattern of light, usually a hollow cone.This pattern is matched by a groove in aglass plate (the retardation plate) in thecondenser. Light that is not diffractedpasses through the holes in the plate. Lightthat has been diffracted by the specimenpasses through the thicker part of the plate,so is slowed down even further. This techniquecan be used to study real-time eventssuch as cell division.phellem (cork) A protective layer of radicallyarranged cells produced to the outsideof the PHELLOGEN (cork cambium).The cork replaces the epidermis in certainwoody plants, forming an impervious layerbroken only by lenticels. The older corkcells are dead, suberized, and frequentlyonly air-filled, although lignin, fatty acids,and tannins often accumulate. The corkoak, Quercus suber, of Southern Europe,forms a very thick layer and is the mainsource of commercial corks.phelloderm The inner layer of the PERI-DERM. During secondary growth, themeristematic PHELLOGEN (cork cambium)gives rise to parenchyma tissue, the periderm,which forms the secondary cortex.The cells of the phelloderm arise in radialcolumns from the phellogen, and this distinguishesthem from the primary cortex.phellogen (cork cambium; periderm cambium)A subepidermal layer of cellsforming a lateral meristem that arises followingthe onset of secondary growth. Thecells of the phellogen (cork cambium) giverise externally to the PHELLEM (cork) andinternally to the PHELLODERM.phelloidcell.A thin-walled unsuberized corkphenolics (phenols) Compounds thatcontain a benzene ring or other aromaticring substituted with one or more hydroxylgroups, e.g. phenol (C 6 H 5 OH). Phenolicsare similar to alcohols but are more solublein water. Many have sharp, spicy odors. Itis thought that most phenolics are secretedby plants to deter herbivores as many areenzyme inhibitors and disrupt digestion.Plant phenolics include the FLAVONOIDS,LIGNIN, and TANNINS.171

phenotypephenolphthalein An acid–alkali indicatorthat is colorless in acids and red in alkalis.It has a pH range from 8.4–10.0 andis a frequently used indicator for the detectionof pH change in acid–alkali titrations.Phenolphthalein, together with borax, isused to test for saccharide derivatives, e.g.glycerol. Such derivatives turn the solutionfrom red to colorless but on boiling the redcolor returns.phenotype The observable characteristicsof an organism, which are determinedby the interaction of the GENOTYPE with theenvironment. Many genes present in thegenotype do not show their effects in thephenotype because they are masked bydominant alleles. Genotypically identicalorganisms may have very different phenotypesin different environments, an effectparticularly noticeable in plants grown invarious habitats.phenylalanine An aromatic AMINO ACIDderived from phosphoenolpyruvate anderythrose 4-phosphate. It is a precursor ofmany aromatic compounds and of alkaloidssuch as curare and morphine. Phenylalaninecan be deaminated to cinnamicacid, required in the synthesis of lignin.phloem Plant vascular tissue in whichfood is transported from areas where it ismade to where it is needed or stored. Itconsists of SIEVE ELEMENTS and COMPANIONCELLS. The PROTOPHLOEM and METAPHLOEMare primary tissues and derived from theprocambium, while the secondary phloemis formed from the VASCULAR CAMBIUM.There are also fibers (sclerenchyma), andparenchymatous packing tissue in thephloem. See also mass flow.phloroglucinol A temporary stain thatdyes lignin magenta red in thin sections ofplant tissue. It is usually acidified with hydrochloricacid before use. See staining.phosphatide A glycerophospholipid. Seelipid.linking a sugar and a phosphate group bymeans of an oxygen bridge, for example,the bonds linking sugars and phosphategroups in the backbone of NUCLEIC ACIDS.Phosphodiester bonds are formed by condensationreactions and may be broken byhydrolysis.phosphoenolpyruvate (PEP) A 4-carbonphosphorylated organic acid that isthe main substrate for carbon dioxide fixationin C 4 PLANTS, being carboxylated tooxaloacetic acid in a reaction mediated bythe enzyme phosphoenolpyruvate carboxylase(PEP carboxylase). It is also an importantintermediate of GLYCOLYSIS, being theimmediate precursor of pyruvic acid. In theGLYOXYLATE CYCLE for the synthesis of glucoseit is generated from oxaloacetate.phosphoglyceric acid See glycerate 3-phosphate.phosphoglyceride (phospholipid) A complexlipid similar to acylglycerides (seeacylglycerol), but with a backbone of glycerol3-phosphate rather than glycerol.Phosphoglycerides are major consitituentsof the membranes of cells and organelles,with the exception of chloroplasts, inwhich GLYCOLIPIDS are more important.They have both hydrophilic and hydrophobicgroups, so are reasonably soluble inboth water and lipids.phospholipidSee lipid.phosphorescence In general usage theterm is applied to the emission of ‘coldlight’ – light produced without a high temperature.The light comes from excitedatoms produced directly in the reaction –not from the heat produced. It is thus anexample of chemiluminescence. In biologicalsystems, the light emitted is often lightthat has been previously absorbed, butwhich is re-emitted at a different wavelength.Phosphorescent seas are due to thebioluminescence of dinoflagllates (Dinomastigota),in particular Noctiluca species.phosphodiester bondA covalent bondphosphorusOne of the essential el-172

photoperiodismements in living organisms, a macronutrientneeded for plant growth. Phospholipidsare important in cell membrane structure,and phosphates are necessary for the formationof the sugar–phosphate backboneof nucleic acids. Phosphates are also necessaryfor the formation of high-energybonds in compounds such as ATP. Phosphorushas other important roles in livingtissues, being a component of certain coenzymes.The phosphate ion, PO 4 3– , is an importantbuffer in cell solutions. Plants takeup phosphorus from the soil water as thePO 4 3- ion. Phosphorus deficiency results instunted roots, bluish, bronzed, or purpleleaves, and poor germination, seed set, andfruit ripening.See kinase; phosphory-phosphorylaselation.phosphorylation The addition of aphosphate group to a compound. Phosphorylationreactions are catalyzed byphosphorylase enzymes. Phosphorylationis the first step in many metabolic pathways,the phosphate bond serving as an energysource in a later stage of the pathway.For example, glycolysis starts with thephosphorylation of glucose and fructose1,6-bisphosphate. Phosphorylation is alsoneeded for the activation or deactivation ofsome enzymes. See also oxidative phosphorylation;photophosphorylation.phosphotransferaseSee kinase.photic zone The surface layer of anocean or lake that is penetrated by sunlightand in which the phytoplankton flourish.Red and yellow wavelengths of light penetrateto about 50 m while blue and violetlight may reach 200 m. The diatoms,which are the main components of phytoplankton,may be found down to 80 m.Most of the plankton lives in the top 10 m.Beyond 200 m the water is perpetuallydark. The depth at which the rate of fixationof carbon dioxide by photosynthesis isequal to the rate at which it is lost by respirationis called the compensation depth.The part of the photic zone above thisdepth is termed the euphotic zone, the partbelow the compensation depth is the dysphoticzone. The depth of the euphoticzone depends on the turbidity of the water.See plankton. Compare aphotic zone.photoautotrophismphototrophism.See heterotro-photoheterotrophismphism; phototrophism.See autotrophism;photolysis Chemical breakdown causedby light. In PHOTOSYNTHESIS the process isimportant in providing hydrogen donorsby the splitting of water, as follows:2H 2 O ® O 2 + 4H + + 4e –In the CHLOROPLAST photolysis of water occursin the oxygen- releasing complexlinked to Photosystem II. The protons arereleased into the lumen of the thylakoid.The high-energy electrons go to replaceelectrons displaced from the reaction centerof Photosystem II by the absorption oflight energy. The electrons are the energizedby light energy and pass to theelectron-transport chain of noncyclic photophosphorylation,which generates ATPand reduced NADP.photomicrographSee micrograph.photonasty (photonastic movements) Anastic movement in response to light.Flowers such as the marigold (Calendulaofficinalis) open on exposure to light. Caremust be taken not to confuse photonastywith nyctinasty, in which day–night opening/closingof flowers is governed by an internalCIRCADIAN RHYTHM. See nasticmovements.photo-oxidation The oxidation of asubstance as a result of the absorption oflight energy. Very high light intensity cancause oxidation (bleaching) of photosyntheticpigments such as chlorophylls, inactivatingthem.photoperiodism The response of an organismto changes in day length (photoperiod).In plants, leaf fall and flowering are173

photophosphorylationcommon responses to seasonal changes inday length.Plants are classified as short-day plants(SDPs), e.g. Chrysanthemum, or long-dayplants (LDPs), e.g. cucumber (Cucumissativus), according to whether they flowerin response to short or long days. Dayneutralplants, e.g. pea (Pisum sativum),have no photoperiodic requirement. Infact, the critical factor is not the length ofthe day, but the length of the dark period,since flowering of SDPs is inhibited by evena brief flash of red light in the dark period(a PHYTOCHROME response), and an artificialcycle of long days and long nights inhibitsflowering in LDPs. Thus, it is theinteraction between light and dark periodsthat in some way affects flowering throughthe mediation of phytochrome. Phytochromeis a protein that can absorb redlight, which converts it to a form (P FR ) thatabsorbs far-red light. Absorption of far-redlight causes the phytochrome to revert tothe red light-absorbing form (P R ). The P FRform of phytochrome inhibits flowering inSDPs (P FR slowly disappears during longnights) and promotes flowering in LDPs(P FR remains at high levels in short nights).The light stimulus is perceived by the leavesand in some unknown way transmitted tothe floral apices. See circadian rhythm;Photorespiration174

photosynthesiscritical day length; thermoperiodism; vernalization.photophosphorylation (photosyntheticphosphorylation) The conversion ofADP to ATP using light-induced electrontransport. See photosynthesis.photoreceptor Any light-sensitive organor organelle. The paraflagellar body, aswelling on the undulipodium of certainspecies of Euglena, is an example.photorespiration (C 2 cycle) A lightdependentmetabolic process of most greenplants that resembles true (or ‘dark’) respirationonly in that it uses oxygen andproduces carbon dioxide. Carbon dioxideproduction during photorespiration maybe up to five times greater than in dark respiration.Photorespiration takes place inPEROXISOMES rather than mitochondria,and it is not coupled to oxidative phosphorylation.It wastes carbon dioxide and energy,using more ATP than it produces. Itssubstrate is glycolate produced in C 3 plantsby the oxidation of ribulose bisphosphateto glycerate 3-phosphate and phosphoglycolicacid. During photorespiration the enzymeglycolate oxidase catalyzes theoxidation, by molecular oxygen, of glycolicacid to glyoxylic acid and hydrogenperoxide in the peroxisomes. The glyoxylateis converted to glycine, and carbondioxide is released when the glycine moleculesare converted to serine in the mitochondria.The term photorespiration isbeing replaced by the C 2 cycle (oxidativephotosynthetic carbon cycle), to better reflectthe fact that unlike respiration it usesrather than releases energy.It is estimated that in C 3 plants 50% ofthe potential yield of photosynthesis can belost through photorespiration in warmweather. It is therefore economically importantand ways of inhibiting the processare being investigated.photosynthesis The synthesis of organiccompounds using light energy absorbedby chlorophyll. With the exceptionof a small group of bacteria, organismsphotosynthesize from inorganic materials.All green plants photosynthesize, as well asalgae and certain bacteria. In green plants,photosynthesis takes place in chloroplasts,mainly in leaves and stems. Directly or indirectly,photosynthesis is the source ofcarbon and energy for all except chemoautotrophicorganisms. The mechanism iscomplex and involves two sets of stages:light reactions followed by dark reactions.The overall reaction in green plants can besummarized by the equation:CO 2 + 4H 2 O ® [CH 2 O] + 3H 2 O + O 2The light reactions of photosynthesisinvolve reaction centers in the chloroplasts.Each reaction center lies in a flat sheet of200-400 pigment molecules in the thylakoidmembrane, the antenna complex,and contains a pair of chlorophyll a molecules.The whole unit is called a photosystem.There are two kinds of photosystemsin green plants, Photosystems I and II.Both are usually involved in the light reactionsof photosynthesis. Light energy absorbedby the pigments of the antennacomplex is passed to the reaction-centerchlorophyll molecules, from which itpasses along an electron-transport chain.Photosystem II contains a type of chlorophylla (P680) that shows maximum lightabsorption at a wavelength of 684 nm.When activated by light, a pair of electronsare excited and leave Photosystem II, beingreplaced by electrons from the photolysisof water:2H 2 O ® O 2 + 4H + + 4e –Molecular oxygen is released, and theprotons pass into the lumen of the thylakoid.The electrons pass through anELECTRON-TRANSPORT CHAIN in the thylakoidmembrane, being accepted first byplastoquinone, then by the cytochrome b/fcomplex (a proton pump that pumps H +into the thylakoid lumen), and on via plastocyaninto Photosystem I, which containschlorophyll a (P700) with a maximumlight absorption at 700 nm. Here the electronsare boosted by more absorbed lightenergy to an even higher energy level andpass through a second electron-transportchain involving the protein ferredoxin, reducingNADP to NADPH + H + for use in175

photosynthesisPhotosynthesis: the Calvin cycle176

photosynthetic pigmentsLight reaction of photosynthesisother reactions, such as various syntheses.This sequence, which involves both photosystems,is termed noncyclic electron flow.However, it is possible for electron flow toinvolve only Photosystem I, and the ferredoxin/cytochromeb/f/ plastocyain electron-transport chain, electrons returningto Photosystem I to be reenergized. Thiscyclic electron flow does not produceNADPH. It simply generates a proton gradientacross the thylakoid membrane. Thisis used to drive ATP synthesis (cyclic photophosphorylation).ATP synthesis associatedwith the noncyclic electron-transportchain is termed noncyclic photophosphorylation.The dark reactions of photosynthesistake place in solution in thechloroplast stroma. Here, ATP andNADPH from the light reactions are usedto reduce carbon dioxide to carbohydrate.Carbon dioxide is ‘fixed’ by combinationwith the 5-carbon sugar ribulose bisphosphate(RuBP), forming two molecules ofphosphoglyceric acid (PGA). This reactionis catalyzed by the enzyme ribulose bisphosphatecarboxylase (rubisco). In a series ofreactions using NADPH and ATP from thelight reactions, PGA is converted to a successionof 3-, 4-, 5-, 6-, and 7- carbon sugarphosphates in a series of reactions collectivelytermed the Calvin cycle. These productsare then used in the synthesis ofcarbohydrates, fats, proteins, and othercompounds, and RuBP is regenerated. SeeC 3 plant; C 4 plant.photosynthetic bacteria A group ofbacteria able to photosynthesize throughpossession of chlorophyll pigments. Theyinclude the Cyanobacteria, green sulfurbacteria (Chlorobia), green nonsulfur bacteria(Chloroflexa), purple sulfur bacteria,177

The arrangephotosystemand purple nonsulfur bacteria (both in thephylum Proteobacteria). The photosyntheticbacteria fix carbon using the Calvincycle, but use a variety of hydrogensources.photosynthetic pigments Pigments thatabsorb the light energy required in photosynthesis.They are located in the chloroplastsof plants and algae, whereas in mostphotosynthetic bacteria they are located inthylakoid membranes, typically distributedaround the cell periphery. All photosyntheticorganisms contain chlorophylls andcarotenoids; some also contain phycobilins.Chlorophyll a is the primary pigmentin plants, algae, and cyanobacteria,and bacteriochlorophyll is the pigmentused by many photosynthetic bacteria,since energy absorbed by this is useddirectly to drive the light reactions of photosynthesis.The other pigments (chlorophyllsb, c, and d, and the carotenoids andphycobilins) are accessory pigments thatpass the energy they absorb on to chlorophylla. They broaden the spectrum of lightused in photosynthesis. See absorptionspectrum.photosystemSee photosynthesis.phototaxis (phototactic movement) ATAXIS in response to light. Many motilealgae are positively phototactic, e.g.Volvox. Chlamydomonas cells are alsopositively phototactic, swimming towardlight to increase their photosynthesis rate,but if the light intensity becomes too great,they show negative phototaxis, swimmingaway to protect their pigments. Chloroplastsalso show phototaxis, and take upspecific orientations in relation to the directionof the incident light.phototrophism A type of nutrition inwhich the source of energy for synthesis oforganic requirements is light. Most phototrophicorganisms are autotrophic (i.e.show photoautotrophism); these comprisethe green plants, Cyanobacteria, and somephotosynthetic bacteria (the purple andgreen sulfur-bacteria).phototropism (phototropic movement)A directional growth movement of part ofa plant in response to light. The phenomenonis clearly shown by the growth ofshoots and coleoptiles toward light (positivephototropism). The stimulus is perceivedin the region just behind the shoottip. If light falls on only one side of theapex then auxins produced in the apextend to diffuse toward the shaded side.Thus more auxin diffuses down the stemfrom the shaded side of the tip. This resultsin greater elongation of cells on the shadedside thus causing the stem to bend towardthe light source. Most roots are aphototropic(light-insensitive) but some (e.g.the adventitious roots of climbers such asivy) grow away from light (negative phototropism).See also tropism.phragmoplast A barrel-shaped bodyappearing in dividing plant cells during lateanaphase and telophase between the twoseparating groups of chromosomes. It consistsof MICROTUBULES associated with theSPINDLE, together with Golgi apparatus, endoplasmicreticulum, and ribosomes, andtransports vesicles that coalesce to formthe early cell plate.phycobilins (phycobiliproteins) A groupof accessory photosynthetic pigmentsfound in Cyanobacteria and Rhodophyta(red algae). They comprise the blue phycocyanins,which absorb extra orange andred light, and the red phycoerythrins,which absorb green light, enabling redalgae to grow at depth in the sea.phycocyaninphycoerythrinphyllocladeSee phycobilins.See phycobilins.See cladode.phyllode An expanded flattened petiolethat acts as the photosynthetic organ if thelamina is missing or very reduced. Phyllodesare seen in various Australian speciesof Acacia. Compare cladode.phyllotaxis (phyllotaxy)178

physiological racePhyllotaxisment of leaves on a stem. There may beone, two, or several leaves at each node.When there are three or more leaves forminga circle around the node the arrangementis said to be whorled, e.g. bedstraws(Galium spp.). When leaves arise singly thearrangement may be spiral, e.g. poplar(Populus spp.), or alternate, e.g. hazel(Corylus spp.), and when they arise in pairsthe arrangement is termed opposite, e.g.lilac (Syringa spp.). Opposite phyllotaxismay be either distichous or decussate dependingon whether the leaf pairs are in thesame plane up the stem or arise alternatelyat right angles to each other.phylogenetic Relating to the study ofevolutionary relationships between andwithin taxonomic levels, in particular therelationships between groups of organismsbased on their past evolutionary history.phylogeny The evolutionary history ofgroups of organisms. See phylogenetic.phylum (pl. phyla) One of the majorgroups into which a kingdom of organismsis classified. Phyla may be divided into subphyla.The names of plant phyla end in-phyta, e.g. Anthophyta, Lycophyta. Insome plant classifications (especially olderones) the term ‘division’ is used instead ofphylum.physiological drought A situation inwhich plants are unable to take up sufficientwater even though there is water inthe soil: they are effectively in a droughtsituation. It occurs when the concentrationof solutes in the soil water is equal to orhigher than that in the root cells, so wateris unable to enter the root by OSMOSIS.Physiological drought occurs in salt179

physiologymarshes and other brackish coastal habitats,and in deserts where there are highconcentrations of salts in the upper layersof the soil. It also occurs in cold weatherbecause the permeability of the endodermalcells decreases rapidly below 5 o C, sothe root resists the entry of water (root resistance).physiological race The existence ofphysiologically distinct but morphologicallyidentical races within a species. Suchphysiological races are important inhost–pathogen studies, particularly inplanning programs to breed for crop resistance.physiology The way in which organismsor parts of organisms function. Comparemorphology.phytoalexin A chemical produced by aplant that inhibits the growth of a pathogenicfungus. For example, when attackedby the fungus Ceratostomella, the sweetpotato (Ipomoea batatus) concentrates theterpenoid ipomeamarone in its tissues, andthis concentration has been shown to be relatedto the degree to which it resists the attack.phytochrome A proteinaceous pigmentfound in low concentrations in most plantorgans, particularly meristems and darkgrownseedlings. It exists in two interconvertibleforms. P R (or P 660 ) has anabsorption peak at 660 nm (red light) andP FR (or P 730 ) at 730 nm (far-red light). Naturalwhite light favors formation of P FR ,the physiologically active form, while farredlight encourages reversion to P R if givensoon after the red light. Light intensities requiredfor conversion are very low and itoccurs within seconds. In some species, P FRmay revert to P R in the dark, a process inhibitedby low temperatures. See circadianrhythm; photoperiodism.phytohormoneSee hormone.pico- Symbol: p A prefix denoting onemillion-millionth, or 10 –12 . For example, 1picogram (pg) = 10 –12 gram. See SI units.pileus (pl. pilei) The cap of the maturemushroom or toadstool (sporophore) incertain basidiomycete fungi (e.g. Agaricus).Its undersurface comprises gills or poreslined with basidia. The upper surface maybe fleshy or leathery, and may retain theflaky remains of the universal veil.pili (sing. pilus) (fimbriae sing. fimbria)Fine, straight, hairlike protein structuresemerging from the walls of certain bacteria.They confer the property of ‘stickiness’whereby bacteria tend to adhere to one another.They are hollow tubes and maynumber from one to several hundred.piliferous layer The region of the rootepidermis that gives rise to the root hairs. Itis located just behind the zone of elongationand is the main absorptive area of theroot.Pinaceae A family of conifers that bearseparate male and female cones, and inwhich the seeds are borne between the conescales and do not have arils. Most speciesare evergreen trees or shrubs, but thelarches or tamaracks (Larix spp.) are deciduous.One of the oldest living trees is abristlecone pine (Pinus aristata), thoughtto be about 4900 years old. The family includescedars (Cedrus), firs (Abies), hemlocks(Tsuga), and larches and pines(Pinus).pin moldpinna (pl. pinnae)See Mucorales.See pinnate.pinnate Describing a compound leafwith leaflets (pinnae) arranged in tworows, one on each side of the midrib (floweringplants, e.g. Vicia) or of a central stalk(ferns, e.g. Dryopteris). The pinnae maythemselves be divided into leaflets, whichare called pinnules.phytoplanktonSee plankton.pinnuleSee pinnate.180

plantpinocytosisSee endocytosis.pioneer species A species that colonizesa new physical environment, for exampleland exposed by retreating glaciers, fellingof forest, or bare mud on coasts or estuaries.Pioneer species are the first stage inplant succession.pistil In angiosperms, the seed-containingstructure. In an apocarpous gynoeciumit corresponds to the carpel, while in a syncarpousgynoecium it is made up of two ormore fused carpels.pit A gap in the secondary cell wall thatenables communication between thickenedcells, e.g. tracheids. According to whetheror not the secondary wall forms a lip overthe pit, pits are described as bordered orsimple, respectively. If a pit occurs singly itis termed a blind pit. Usually pits occur inpairs so that the only barrier separating adjacentcells is the middle lamella and the respectiveprimary cell walls. The primarycell wall adjacent to the cavity is called thepit membrane.pitcher plant Any carnivorous plantwith pitcher-shaped leaves. Insects andother small animals fall into the pitchers,which are usually filled with rainwater orplant digestive enzymes, and drown; theplant obtains nitrates and perhaps othernutrients from their decomposing bodies.pith (medulla) The central region of thestem and, in some herbaceous plants, rootthat is normally composed of parenchymatoustissue. It occurs to the inside of thestele.pitted thickening The most extensiveform of thickening found in XYLEM vesselsand tracheids, where the secondary cellwall is continuous apart from small areascalled PITS. The pits may be arranged in analternate or opposite pattern. If they areelongated horizontally and parallel to eachother, this is described as scalariformthickening.placenta (pl. placentae) 1. The region oftissue occurring on the inner surface of theovary wall of the carpels of floweringplants where the ovules develop. Thearrangement of ovules within the ovary(see placentation) depends on whetherthere are one or many carpels and whetherthe carpels are free or fused.2. A central swelling on the lower surfaceof fern pinnules linking clusters of sporangiawith the main frond. See also sorus.placentation The position of the ovulebearingplacentae in angiosperm seeds. Placentationvaries according to whether thereare one or many carpels. If there is only oneovule in the ovary, it may be attached atthe base of the ovary wall (basal placentation),or at the apex (apical placentation).If there is more than one ovule, and there isonly one carpel in the ovary, as in pea(Pisum sativum), placentation is along theventral suture and is termed marginal placentation.Marginal placentation also occursin apocarpous polycarpellary ovariesof LEGUMES but various types of placentationare seen in syncarpous ovaries. Wherethe carpels are fused to give a unilocularovary, the ovules may be found along thetwo placentae at each line of fusion givingparietal placentation, as in violet (Violaspp.) and passionflower (Passiflora). Ifinstead the ovules are borne on a centralcolumn, placentation is termed central placentation;a modification of this is freecentralplacentation, in which the columndoes not extend to the top of the ovary,as in primrose (Primula vulgaris). Whereother carpels to give a multilocular ovary,the marginal placentae of the carpels mayfuse in the center of the ovary, giving axileplacentation, as in tulip (Tulipa spp.).plagiotropismSee tropism.plankton A varied collection of aquaticorganisms that drift freely, not being attachedto any substrate and not possessingany organs for locomotion. The most importantcomponents of the plant plankton(phytoplankton) are the diatoms uponwhich the planktonic animals (zooplank-181

plant hormoneton) feed. Other members of the phytoplanktoninclude other microscopic algaeand cyanobacteria.The plankton form thebasis of the food chain in the sea. Comparebenthic.plant An organism that can (usually)make its own food by taking in simple inorganicsubstances and building these intocomplex molecules by a process termedphotosynthesis. This process uses light energy,absorbed by a green pigment calledchlorophyll, which is found in all plantsbut no animals. There are a few exceptions,in the form of certain parasitic plants.Most plants have cellulose cell walls, andhave starch as a storage polysaccharide,whereas animals have no cell walls, storeglycogen, and do not have plastids orchlorophyll; and fungi usually lack celluloseand also do not form plastids. Plantslack motility, with the exception of the gametesof many species. One major characteristicthat distinguishes plants from otherplantlike organisms, such as algae or fungi,is the possession of an embryo that is retainedand nourished by maternal tissue.Fungi and algae lack embryos and developfrom spores. Plants also differ from fungiin having a regular alternation of diploidand haploid generations. Plants also differfrom animals by generally responding tostimuli very slowly, the response often takinga matter of days and occurring only ifthe stimulus is prolonged. Compare algae;Fungi; Protoctista.plant hormoneSee hormone.plasmagel (ectoplasm) The gel-like regionof cytoplasm located in a thin layerjust beneath the plasma membrane, particularlyof unicells that move in an ameboidfashion, and cells that exhibit cyclosis.plasmagene A gene contained in astructure outside the nucleus, e.g. the genesin mitochondria and chloroplasts. See cytoplasmicinheritance.plasma membrane (cell membrane; plasmalemma)The membrane that surroundsall living cells. It ranges from7.5–10 nm in thickness and consists ofphosphoglyceride molecules (about 40%)and proteins (about 60%). The membranebackbone consists of phosphoglyceridemolecules arranged in two rows with theirhydrophilic polar heads facing outwardsand the hydrophobic hydrocarbon tails inthe center. This lipid layer is penetrated byglobular proteins, which in some places extendfrom one side of the membrane to theother. The whole structure has fluidlikeproperties, as the molecules are able tomove laterally – the so-called fluid-mosaicmodel. The membrane is probably beingconstantly renewed. The plasma membraneis selectively permeable, and controlswhich substances can enter and leave thecell.plasmasol (endoplasm) The sol-likeform of cytoplasm, located inside the PLAS-MAGEL. It is free-flowing and contains thecell organelles.plasmid An extrachromosomal geneticelement found within bacterial cells thatreplicates independently of the chromosomalDNA. Plasmids typically consist of circulardouble-stranded DNA molecules ofmolecular weight 10 6 –10 8 . They carry avariety of genes, including those for antibioticresistance (R factors), toxin production,and enzyme formation, and may beadvantageous to the cell. Some plasmids,called episomes, can reversibly insert themselvesinto the bacterial DNA, replicatingwith it.plasmodesma (pl. plasmodesmata) Afine strand of cytoplasm that serves to connectthe protoplasm of adjacent plant cells,allowing material to pass from one cell toits neighbor.See Myxomy-plasmodial slime moldscota.plasmalemma (pl. plasmalemmae)plasma membrane.SeePlasmodiophora A phylum of protoctiststhat are obligate endoparasites of182

pneumatophoreplants, fungi, and algae. Most plasmodiophoranscause little damage to the hostplant, but a few cause serious diseases, e.g.Plasmodiophora brassicae causes clubrootof cabbage and related brassicas.See Myx-plasmodium (pl. plasmodia)omycota.plasmogamy Fusion of protoplasm,usually referring to the fusion of cytoplasmbut not nuclei. It usually occurs betweengametes or protoplasts. Plasmogamy in theabsence of karyogamy (fusion of nuclei)occurs between fungal mycelia of differentstrains of some ascomycetes and basidiomycetesto form a heterokaryon.plasmolysis Loss of water from awalled cell (e.g. of a plant or bacterium)due to osmosis to the point at which theprotoplast shrinks away from the cell wall.The point at which this is about to happenis called incipient plasmolysis. Here the cellwall is not being stretched; i.e. the cell haslost its turgidity or become flaccid (wallpressure is zero). Wilting of herbaceousplants occurs at this point.plastid An organelle enclosed by twomembranes (the envelope) that is found inplants and certain protoctists (e.g. algae),and develops from a proplastid. Varioustypes exist, but all contain DNA and ribosomes.Plastids range from 10 m m to lessthan 1.0 m m in diameter. They includechloroplasts, chromoplasts, amyloplasts,etc., and are often interconvertible.plastid inheritanceSee plastogene.plastocyanin An electron carrier inPHOTOSYNTHESIS, involved in the transfer ofelectrons from photosystem II to photosystemI.plastogene A gene present in a selfreplicatingplastid. Inheritance studies haveshown that plastogenes control leaf colorin some plants, e.g. yellow-leaved Primulasinensis, is an example of plastid inheritance.See cytoplasmic inheritance; plasmagene.pleiomorphism The occurrence of differentmorphological stages during the lifeof an organism. Examples are the differentspore forms of the rust fungi. Comparepolymorphism.Pleistocene The first epoch of the Quaternaryperiod, which started with a glaciationabout two million years ago and endedwith the last glaciation about 10 000 yearsago. Several ICE AGES drove many organismstoward the equator while others becameextinct. For example, tundra coveredtemperate parts of the USA and CentralEurope where today deciduous forests arethe natural vegetation. The boreal forestsextended south almost to northernLouisana and the Mediterranean. Fossilpollen deposits provide evidence for thepast distribution of plants. Some migratingplants found the tops of mountains furthersouth resembled the conditions they hadleft, and survived in isolated areas (refugia).When the climate warmed again,these became centers of diversity. The cycleof southward then northward migrationled to the mixing of plants from bothcooler and warmer regions, leading to newpatterns of competition and reoorganizationof plant communities. Modern man(Homo sapiens) evolved during this period,and probably caused further disruption ofplant communuties by his use of fire andthe start of forest clearance. See also geologicaltime scale; pollen analysis.pleromeSee histogen theory.pleuropneumonia-like organismsmycoplasmas.SeePliocene The epoch of the Tertiary period,about 7 to 2 million years ago, whichfollowed the Miocene. In the Pliocene thehominids, such as Australopithecus andHomo, became clearly distinguishablefrom the apes. Grasses became more abundant,perhaps because the climate wasbecoming drier. Modern conifers were183

Poaceaespreading, and leptosporangiate ferns underwentadaptive radiation. See also geologicaltime scale.plumule The shoot apex and first rudimentaryleaves in the mature embryo andthe seedling. In seedlings showing epigealgermination, the plumule is taken aboveground between the cotyledons. When germinationis hypogeal, only the plumuleemerges from the soil; it has a hooked tipto protect it as it pushes up through thesoil. Compare radicle.pneumatophore (aerophore; breathingroot) A specialized negatively geotropicroot produced by certain aquatic vascularplants (especially trees such as mangroves,e.g. Sonneratia) that protrudes well abovesoil level. The aerial part is covered withLENTICELS through which gases can diffuseto and from the highly developed systemof intercellular airspaces. Pneumatophoresare an adaptation to waterlogged, poorlyaerated soils, allowing roots to exchangegases.Poaceae (Gramineae, grasses) Thegrasses, a family of monocotyledons with9500 species. Most are annual or perennialherbs, although some are woody (but donot have secondary thickening), e.g. bamboos.Grasslands are the dominant vegetationon the prairies, steppes and savannas,and in times past supported vast herds ofgrazing mammals. Today these areas arealso important are for growing cerealgrasses, such as barley (Hordeum vulgare),maize (Zea mays), millet (Panicum miliaceum),oats (Avena sativa), rye (Secalecereale), sorghum (Sorghum), and wheat(Triticum). Rice (Oryza sativa) and sugarcane (Saccharum officinarum) are alsograsses.podSee legume.Podocarpaceae A family of evergreenconifers. It includes one of the oldest livingorganisms, a huon pine, Dacrydiumfranklinii, from Tasmania, possibly 10 000years old.podsol (podzol) The type of SOIL foundunder heathland and coniferous forests intemperate and boreal climates where thereis heavy rainfall and long cool winters,such as the coniferous forests of northernNorth America and Eurasia. It is stronglyacid and often deficient in nutrients, especiallyiron compounds and lime, as a resultof leaching by the heavy rain or snow-meltin spring. Beneath the humus layer lies aleached horizon (A horizon) composedmainly of quartz sand. In the clay-rich Bhorizon below the leached layer iron compoundsaccumulate, staining the layerbrown and forming an impermeable hardpan that prevents drainage. Where thislayer is sufficiently impermeable to maintainthe A horizon in a waterlogged condition,the soil is termed a gley podsol.point mutationSee gene mutation.polarity A lack of symmetry: a definiteorientation that arises during the differentiationof a cell, tissue, or organ. For example,in the development of the plantembryo from the zygote, one end becomesthe radicle, the other gives rise to cotyledons.polarizing microscope A light MICRO-SCOPE that uses polarized light to illuminatethe specimen. The orientation of moleculesin certain crystalline substances affects thetransmission of polarized light.polar nuclei The, usually two, haploidnuclei found midway along the EMBRYOSAC, forming a binucleate central cell. Theymay fuse to form the diploid definitive nucleus.The endosperm is formed from thefusion of one or both polar nuclei (occasionallyseveral) with one of the male gametesfrom the pollen tube.pollen The MICROSPORES of seed plants,produced in large numbers in the pollensacs, or stamens. They are formed by meiosisof somatic pollen mother cells. Pollengrains are rich in mitochondria, Golgi apparatus,and endoplasmic reticulum. Inmany gymnosperms the nuclei dividewithin the pollen grain to produce a small184

pollutionPollen: section of a pollen grain showing the exinenumber of cells (2–40 depending on thegroup) representing the male prothallus. Inangiosperms and some conifers (e.g. Cupressus)the pollen grains contain just twonuclei: a GENERATIVE NUCLEUS and a VEGE-TATIVE NUCLEUS (tube nucleus).When apollen grain germinates, a pollen tubepushes its way out through an aperture inthe pollen grain wall, and the nuclei migrateinto the tube. The generative nucleusdivides to produce two sperm cells consistingof only nucleus and cytoplasm, one ofwhich will fertilize the ovum, while theother will fuse with the POLAR NUCLEI toform the endosperm. The vegetative nucleusdegenerates after fertilization. Pollengrains are highly variable in size and shaperanging from 5m to over 200m in diameterand from circular to tetrahedral or dumbbellshaped. The wall of the pollen grainconsists of two layers, a highly resistantouter exine, which may be intricatelysculpted and is punctured by apertures,and an inner intine, which sometimes protrudesthrough these apertures. The sizeand shape of the pollen grains and thenumber and orientation of the aperturesand sculpturing is characteristic of particularfamilies, genera, or sometimes species.Pollen grains are adapted according to themethod of pollination, those carried by insectsoften being sticky or barbed, whilewind-pollinated plants generally producesmooth light pollen. See also pollen analysis.pollen analysisA means of obtaininginformation on the composition and extentof past floras and environments by examiningfossil pollen grains, spores, and similarstructures in peat and sedimentarydeposits. The outer wall (exine) of thepollen grain is often diagnostic (see pollen)and very resistant to decay. See also palynology.pollen cultureSee anther culture.pollen mother cell (PMC) In angiospermsand gymnosperms, a somaticcell that gives rise to four haploid pollengrains by meiosis.pollen sac A chamber in which thepollen is formed in the angiosperms andconifers.pollen tube A filamentous outgrowth ofthe pollen grain that in most seed plantsgrows out through a pore in the exine andtransports the male gametes to the ovule.Germination of the pollen grain to give thepollen tube usually takes place only whenthe pollen is compatible with the stigma tissue.The pollen tube may grow down thestylar canal, or it may secrete enzymes thatdigest the tissues of the style, creating apath for the tube. It usually enters the ovulethrough the micropyle, but in some species,such as beech (Fagus) it may enter throughthe chalaza (chalazogamy).pollination The transfer of pollen fromthe anther to the stigma in angiosperms,185

polyembryonyand from the sporangiophores to the micropylein gymnosperms. If the pollen iscompatible (i.e. of the right type) then thepollen grains germinate, producing apollen tube that grows down the stylecarrying the pollen nuclei to the ovule.Plants may be self-pollinating, e.g. barley(Hordeum vulgare), thus ensuring thatseed will be set, even in the absence ofother members of the same species. However,self-pollination also leads to homozygosityand, potentially, less adaptableplants in the long term; thus, in many plantspecies mechanisms exist to prevent it andpromote CROSS-POLLINATION. In some speciesof angiosperms stamens and stigmasmature at the same time and pollen mayfall directly onto the stigma of the sameflower, or the stamens and stigmas may actuallytouch. See also cleistogamy; dichogamy;homogamy; incompatibility;protandry; protogyny.pollution Any damaging or unpleasantchange in the environment that resultsfrom the physical, chemical, or biologicalside-effects of human activities. Pollutioncan affect the atmosphere, rivers, seas, andthe soil.Air pollution is caused by the domesticand industrial burning of carbonaceousfuels, by industrial processes, and by vehicleexhausts. Industrial emissions of sulfurdioxide cause acid rain, and the release intothe atmosphere of chlorofluorocarbons,used in refrigeration, aerosols, etc., leads tothe depletion of ozone in the stratosphere(see acid rain; ozone layer). Carbon dioxide,produced by burning fuel and bymotor vehicle exhausts, is slowly buildingup in the atmosphere, and is one of the factorsleading to an overall increase in thetemperature of the atmosphere (see greenhouseeffect). Vehicle exhausts also containcarbon monoxide, nitrogen oxides, andother hazardous substances, such as fineparticulate dusts. Water pollutants includethose that are biodegradable, such assewage effluent and nitrates leached fromagricultural land, which if allowed to enterwater courses can lead to eutrophication(see eutrophic) and algal blooms. Nonbiodegradablepollutants, such as certainchlorinated hydrocarbon pesticides (e.g.DDT) and heavy metals, such as lead, copper,and zinc in some industrial effluents,accumulate in the environment. Heavymetals, even when present at low concentrations,are toxic to plants, inhibitingwater or nutrient uptake, damaging cellmembranes, and inhibiting enzymes.polyembryony The occurrence of manyembryos in one ovule. See apomixis;parthenogenesis.polymer A molecule made up of a seriesof similar units joined together, e.g. starch,which is made up of glucose units.polymerase An enzyme that regulatesthe synthesis of a polymer. Examples includeRNA polymerases and DNA polymerases.polymerase chain reaction (PCR) Atechnique for amplifying small samples ofDNA rapidly and conveniently. It is usedwidely in research and forensic science, e.g.to clone genes or to produce a suitablequantity of DNA for genetic fingerprintingfrom the minute amounts present in tracesof blood or other tissue. See also geneticfingerprinting.polymorphism A distinct form of variationin which significant proportions ofdifferent types of individuals exist within apopulation of a species at the same timeand in the same place, such that the frequencyof the rarest form cannot be explainedon the basis of recurring mutation.The term is generally used where the frequencyof the least common morph exceeds1 in 20, less frequent occurrencesbeing usually due to rare mutation or recombinationevents. If the differences persistover many generations then there is abalanced polymorphism. Compare pleiomorphism.polypeptide A compound that containsmany amino acids linked together by peptidebonds. The three-dimensional shape186

potometer(conformation) of the poplypeptide chainis determined by its amino acid sequence,but can be disrupted by extremes of pH orheat. Proteins are made up of one or morepolypeptides. See peptide; denaturation;conjugated protein.polyploid The condition in which a cellor organism contains three or more timesthe haploid number of chromosomes. It occurswhen chromosomes fail to separateduring anaphase I of meiosis, giving rise togametes that are diploid instead of haploid.Fertilization results in triploid or tetraploidindividuals. Polyploidy is far more commonin plants than in animals and veryhigh chromosome numbers may be found;for example in octaploids and decaploids(containing eight and ten times the haploidchromosome number). Some 60% ofmonocotyledons and 40% of dicotyledonsare polyploid, as are 90% of ferns, manybryophytes, and some algae. See allopolyploidy;autopolyploidy; heterosis.Polypodiaceae A family of terrestrial orepiphytic leptosporangiate ferns.polyribosomeSee ribosome.polysaccharide A polymer of 10 ormore monosaccharides joined by glycosidiclinks (see glycoside). They containmany repeated units in their molecularstructures and are of high molecularweight. They can be broken down tosmaller polysaccharides (oligosaccharides),DISACCHARIDES, and MONOSACCHARIDES byhydrolysis or by the appropriate enzyme.Important polysaccharides are inulin (hydrolyzedto fructose), starch (hydrolyzed toglucose), and cellulose (hydrolyzed to glucose).Polysaccharides may form branchingchains, or they may be unbranched. Theyhave many roles in plants, as storage molecules(e.g. starch), structural elements incell walls (cellulose), and be components ofgums and mucilages (glucuronic acid). Apolysaccharide containing only one type ofsugar residue is called a homopolysaccharide,e.g. starch, which contains only glucoseunits. Heteropolysaccharides containtwo or more different monosaccharides,e.g. hemicelluloses. See also carbohydrates;sugar.polysomepolysomySee ribosome.See aneuploidy.polystely Having many steles, as in thestems of some Selaginella species. See alsodistely.pome A fleshy pseudocarp fruit, characteristicof the family Rosaceae, e.g. apple(Malus). The fleshy part of the fruit is derivedfrom the greatly enlarged urn-shapedreceptacle, which encloses the entire fruit,and which is fused to the tough carpelwalls. Thus the apple core is the true fruit.population dynamics The study of thefactors influencing the fluctuations in numbersin a population or of its gene pool: thefactors affecting birth and mortality rates,immigration and emigration, and reproductivepotential (for example, seed size,dispersal, and dormancy).population A group of interbreedingorganisms of the same species (or othergroups within which individuals may exchangegenetic information) occupying aparticular space. A population is continuallymodified by increases (birth and immigration)and losses (death and emigration),and is limited by the effects of environmentalfactors such as disease. Many plantscannot be divided strictly into populationsas they may reproduce vegetatively, withthe result that one individual can occupy alarge area and parts can detach.porogamy The usual method of fertilizationin angiosperms in which the pollentube enters the ovule by the micropyle.Compare chalazogamy.porphyrins Cyclic organic structurescontaining tetrapyrrole rings (four joinedrings, each consisting of four CH units andone NH unit) that have the important characteristicproperty of forming complexes187

PPLOwith metal ions. Examples of such metalloporphyrinsare the iron porphyrins (e.g.heme in hemoglobin) and the magnesiumporphyrin, chlorophyll, the photosyntheticpigment in plants. In nature, the majorityof metalloporphyrins are conjugated toproteins to form a number of very importantmolecules, such as the cytochromes.potassium One of the essential elementsin plants. It is absorbed by plant roots asthe potassium ion, K + , and in plants is themost abundant cation in the cell sap. Potassiumions are required in high concentrationsin the cell for efficient proteinsynthesis, and for glycolysis in which theyare an essential cofactor for the enzymepyruvate kinase. Potassium ions balanceanions and organic acids, and are thus importantin osmoregulation, the control ofstomatal opening, and leaf movements(through their effects on the turgor of pulvini).A deficiency of potassium leads to acharacteristic red or purple coloration ofthe leaves, poor root growth, and flowerand fruit formation.potometer An apparatus for measuringthe rate of water uptake by a cut shoot orwhole plant. This is normally closely relatedto the rate of water loss by transpiration,and the potometer can be used tocompare transpiration rates under differentconditions. The rate of uptake is measuredby the progress of an air bubble in acapillary tube along a scale. More meaningfulcomparisons can be made by comparingwater loss from the potometer withthat from an atmometer (an apparatusused to measure the rate of evaporationfrom a porous pot or other nonliving wetsurface), which allows uncontrolled evaporation.PPLOP R (P660)See mycoplasmas.See phytochrome.Precambrian The time in the earth’s geologicalhistory that precedes the Cambrianperiod, i.e. from the origin of theearth, nearly 5 billion years ago, to thestart of the Cambrian, around 570 Ma(million years ago). The term ‘Precambrian’is now used mainly descriptively,and has been largely discarded as a geologicalterm in the light of greater knowledgeof the early evolution of life. Precambriantime is now divided into three eons:Hadean, from the earth’s origin to about3900 Ma; Archaean, 3900–2390 Ma; andProterozoic, 2390–570 Ma (the Cambrianmarks the start of the Phanerozoic eon,which extends to the present day).The oldest fossils discovered so far areremains of bacterialike organisms, datingfrom about 3500 Ma. There is abundantevidence of flourishing colonies of cyanobacteriaand other bacteria throughout theArchaean and Proterozoic eons. This takesthe form of STROMATOLITES, rock structuresrepresenting the remains of sedimenttrapped or precipitated by bacterial communities.Stromatolites are still found in afew special locations today. The ancientrocks also contained spherical carbonaceousaggregates up to 20 mm diameter,that resemble algae or the cysts of flagellates,and delicate carbonaceous filamentsup to 150 m m long. The first appearanceof multicellular animals is in the so-calledEdiacara fauna, in rocks dated to the last100 million years of Precambrian time.Around this time the stromatolites beganto decline, perhaps as they were beingeaten by newly evolved multicellular animals.pressure potentialwater potential.See turgor pressure;prickle A protective multicellular outgrowthfrom the surface of a plant. It is amodified trichome and thus completelyepidermal in origin. Compare spine, thorn.See en-primary endosperm nucleusdosperm.primary growth Growth derived solelyfrom meristems present in the embryo, i.e.apical meristems. Such growth generallyincreases the length of plant organs. All thetissues of the young plant are the result of188

promoterprimary growth. Compare secondarygrowth.primary phloem PHLOEM derived fromthe procambium in the young plant. It consistsof the PROTOPHLOEM (formed first)and METAPHLOEM. In nonwoody plants it isthe only food-conducting tissue, but in matureplants with secondary growth thisfunction is taken over by the secondaryphloem. See secondary growth.primary structure The sequence ofamino acids in a polypeptide chain. Seealso conformation; secondary structure,tertiary structure; quaternary structure.primary tissue Plant tissue that is derivedsolely from meristems present in theembryo and their derivatives. See primarygrowth.primary xylem Xylem derived from theprocambium in the young plant. It consistsof the protoxylem (formed first) andmetaxylem. In nonwoody plants the primaryxylem is the only water-conductingtissue, but in mature plants with secondarygrowth this function is mostly taken overby the secondary xylem. See secondarygrowth.primordium (pl. primordia) A collectionof cells that differentiates into a specializedcell, tissue or organ, e.g. the apicalshoot and apical root primordia of the embryo.The term is used particularly in relationto a part of the apical meristem thatlater differentiates further.probability The chance that a givenevent will occur, or that over a series of observationsa particular kind of observationwill occur regularly as a given proportionof the total number of observations. Statisticalprobability is usually based on aninfinite number of observations. For example,in genetics, if a heterozygous plant isselfed, the probability of finding the doublerecessive is 1 in 4, or 25%. The greater thenumber of offspring the better the chancethat this actual percentage will be achieved.procambium (procambial strand) Thelayer of cells that gives rise to the vasculartissue. Part of the apical meristem, it is discernablejust below the apex as a strand offlattened cells which, if traced back alongthe shoot or root, may be seen to give riseto the primary vascular tissues. It is continuouswith the intrafascicular cambium. Inroots it may also be called plerome. See histogentheory; primary growth.procaryoteSee prokaryote.producer The first TROPHIC LEVEL in afood chain. Producers are those organismsthat can build up foods from inorganic materials,i.e. green plants, algae, and photosyntheticand chemosynthetic bacteria.Producers are eaten by herbivores (primaryCONSUMERS).proembryo A young plant after fertilizationbut before differentiation into embryoand suspensor tissue.profundal The deepwater zone of a lakebeyond a depth of 10 meters. Little lightpenetrates this zone and thus the inhabitantsare all heterotrophic, depending onthe littoral and sublittoral organisms forbasic food materials. Commonly found inhabitantsinclude bacteria, fungi, mollusks,and insect larvae. Species found in the profundalzone are adapted to withstand lowoxygen concentration, low temperatures,and low pH. Compare littoral; sublittoral;photic zone.prokaryote (procaryote) An organismwhose genetic material (DNA) is not enclosedby membranes to form a nucleus butlies free in the cytoplasm. Organisms canbe divided into prokaryotes and EUKARY-OTES, the latter having a true nucleus. Inprokaryotes the nuclear material is simplya circular strand of DNA, not complexedwith histone proteins, and cell divisionis amitotic. This is a fundamental divisionbecause it is associated with other majordifferences. Prokaryotes constitute thekingdom Bacteria (formerly the Prokaryotae).Recent molecular evidence suggests189

prophagethe prokaryotes actually comprise two distinctkingdoms, or even domains, the Archaeaand the Eubacteria or Bacteria.Eukaryotes comprise all other organisms.Prokaryote cells evolved first and gave riseto eukaryote cells. They are normally muchsmaller (about 1m m in diameter) than eukaryotecells (about 20m m). Prokaryoteslack organelles such as nucleoli, mitochondria,plastids, Golgi apparatus, endoplasmicreticulum, and Golgi apparatus, anddo not exhibit cyclosis. Their ribosomesare smaller (70S) than those of eukaryotes(80S) – similar in size to the ribosomes ofchloroplasts and mitochondria. This hasled to the theory (endosymbiont theory)that eukaryotes arose as a result of symbioticassociations between prokaryotes. SeeBacteria; cell; endosymbiont theory.proline A nonpolar molecule synthesizedfrom glutamic acid and broken downto glutamic acid by the same pathway. Thederivative 4-hydroxyproline is an importantcomponent of cell walls. Proline hasbeen found in every protein studied to date.See amino acids.promoter A specific DNA sequencewithin an OPERON that initiates TRANSCRIP-TION by binding RNA polymerase. Thepromoter is situated between the operatorand the structural genes or genes. It is thestarting point for transcription of the structuralgenes in the operon, but is not itselftranscribed. In Escherichia coli the RNApolymerase has a protein ‘sigma factor’that recognizes the promoter; in the absenceof this factor the enzyme binds to,and begins transcription at, random pointson the DNA strand. In eukaryotic cells,binding of RNA polymerase to the promoterinvolves proteins called transcriptionfactors.prophageSee lysogeny.prophase The first stage of cell divisionin meiosis and mitosis. During prophasethe chromosomes become coiled, shorteningand thickening and becoming visible,and the nuclear membrane dissolves.Prophase may be divided into successivestages termed leptotene, zygotene, pachytene,diplotene, and diakinesis. In leptotenethe chromosomes, already replicated, startto condense and appear as fine threads, althoughsister chromatids are not yet distinct.The spindle starts to form around theintact nucleus. Zygotene is characterizedby the active and specific pairing (synapsis)of homologous chromosomes leading tothe formation of a haploid number of bivalents.In pachytene the paired homologouschromosomes contract. At this point eachchromosome consists of a pair of chromatidsand the two associated chromosomesare termed a tetrad. In diplotene thepairs of chromatids begin to separate fromthe tetrad formed by the association of homologouschromosomes. Chiasmata canoften be seen at this stage. At diakinesis thesister chromatids of homologous pairs ofchromosomes complete their separation,and the chromosomes coil tightly, shorteningand thickening. In practice, there is noclear demarcation between these stages:the process is continuous. The events occurringduring these stages differ in meiosisand mitosis, notably in that bivalents (pairsof homologous chromosomes) are formedin meiosis, whereas homologous chromosomesremain separate in mitosis. See alsomeiosis; mitosis.proplastid A self-duplicating undifferentiatedplastid, about 0.5–1 m m in diameterand found in the meristematic regionsof plants. They grow and develop into plastidsof different types.prop root An adventitious root, arisingfrom one of the lower nodes of the stemclose to the soil surface, found at the junctionof stem and soil that grows down tothe ground and gives additional support tothe stem, as seen in maize (Zea mays).Some palms, mangroves and other tropicaltrees form woody prop roots, often calledstilt roots. See also buttress root.prosthetic group The nonprotein componentof a conjugated PROTEIN. Thus theheme group in hemoglobin is an example190

Protistaof a prosthetic group, as are the coenzymecomponents of a wide range of enzymes.e.g. the heme group is an iron–porphyrincomplex that acts as a prosthetic group inthe cytochromes, important components ofelectron-transport chains in both plantsand animals. See enzyme.protandry The maturation of the anthersbefore the stigma, a very commoncondition seen in many specialized insectpollinatedflowers, especially those pollinatedby bees. It helps to promotecross-pollination and is often combinedwith self-incompatibility. Compare protogyny.See also dichogamy.protease (proteinase) An enzyme thatcatalyzes the hydrolysis of peptide bonds inproteins to produce peptide chains andamino acids. Individual proteases arehighly specific in the type of peptide bondthey hydrolyze.protein A large, complex moleculemade up of one or more polypeptidechains, i.e. it consists of amino acid moleculesjoined together by peptide links. Themolecular weight of proteins may varyfrom a few thousand to several million.About 20 amino acids are present in proteins.Simple proteins contain only aminoacids. In conjugated proteins, the aminoacids are joined to other groups.The primary structure of a protein is theparticular sequence of amino acids present;this determines the three-dimensionalshape of the molecule. The secondarystructure is the way in which this chain isarranged; for example, coiled in an alphahelix or held in beta-pleated sheets. Thesecondary structure is held by hydrogenbonds. The tertiary structure of the proteinis the way in which the protein chain isfolded. This may be held by cystine bonds(disulfide bridges) and by weak polar andhydrophobic attractive forces betweenatoms. The quaternary structure is thecombination of more than one polypeptidechain. There are two main groups of proteins:globular proteins and fibrous proteins.Globular proteins have the morecomplex tertiary structure, and are mostlyenzymes. Fibrous proteins usually have astructural or contractile role. Proteins arevital compounds found in all living organisms.Their functions range from enzymesand hormones to electron-transfer molecules,contractile elements, and importantgenetic elements such as the histones thatcoat DNA in eukaryotic chromosomes. Bycoding for structural proteins and for theenzymes that catalyze the synthesis ofother proteins and biological molecules,genes control the functioning and developmentof all living organisms.proteinaseSee protease.protein sequencing The determinationof the primary structure of proteins, i.e. thetype, number, and sequence of amino acidsin the polypeptide chain. The most widelyused procedure is the Edman degradation.This uses a series of three chemical reactions,each taking place at a specific pH, toremove one amino acid at a time from theamino terminus of a protein. The aminoacids are then identified by ion-exchangechromatography. The process is automatedin an amino acid analyzer. TheEdman degradation can sequence peptideswith only 5–10 picomoles of protein present.protein synthesis The process wherebyproteins are synthesized on the ribosomesof cells. The sequence of bases in MESSEN-GER RNA (mRNA), transcribed from DNA,determines the sequence of amino acids inthe polypeptide chain: each CODON in themRNA specifies a particular amino acid.As the ribosomes move along the mRNA inthe process of translation, each codon is‘read’, and amino acids bound to differenttransfer RNA molecules are brought totheir correct positions along the mRNAmolecule. These amino acids are polymerizedto form the growing polypeptidechain. See also transfer RNA; translation.proteolysis The hydrolysis of proteinsinto their amino acids. Enzymes that cat-191

protocooperationalyze this are proteases or proteolytic enzymes.proteomics The compilation and characterisationof all the proteins in an organism– its proteome.proteoplast A colorless plastid (leukoplast)that stores protein.ProterozoicSee Precambrian.prothallus (pl. prothalli) A flatteneddisk of cells that forms the free-living haploidgametophyte generation of certainferns, e.g. Dryopteris. In homosporousplants, there is only one type of prothalluswith both male and female sex organs. Inheterosporous plants, the microspores giverise to small male prothalli bearing malesex organs (antheridia), and larger femaleprothalli bearing female sex organs(archegonia). The prothallus is greatly reducedin spermatophytes. See alternationof generations.Protista In some classifications, a kingdomof simple eukaryotic organisms includingthe algae and protozoans. It wasintroduced to overcome the difficulties ofassigning such organisms that may showboth animal and plantlike characteristicsto the kingdoms Animalia or Plantae.Today the grouping is considered to be artificialand many taxonomists support theFIVE KINGDOMS CLASSIFICATION whereby thebacteria and fungi are both assigned to separatekingdoms, while algae and protozoansconstitute various phyla of thekingdom PROTOCTISTA.protocooperationSee mutualism.Protoctista A kingdom of simple eukaryoticorganisms that includes the algae,slime molds, funguslike oomycetes, and theorganisms traditionally classified as protozoa,such as flagellates, ciliates, and sporozoans.The members of this kingdom donot share clear phylogenetic links, but aregrouped together simply because they donot not belong in any of the other fourkingdoms. Most are aerobic, some are capableof photosynthesis, and most possessundulipodia (flagella or cilia) at some stageof their life cycle. Protoctists are typicallymicroscopic single-celled organisms, suchas the amebas, but the group also has largemulticellular members, for example theseaweeds and other large algae, some ofwhich are sometimes classified as plants.Protoctists show a wide range of nutritionalhabits, including photoautotrophs,heterotrophs, phagotrophs and mixotrophs.protoderm The outermost layer of theapical meristem: the tissue that developsfrom the tunica initials of the apical meristemand gives rise to the epidermis and, insome plants, the root cap. In the root itmay also be called the dermatogen. See histogentheory.protogyny The maturation of thestigma before the anthers. It is a mechanismthat helps prevent self-pollinationand is most common among wind-pollinatedand unspecialized insect-pollinatedflowers. as in figwort. It is less commonthan protandry. Compare protandry. Seealso dichogamy.protonema (pl. protonemata) The youngbryophyte gametophyte that develops followingspore germination. The mature gametophyteplants develop from buds thatform at several points along the protonema.proton pump An energy-requiring metabolicprocess that causes protons (hydrogenions) to move across a membraneagainst their concentration gradient. Thiscreates a potential difference across themembrane, a source of potential energythat can be used to drive other metabolicprocesses. For example, the inner membraneof the mitochondrion is impermeableto the movement of protons. Duringrespiratory electron transport, protons aredrawn from the internal matrix into thespace outside the membrane, creating a potentialdifference across the membrane. In192

proximalrespiration and photosynthesis, the movementof protons back across a membranedown a diffusion gradient takes place atsites where it can be coupled to ATP synthesis,using the enzyme ATPase. The energythus trapped can be transported toother parts of the cell. See chemiosmotictheory; electron-transport chain.Pseudocarp: various types of pseudocarp193

pseudoalleleprotophloem The first-formed PRIMARYPHLOEM, differentiated from the procambiumin the region just behind the meristem.protoplasm The living contents of acell, comprising the cytoplasm plus nucleoplasm.The term usually excludes the largevacuoles. See cytoplasm; nucleus.protoplast The protoplasm and plasmamembrane of a cell after removal of the cellwall, where present. This can be achievedby physical means or by enzymic digestion.protostele A simple form of stele, uninterruptedby leaf gaps, consisting of xylemin the center completely surrounded byphloem. Most roots have protosteles. Thestems of some lower plants (e.g. Lycopodium)also have protosteles.protoxylem The PRIMARY XYLEM elementsthat are formed from the procambiumfirst. In seed plants the protoxylemoccurs at the innermost edge of the vascularbundles in stems, but external to thelater-formed xylem in roots.protozoa An old name for animal-likePROTOCTISTA that included some photosyntheticorganisms (e.g. Euglena, Volvox).proximal Denoting the part of an organthat is nearest the origin or point of attachment.Compare distal.pseudoallele A mutation in a gene thatproduces an effect similar to another mutationat a different site in the same genelocus. They are closely linked, so recombinationsbetween them are rare. See cis–trans effect.Ptyxis: various types of leaf ptysis194

purinepseudocarp (false fruit) A fruit that includesparts of the flower, e.g. the bracts,inflorescence, or receptacle, other thanthose derived from the ovary. See also hip;fruit; pome. See illustration on page 193.pseudogene A DNA sequence that cannotbe transcribed. Although they have noimmediate function, pseudogenes havehigh potential to form new genes by furthermutation as they already have useful sequences,such as those signaling transcription.Pseudogenes are genes that havebecome ‘switched off’ during the course ofevolution.pseudoparenchyma A fungal or algaltissue resembling parenchyma but made upof interwoven hyphae (fungi) or filaments(algae). The stipe of the mushroom and thethallus of red algae (e.g. Porphyra) arepseudoparenchymatous tissues.pseudoplasmodium See Acrasiomycota.pseudopodium (pl. pseudopodia) 1. Aleafless stalk that bears the capsule inSphagnum and other mosses that lack aseta.2. In slime molds, a projection of the bodyused for locomotion.3. A temporary fingerlike projection orlobe on the body of an ameboid cell. It isformed by a flowing action of the cytoplasmand functions in locomotion andfeeding.Psilophyta (whisk ferns) In the FiveKingdoms classification scheme, a phylumof vascular seedless plants, the whiskferns. In most modern classifications theyare placed in the class Psilopsida of theFilicinophyta. There are only two livingsubtropical genera (Psilotum and Tmesipteris).psychrophilic Describing microorganismsthat can live at temperatures below20°C. Compare mesophilic; thermophilic.pteridophyte (Pteridophyta) A generalterm, now largely obsolete, to include anyvascular nonseed- bearing plant. Pteridophytesinclude the club mosses (phylumLycophyta), horsetails (Sphenophyta),ferns (Filicinophyta), and whisk ferns(Psilophyta).ptyxisThe way in which young leavesPunnet squares195

pyramid of biomassPurineare folded or rolled in the bud. See also vernation.pulvinus (pl. pulvini) 1. A specializedgroup of cells with large intercellularspaces that are located at the bases ofleaves or leaflets in certain plants. They areinvolved in nongrowth nastic movements,bringing these about by rapid changes inturgor through loss of water to the intercellularspaces. See nyctinasty.2. A thickened region at the node of a grassstem, which often contains an INTERCALARYMERISTEM.punctuated equilibrium A theory ofevolution proposing that there have beenlong periods of geological time, lasting forseveral million years, when there is littleevolutionary change, punctuated by shortperiods of rapid speciation of less than100 000 years. This is in contrast to thetraditional theory (see neo-Darwinism) inwhich it is postulated that species haveevolved gradually throughout geologicaltime.Punnett square A chequerboard diagramused in genetics to illustrate the frequenciesof various genotypes in theoffspring of a particular cross.pure line The succession of descendantsof an individual that are identical to eachother and continue to breed true, i.e. theyproduce genetically identical offspring.They are assumed to be homozygous.purine A simple nitrogenous organicmolecule with a double-ring structure – a6-membered pyrimidine ring fused to a 5-membered imidazole ring. Members of thepurine group include adenine and guanine,which are constituents of the nucleic acids,and certain plant alkaloids, e.g. caffeineand theobromine.pyramid of biomass A type of ecologicalpyramid based on the total amount ofliving material at each trophic level in thecommunity, which is normally measuredby total dry weight or calorific value perPyrimidineunit area or volume, and shown diagrammatically.The biomass depends on theamount of carbon fixed by green plantsand other producers. The pyramid of biomassusually has a more gentle slope thanthe PYRAMID OF NUMBERS because organismsat successively higher levels in thepyramid tend to be larger than thosebelow.pyramid of numbers A type of ecologicalpyramid in which the number of individualorganisms at each stage in the foodchain of the ecosystem is depicted diagrammatically.The producer level formsthe base, and successive levels the tiers. Theshape of the pyramid of numbers dependsupon the community considered; generally,the organism forming the base of a foodchain is numerically very abundant, and196

Qquadrat A square area of variable sizedepending on the study (standard sizes are0.5 or one meter square to 10 m square ormore) taken at random, within which thecomposition of organisms is noted. Thequadrat sampling technique is mostly usedin plant ecology to study the distributionand abundance of species in plant communities.See discontinu-qualitative inheritanceous variation.See discontinu-qualitative variationous variation.See continu-quantitative inheritanceous variation.quantitative variationvariation.See continuousquaternary structure The pattern inwhich two or more polypeptide chains arelinked together to form a protein molecule.This involves hydrogen bonds, ionicbonds, disulfide bridges and hydrophobicinteractions. See conformation; protein.Compare primary structure; secondarystructure; tertiary structure.Quaternary (Neogene) The most recentperiod of the Cenozoic era from about twomillion years ago to the present day andcomposed of the Pleistocene and Holoceneepochs. It was a period of cyclical climatechange, including several ice ages in highand mid-latitudes, cycles of varying aridityand humidity in low latitudes, and significantfluctuations in sea level. This had importantimplications for plant speciation,as species ranges expanded and contracted,new species radiated out as climate ameliorated,and retreating ice afforded opportunitiesfor colonization of newly exposedland. The Quaternary is also characterizedby the emergence of humans and the startof human modification of the environment.Increased rates of evolution and extinctionof mammals (due to both climatechange and human activity) also affectedplant distribution. See also Ice Age.quiescent center A group of cells in thecenter of the apical meristem in which mitoticdivisions are rare or absent. The cellsmay begin dividing if another part of themeristem is damaged.quillwortsSee Isoetales.197

Rrace In classification, a taxon below therank of SPECIES, sometimes being placed betweensubspecies and variety. The term issometimes used instead of ECOTYPE, forgroups of individuals that have uniformecological preferences or physiological requirements.raceme (racemose inflorescence; indefiniteinflorescence) See inflorescence.rachis 1. The main stalk on which theleaflets are borne in a compound leaf, or onwhich the pinnae are attached in the fernleaf.2. The central axis of certain inflorescences,e.g. wheat (Triticum), that bearsthe spikelets.radial 1. Aligned along the radius of acircle or cross-section.2. See anticlinal.radial symmetry The arrangement ofparts in an organism in such a way thatcutting in any plane across the diametersplits the structure into similar halves (mirrorimages). Many stems and roots showradial symmetry. The term actinomorphyis generally used to describe radial symmetryin flowers. See also bilateral symmetry.radicle The embryonic root. It is the firstorgan to emerge from the seed on germination.The radicle is joined to the hypocotyland both tissues are derived from the fouroctants nearest the suspensor. The root capover the tip of the radicle is derived fromthe closest cell of the suspensor. The radiclemay persist to form a taproot, or it maybe replaced by adventitious roots.See radiometric dat-radioactive datinging.radiometric dating (isotopic dating; radioactivedating) Any method of datingthat uses the decay rates of naturally occurringradioactive isotopes to assess theage of a specimen or material. Organicmatter less than 7000 years old can bedated using radiocarbon dating. This usesthe fact that the isotope carbon-14 is foundin the atmosphere and taken in by plantswhen they photosynthesize, and subsequentlyassimilated by the animals thatfeed on them. When plants and animalsdie, no more carbon is taken in and the existing14 C decays to the nonradioactive isotopecarbon-12. If the proportion of 14 C to12 C in the atmosphere and the decay rate of14 C to 12 C are both known, as they are,then the sample may be dated by findingthe present proportion of 14 C to 12 C. Specimensof rocks over 7000 years old canbe dated by other radioisotope methods,e.g. potassium–argon dating and rubidium–strontium dating.rainforestSee forest.random amplified polymorphic DNA(RAPD) The generation of copies of segmentsof DNA by means of the polymerasechain reaction, whereby the segments multipliedare those that lie between two identicalbinding sites for a particular primer,usually about 10 base pairs long. Severaldifferent segments of DNA may fall intothis category, and the amplified DNA canbe sorted using ELECTROPHORESIS. Differentindividuals may differ in the number andlocation of such sites, giving rise to a kindof genetic fingerprint. See genetic finger-198

eceptacleprinting; molecular systematics; polymerasechain reaction.Ranunculaceae A highly variable familyof about 2450 species of dicotyledonousplants that includes the buttercups, monkshoods,anemones, clematis, delphiniums,and hellebores.raphe 1. In some seeds, a ridge formedby the portion of the funicle that is fusedwith the integument.2. A slit in the valve of motile diatoms,thought to be involved in movement by allowingcontact between cytoplasm andsubstrate.raphides Bunches of needlelike crystalsof calcium oxalate found in certain plantcells.Raunkiaer’s plant classification Aclassification of growth forms based on thepersistence of the shoots and the positionof the resting buds. The system simplifiesassessment of the percentages of differentplant forms in any given type of vegetation,specially in temperate communities. Seealso chamaephyte; cryptophyte; helophyte;hemicryptophyte; phanerophyte; therophyte.Ray, John (1627–1705) English naturalistand taxonomist. Ray, from a humblebackground, was appointed a Cambridgefellow in 1648 but lost his fellowship in1662, having refused to take the oath requiredby the Act of Uniformity. Hisfriends at Cambridge, especially FrancisWilloughby, helped to finance him in hisfurther studies. His life’s work was an attemptto describe all the known flora andfauna. With Willoughby, he traveledthroughout Europe to study a wide rangeof plants and animals. In 1667 he publisheda catalog of British plants and in thesame year was elected to the Royal Society.On Willoughby’s death in 1672 he inheritedenough money to continue theirendeavor. His three-volume Historia plantarumwas published between 1686 and1704. He is best known for fixing thespecies as the basic unit in taxonomy andalso for realizing the importance of distinguishingbetween monocotyledons and dicotyledons.ray floretray fungiray initialSee inflorescence.See Actinobacteria.See initial.reaction center In PHOTOSYNTHESIS, ahypothetical site at which absorbed lightenergy is used to transport electrons forphotophosphorylation. It contains a moleculeof the primary pigment (in plants thisis chlorophyll a, which transfers the electrons.Plants have two reaction centers inthe thylakoids of the chloroplast: PhotosystemsI and II.RecentreceptacleSee Holocene.1. The tip of the angiospermRaunkiaer’s plant classification199

ecessiveflower stalk upon which the floral organsare inserted. If the ovary is on top of the receptaclewith the other organs it is said tobe superior but if it is sunk in the center,e.g. in the daffodil (Narcissus), it is said tobe inferior. See also epigyny; hypogyny;perigyny.2. The swollen portion of the lamina(blade) bearing the conceptacles in certainalgae (e.g. Fucus).recessive An allele that is only expressedin the phenotype when it is in the homozygouscondition. Compare dominant. Seealso double recessive.reciprocal cross A cross that testswhether the inheritance of a particularcharacter is affected by the sex of the parent.The cross is thus made both ways, i.e.the character under consideration is carriedby the female in one cross and by themale in the second cross. The procedurecan demonstrate which characters are controlledby sex-linked genes.recombinant DNA 1. DNA producedby recombinant DNA technology.2. DNA formed naturally by recombination,e.g. by crossing over in meiosis or byconjugation in bacteria. See recombination.recombinant DNA technology Thetechnique by which foreign DNA, whetherfrom another organism or genetically engineered,is inserted into another DNA fragmentor molecule. The product –recombinant DNA – is fundamental tomany aspects of GENETIC ENGINEERING, particularlythe introduction of foreign genesto cells or organisms. There are now manytechniques for creating recombinant DNA,depending on the nature of the host cell ororganism receiving the foreign DNA. Particulargenes or DNA sequences are cutfrom the parent molecule using specifictype II RESTRICTION ENDONUCLEASES, or areassembled using a messenger RNA templateand the enzyme REVERSE TRANSCRIP-TASE. In GENE CLONING using cultures ofbacterial or eukaryote tissue cells, the foreigngene is inserted into a VECTOR, e.g. abacterial plasmid or virus particle, whichthen infects the host cell. Inside the hostcell the recombinant vector replicates andthe foreign gene is expressed. One of themost common vectors used with plantsis the Ti (tumor-inducing) plasmid of thebacterium Agrobacterium tumefaciens.This causes crown gall tumors in plants,and its plasmid has been used on a range ofcrop plants.recombination The regrouping of genesthat regularly occurs during meiosis as a resultof the independent assortment of chromosomesinto new sets, and the exchangeof pieces of chromosomes (crossing over).Recombination results in offspring thatdiffer both phenotypically and genotypicallyfrom both parents (recombinants)and is the most important means of producingvariation in sexually reproducingorganisms.red algaeSee Rhodophyta.Red Data Book An updatable set of informationon rare, threatened, and endangeredspecies worldwide, compiled by theWorld Conservation Monitoring Center,and published at regular intervals by theSpecies Survival Commission of the WorldConservation Union (WCU).red desert soil A type of coarse SOIL richin salts and lime but poor in humus,formed in hot deserts.red light Electromagnetic radiation ofwavelength approximately 630 nm. It isthe most effective wavelength for initiatingmany light-dependent reactions in plants,such as the germination of light-sensitiveseeds. See photoperiodism; phytochrome.red tropical soil A type of SOIL formedunder rainforests and in savannas in areasof alternating wet and dry seasons. It isheavily leached, but nutrients released bydecomposition help to maintain the mineralbalance. The red color is due to oxidesof iron, magnesium, and aluminum, which200

eplicationin some such soils form a hard crust (LAT-ERITE) at the soil surface.reducing sugar A sugar that can reducecertain inorganic ions in solution, such asthe cupric ions in Benedict’s and Fehling’ssolutions. Examples are glucose, lactose,and meltose.reduction division The first division ofmeiosis, including prophase, metaphase I,and anaphase I. It results in a haploid numberof chromosomes gathering at each endof the nuclear spindle. See meiosis.reflexedBent backwards.regeneration The regrowth by an organismof an organ or tissue that has beenlost by injury or other means. It applies tonatural regrowth from living roots or coppicedstems as well as artificial regrowth inthe laboratory. Many plant propagationtechniques make use of the ability of cuttingsof shoots, roots, or leaves to regenerateinto complete plants.regulator gene A gene whose productcan promote or prevent the TRANSCRIPTIONof structural genes, which may or may notbe adjacent to other regulator genes, andmay even be on another chromosome. Theregulator may code for a REPRESSOR proteinthat switches off an operator gene (in bacteria).In eukaryotes it may bind to proteinsin a transcription complex to inhibit its action,or it may bind to a TRANSCRIPTION factor.See also operon; promoter.relative humidity The ratio of theamount of water vapor present in a body ofair at a given temperature compared to themaximum quantity of water vapor thatbody can contain at the same temperature,i.e. when saturated at that temperature.Relative humidity affects the diffusion gradientfor TRANSPIRATION and evaporation,which proceed more slowly with increasein relative humidity.release factorSee translation.rendzina A type of brown earth soil typicalof humid to semi-arid grasslands overcalcareous rocks. The rendzina is rich inlime and has an upper humus-rich horizon.repetitive DNA DNA that consists ofmultiple repeats of the same nucleotide sequences(repeated sequences). In someplants, such as lilies, repeats may constituteover 50% of the genome, leading to unexpectedlylarge genomes. The repeats mayoccur together, or they may be widely scattered.Unlike prokaryotic cells, eukaryoticcells contain appreciable amounts of repetitiveDNA. For example, satellite DNA(tandem DNA) is made up of up to onemillion adjacent repeats of the same shortsequence, usually not more than 10 basepairs long. It occurs in the region of centromeresand telomeres, and is described ashighly repetitive DNA. This DNA oftenhas a nucleotide composition significantlydifferent from the rest of the cell’s DNA,giving it a different density, so it shows upas a separate band (the ‘satellite’) whencentrifuged. Microsatellite DNA, withshort sequences repeated only 10 to 100times, is used in DNA fingerprinting. Somerepetitive DNA is accounted for by multiplecopies of particular genes, e.g. genes encodinghistones and ribosomal RNA. Seealso junk DNA; transposon.replica In a general sense an exact copy.In biology specifically used for a thin detailedcopy of a biological specimen, obtainedby spraying the surface with a layerof plastic and carbon. Replicas are used inelectron-microscope work.replication The mechanism by whichexact copies of the genetic material areformed. Replicas of DNA are made whenthe double helix unwinds as a result of helicaseenzyme action and the separatedstrands serve as templates along whichcomplementary nucleotides are assembledthrough the action of the enzyme DNApolymerase. The result is two new moleculesof DNA each containing one strandof the original molecule, and the processis termed semiconservative replication.201

epressorReplication of DNA occurs in interphase;by the end of interphase the replicatedDNA and associated histones are in theform of chromosomes each consisting oftwo chromatids joined at the centromere.In RNA viruses an RNA polymerase is involvedin the replication of the viral RNA.repressor A protein molecule that preventsprotein synthesis by binding to theoperator sequence of the gene and preventingtranscription. The molecule is producedby a regulatory gene and may acteither on its own or in conjunction with asecond protein called a corepressor. Insome cases another molecule, an inducer,may bind to the repressor, weakening itsbonds with the operator and derepressingthe gene, allowing transcription to proceed.Such a molecule is termed an effector.See operon.reproduction See asexual reproduction;sexual reproduction.resin One of a group of substances occurringin many trees and shrubs, includingconifers and certain families of floweringplants. They are usually secreted by specialcells into long resin ducts or canals. Sometimesthey are produced in response to injuryor infection. They sometimes form asticky covering to buds (e.g. horse chestnut),reducing transpiration and givingprotection.resolving power (resolution) The abilityof an optical system to form separateimages of closely spaced objects. It dependsupon the perfection of the lens, and is theoreticallylimited by the wavelength of light,being greater with decreasing wavelength.See microscope.respiration The oxidation of organicmolecules to provide energy in plants andanimals. Autotrophic plants respire moleculesthat they have synthesized by photosynthesis.Respiration occurs in all cells,whether or not they are capable of photosynthesis.The energy from respiration isused to attach a high-energy phosphategroup to ADP to form the short-term energycarrier ATP, which can then be usedto power energy-requiring processes withinthe cell. ATP is not transported betweencells, but is made in the cell where it is required.The chemical reactions of respirationnormally require oxygen from the environment(AEROBIC RESPIRATION). Some organismsare able to respire, at least for a shortperiod, without the use of oxygen (ANAER-OBIC RESPIRATION), although this processproduces far less energy than aerobic respiration.A few bacteria can survive indefinitelyin anaerobic conditions. Thecomplex reactions of cell respiration fallinto two stages, GLYCOLYSIS and the KREBSCYCLE. The first stage, glycolysis, occurswithin the cytoplasm, but the Krebs cycleenzymes are localized within the mitochondriaof eukaryotes; cells with high rates ofrespiration have many mitochondria. Glycolysisresults in partial oxidation of therespiratory substrate to the 3-carbon compoundpyruvate. It does not require oxygen,so can occur in anaerobic conditions,when the pyruvate is converted to ethanolin plants and most bacteria, or to lacticacid in certain bacteria. The yield of glycolysisis 2 molecules of ATP plus 2 moleculesof the reduced coenzyme NADH 2for each molecule of glucose respired. Inthe presence of oxygen, the pyruvate is furtheroxidized in the Krebs cycle. In theKrebs cycle, which requires free oxygen,pyruvate is converted into the 2-carbonacetyl group, which becomes attached to acoenzyme forming acetyl coenzyme A. Thisthen enters a cyclic series of reactions duringwhich carbon dioxide is evolved andhydrogen atoms are transferred to thecoenzymes NAD and FAD. The energy releasedby the Krebs cycle is transferred viathe reduced coenzymes NADH 2 andFADH 2 to an ELECTRON-TRANSPORT CHAINembedded in the inner mitochondrialmembrane. According to the chemiosmotictheory, electrons flow through the variouscomponents of the chain, while a gradientof hydrogen ions is maintained by an energy-requiring(active) proton pump thatdrives protons into the intermembrane202

Rf valuespace. This gradient drives the formationof ATP. Overall, 38 molecules of ATP aregenerated for each molecule of glucose oxidizedduring aerobic respiration, comparedwith only 2 molecules of ATP duringanaerobic respiration. At the end of theelectron-transport chain the electrons reactwith protons and oxygen to give water. Inaerobic respiration, therefore, pyruvate iscompletely oxidized to carbon dioxide andwater.See electron-trans-respiratory chainport chain.respiratory quotient (RQ) The ratio ofthe volume of carbon dioxide evolved duringRESPIRATION to the volume of oxygenabsorbed:RQ = volume CO 2 /volume O 2The complete breakdown of sugars by aerobicrespiration gives an RQ of 1. Thisrises if anaerobic respiration is involved.The respiration of proteins or fats producesless carbon dioxide, so the RQ fallsbelow one.response A change in an organism or inpart of an organism that is produced as areaction to a stimulus.restriction endonuclease (restriction enzyme)A type of enzyme, found mainly inbacteria, that can cleave and fragmentDNA internally (see endonuclease). Theirfunction is to protect the cell against foreignDNA, such as the DNA of an invadingvirus. Some restriction endonucleasescleave DNA at random, but a particulargroup of enzymes, known as class IIrestriction endonucleases, cleave DNA atspecific sites. The discovery of these enzymesformed the basis for the developmentof genetic engineering, since theyenable the isolation of particular gene sequencesand the DNA fragments are replicatedby means of base pairing and DNAligases. See also recombinant DNA technology.restriction fragment length polymorphism(RFLP) Variation among themembers of a population in the sites atwhich restriction enzymes cleave the DNA,and hence in the size of the resulting DNAfragments. It results from differences betweenindividuals in nucleotide sequencesat the cleavage sites (restriction sites). Thepresence or absence of particular restrictionsites can be ascertained using DNAprobes in the technique called SOUTHERNBLOTTING. Restriction sites vary enormously,and this variation is exploited inanalyzing and comparing the genomes ofdifferent individuals, e.g. to establish howclosely related they may be. Restrictionsites are also invaluable as genetic markersin chromosome mapping, and can be usedto track particular genes. RFLPs are used inGENETIC FINGERPRINTING. See chromosomemap.restriction map A map of a segment ofDNA showing the cleavage sites of restrictionendonucleases and their physical distanceapart, usually measured in basepairs. It can be used to reveal variations inrestriction sites between individuals of thesame species or between different species(see restriction fragment length polymorphism).This variation serves as a key to theorganism’s genes, since the restriction sitescan be used as markers to identify closelylinked genetic loci and allow investigationof deletions, insertions, or other mutations.RFLPs are an essential tool in chromosomemapping.reticulate thickening The type ofsecondary-wall formation in which an irregularlybranching mass of lignified depositscovers the inner wall of the cell. It isusually found in metaxylem tracheids andvessels. Compare annular thickening;scalariform thickening.reverse transcriptase An enzyme thatcatalyzes the synthesis of DNA from RNA(i.e. the reverse of transcription, in whichmRNA is synthesized from a DNA template).The enzyme occurs in certain RNAviruses and enables the viral RNA to be‘transcribed’ into DNA, which is then integratedinto the host DNA and replicates203

Rhizobiumwith it. It is also used in genetic engineeringto make complementary DNA (cDNA)from an RNA template.RFLP See restriction fragment lengthpolymorphism.Rf value In chromatography, a ratiothat expresses the degree to which a solutemoves in a solvent. It is calculated by dividingthe distance moved by the spot ofsolute on the chromatogram column by thedistance traveled by the solvent front. TheRf value is constant for a particular moleculeand serves to identify it.Rhizobium A spherical or rod-shapednitrogen-fixing bacterium that can live eitherfreely in the soil or symbiotically in theroot nodules of leguminous plants and afew other species, such as alder (Alnus). Seealso nitrogen cycle; nitrogen fixation.rhizoid A unicellular rootlike structurefound in certain algae and in the gametophytegeneration of the bryophytes andsome ferns. It serves to anchor the plantand absorb water and nutrients.rhizome A stem that grows horizontallybelow ground. Rhizomes may be fleshy,e.g. Iris, or wiry, e.g. couch grass (Elytrigiarepens) and may serve as an organ ofperennation or vegetative propagation, oroccasionally both. Compare corm; tuber;stolon. See also rootstock.Rhodophyta (red algae) A phylum ofprotoctists comprising aquatic, mainly marine,algae characterized by their red color.The color results from photosynthetic pigments,phycoerythrin and phycocyanin(see phycobilins). Most species are marine,a few freshwater or terrestrial; their distributionranges from the shore to quite deepwater.rhytidomeSee bark.riboflavin (vitamin B 2 ) A flavin pigmentthat is thought to be involved in the receptionof the light stimulus in phototropism,and in the photooxidation of endogenousauxins. It is one of the water-solubleB-group of VITAMINS. It is found in cerealgrains, peas, and beans. Riboflavin is aconstituent of several enzyme systems(flavoproteins), acting as a coenzyme forhydrogen transfer in the reactions catalyzedby these enzymes.ribonuclease (RNase) Any nuclease enzymethat cleaves the phosphodiesterbonds between adjacent nucleotides inRNA. Exoribonucleases cleave nucleotidesfrom one or both ends of the RNA molecule,while endoribonucleases cleavebonds within the molecule. RNase isthought to be involved in the incompatibilitysystem that prevents pollen of certaingenetic composition from germinating onthe style of a flower.ribonucleic acidSee RNA.ribose A monosaccharide, C 5 H 10 O 5 ; acomponent of RNA.ribosomal RNA (rRNA)See ribosome.ribosome A small organelle that is thesite of protein synthesis in the cell. Ribosomesare found in large numbers in allcells, free in the cytoplasm or bound to EN-DOPLASMIC RETICULUM. In most species theyare composed of roughly equal amounts ofprotein and ribosomal RNA (rRNA). Theribosome consists of two unequally sizedrounded subunits arranged on top of eachother like a cottage loaf. Eukaryotic cellshave larger ribosomes than prokaryoticcells but the ribosomes in mitochondriaand chloroplasts are about the same size asprokaryotic ribosomes.The ribosome comprises different typesof rRNA and associated proteins. TherRNA interacts directly with the mRNAduring initiation of translation, and thereare specific sites on the rRNA that interactwith anticodons on the transfer RNAs(tRNA). The proteins have various roles,including keeping the mRNA in a singlestrandedstate so that the bases are exposedfor TRANSLATION. During translation the204

ootribosome moves along the messenger RNA(mRNA), enabling the peptide linkage ofamino acids delivered to the site by transferRNA molecules according to the codein mRNA. Several ribosomes may be activelyengaged in protein synthesis alongthe same mRNA molecule, forming apolyribosome, or polysome.ribozyme Any RNA molecule that actsas an enzyme.ribulose A 5-carbon sugar that is phosphorylatedto form RIBULOSE BISPHOSPHATE.ribulose bisphosphate (RUBP) A 5-carbon compound that accepts carbondioxide during PHOTOSYNTHESIS. Each moleculeis then converted into two moleculesof 3-carbon phosphoroglyceric acid. Theribulose bisphosphate is regenerated in theCalvin cycle when the carbon dioxide isconverted into carbohydrate.ribulose bisphosphate carboxylase(RuBP carboxylase; rubisco) An enzymethat catalyzes the carboxylation of ribulosebisphosphate to two molecules of glycerate3-phosphate, the carbon dioxide-fixingfirst step in the Calvin cycle of PHOTOSYN-THESIS. It may be as much as 15% of thetotal protein in the chloroplast. Rubiscocan also act as an oxygenase, breakingdown ribulose bisphosphate in the presenceof oxygen to glycerate 3-phosphateand phosphoglycolic aid. This reactionforms the basis of PHOTORESPIRATION. Therelative concentrations of oxygen and carbondioxide in the surrounding air determinethe balance of photosynthesis andphotorespiration and thus the efficiency ofcarbon fixation. See also C 3 plant; C 4plant.ring-porous Describing wood in whichthe largest vessels are in the early wood,giving distinct growth rings, as seen in elm.Compare diffuse-porous. See also annualring.RNA (ribonucleic acid) A nucleic acidcomprising a single polynucleotide chainsimilar in composition to a single strand ofDNA except that the sugar ribose replacesdeoxyribose and the pyrimidine base uracilreplaces thymine. RNA is synthesized inthe nucleus, using DNA as a template, andexists in three main forms (messengerRNA; transfer RNA; ribosomal RNA). Incertain viruses RNA is the genetic material.RNA polymeraseSee polymerase.RNase (ribonuclease) An enzyme thatcatalyzes the hydrolysis of the sugar–phosphate bonds of RNA. There are severaltypes, each having a specific action.For example ribonuclease T1 degradesRNA to mono- and oligonucleotides terminatingin a 3¢-guanine nucleotide, whilethose produced by ribonuclease T2 terminatein a 3¢-adenine nucleotide.RNA splicing In eukaryote cells, theprocess whereby noncoding segments (IN-TRONS) of the primary RNA transcript areremoved and the remaining coding segments(EXONS) joined together to form thefunctional mRNA molecule. It is catalyzedby a complex of proteins and RNA molecules(a spliceosome), or sometimes bymRNA itself (see ribozyme). See transcription.root The organ that anchors a plant tothe ground and that is responsible for theuptake of water and mineral nutrientsfrom the soil. Roots develop from the radicleof the embryo and, according to thenature of branching from the seedling root,a fibrous or tap root system develops.Roots differ from shoots in lacking chlorophylland not producing buds or leaves.Roots also differ from shoots in thearrangement of xylem and phloem, havinga solid central strand of vascular tissuerather than a hollow cylinder of conductingtissue.The growing point of the root is protectedby a root cap to withstand the abrasionthat occurs as the root grows throughthe soil. The direction of root growth iscontrolled by both gravity and water supply.The main absorptive region of the root205

oot hairis just beyond the zone of elongation behindthe root tip, where the ROOT HAIRS areformed. Further back along the root, lateralroots are formed, which develop fromwithin the vascular tissue and grow outthrough the cortex (endogenous root formation).All roots whose derivation caneventually be traced back to the radicle arecalled primary roots. Roots that arise inany other way are termed adventitiousroots, for example those that develop fromthe stem in bulbs and corms. In manyplants, particularly biennials, the root maybecome swollen with carbohydrates andact as an underground food store duringthe winter. Many other root modificationsare seen (see buttress root; contractile root;prop root). The roots of many plant speciesexist in association with fungi as mycorrhizaeor, as in the Fabaceae, with nitrogen-fixingbacteria to form root nodules(see nitrogen fixation).root hair An outgrowth, or trichome,from the piliferous layer of the root. Roothairs are projections of single epidermalcells that make contact with the soil for uptakeof water and solutes, increasing thesurface area of the root for absorption.They may also have an anchoring role,helping to retain contact between the roottip and the soil under dry conditions whenthe soil tends to shrink away from theroots.root caproot noduleSee calyptra.See nitrogen fixation.root pressure The pressure that maybuild up in a plant root system due to theosmotic potential of the root cells, which isthought to help force water upward in thexylem vessels. It is demonstrated by theoozing of water from the surface of a cutstem. Root pressure tends to build up atnight, when the rate of transpiration is low,and is a cause of GUTTATION in somespecies, especially grasses. See osmosis;water potential.rootstockA vertical, usually short, undergroundstem or rhizome. It is found inmany angiosperms, e.g. rhubarb (Rheumspp) and strawberry (Fragaria ananassa),and in certain ferns, e.g. Osmunda. It canreproduce vegetatively.Rosaceae A large family of dicotyledonoustrees, shrubs, and herbs with alternatelyarranged (opposite in Rhodotypos),simple, compound, or dissected leaves withstipules, and actinomorphic flowers withparts in fives and stamens in one to severalwhorls of five or more. The ovary is inferioror partly inferior or the carpels lie freein a hollow receptacle. The fruit may be ahead of follicles or achenes, as in avens(Geum), achenes on a fleshy receptacle, asin strawberry (Fragaria); a drupe, as incherries, plums, and peaches (Prunus); ahead of drupelets, e.g. blackberry and raspberry(Rubus); or a pome, as in apple(Malus). There are about 2825 species.Many species and cultivated hybrids, especiallyroses and cherry trees, are grownas ornamentals for their showy flowers.Cotoneaster and Spiraea are also populargarden shrubs. Other edible fruits includeloquats (Eriobotrya japonica), pears(Pyrus communis), quinces (Cydonia oblonga),and strawberries (Fragaria).rosette plant Any plants whose leavesradiate outward from a short stem at soillevel, e.g. daisy (Bellis perennis). Thisgrowth form helps plants to withstandtrampling or grazing and in exposed habitatsavoids exposure to strong winds.royal fernsSee Osmundales.Rubiaceae A large family of dicotyledonoustrees, shrubs, and herbs, includingsome lianas, epiphytes, and ant plants (e.g.Myrmecodia). They have decussatelyarranged simple leaves often bearing elaboratestipules, and flower parts fused, infours or fives (sometimes including oneextra-large, brightly colored sepal), and4–5 stamens that alternate with the corollalobes. The ovary is usually inferior, withtwo fused carpels. The fruit is a capsule,berry, or schizocarp. There are about206

Rutaceae10 200 species, mostly tropical, but withsome in temperate and arctic regions.Many species have long corolla tubes, anadaptation to pollination by butterflies,moths, or birds. Most temperate speciesare herbs, e.g. bedstraws (Galium spp.),typically with square stems and whorls ofleaves. The family includes a few light timbertrees, coffee (Coffea arabica), and Cinchona,whose bark is a source of quinine.There are some cultivated ornamentals,such as Gardenia jasminoides and Ixora.rubisco See ribulose bisphosphate carboxylase.runner A branch, formed from an axillarybud, that grows horizontally along theground. Runners often have greatly elongatedinernodes. The axillary buds of therunner may develop into daughter plantswith adventitious roots growing from thenode, as seen in the creeping buttercup.Conversely, only the terminal bud mayform a new plant, as in the strawberry. Anew runner then develops from a branch ofthis daughter plant. If the intervening runnerbreaks or rots away, the daughterplants become independent, so this is alsoa means of vegetative reproduction. Seealso offset; stolon.See ribulose bispho-RuBP carboxylasesphate carboxylase.rusts Parasitic basidiomycete fungi ofthe order Uredinales. The name derives207

SsaccharideSee sugar.S allelesSee multiple allelism.Saccharomycetales An order of the Ascomycota,containing the yeasts, in whichthe mycelium is poorly developed or absent,and the vegetative stage comprisesmainly single cells that reproduce asexuallyby budding or fission. The commerciallyimportant yeasts of the genus Saccharomycesare used in brewing (S. cerevisiae),wine-making (S. ellipsoideus), and breadmaking(s.cervisiae): they feed on sugars,and in anaerobic conditions produceethanol (alcohol) and carbon dioxide.Sachs, Julius von (1832–97) Germanbotanist. Having obtained his PhD inbotany at the University of Prague in 1856,Sachs occupied a number of posts, culminatingin his appointment as professor ofbotany at Würzburg University. He establishedthat chorophyll is not univerally distributedin plants, but is confined insidedefinite structures that he named ‘chloroplasts’,and showed that starch is the firstproduct of photosynthesis. He also discoveredthat plants respire in the same way asanimals, taking up oxygen and giving outcarbon dioxide. His work on plant growthmovements led him to invent the KLINOS-TAT.safranin In microscopy, a permanentred stain used to stain nuclei in plant cells.It also stains lignin and cutin red andchloroplasts pink. It is often used with agreen counterstain such a fast green or ablue counterstain, e.g. hematoxylin. Seestaining.Salicaceae A family of dicotyledonoustrees, shrubs, and creeping forms includingthe willows (Salix) and poplars (Populus).salt glandSee hydathode.saltmarsh Vegetation growing in salineor brackish marshy places, such as river estuariesand sheltered, muddy coasts, andconsisting mainly of HALOPHYTES. Saltmarshes can be productive grazing land,and serve as important barriers against inundationof the hinterland by the sea.samara A nut or achene whose pericarpis extended to form a wing for wind dispersal,e.g. ash (Fraxinus excelsior).sand Mineral particles consisting mainlyof quartz, felspar, and mica, and measuringbetween 2 and 0.05 mm in diameter.Sandy soils contain at least 85% sand andnot more than 10% clay. They are lightand drain well. However, retention of nutrientsand water is poor. Compare silt.sap 1. The solution of mineral salts andsugars that is found in xylem and phloemvessels and oozes out of cut stems.2. The liquid contents of a plant cell vacuole.saponins Any of a class of bitter-tastingGLYCOSIDES that form colloidal solutions inwater and foam when shaken. Saponinsoccur in many plants, such as soapwort(Saponaria offinicalis).saprobe (saprophyte) An organism thatderives its nourishment by absorbing theproducts or remains of other organisms.Saprobes usually secrete enzymes on theirfood and absorb the soluble breakdownproducts. Many fungi and bacteria aresaprobes, and are important decomposers208

Schultze’s solutionin food chains, returning nutrients to thesoil by putrefaction and decay.See mi-scanning electron microscopecroscope.Saprolegniales (water molds) An orderof the OOMYCOTA containing aquatic orsoil-dwelling funguslike microorganisms,e.g. the water mold, Saprolegnia, whichmay be saprobes or parasites. Some formmycelia; others are unicellular.saprophyteSee saprobe.saprotrophic (saprophytic) The modeof nutrition of a SAPROBE. See decomposer.sapwood (alburnum) The outer livingxylem cells in a tree trunk, consisting ofsecondary xylem elements, parenchyma,and medullary rays, that are actively involvedin water transport and food storage.Compare heartwood.satellite DNASee repetitive DNA.savanna Tropical GRASSLAND with scatteredtrees and/or tall bushes, found in allparts of the tropics where there is a pronounceddry season. Some savannas havemany grazing animals that affect the vegetationstructure; these and other savannashave periodic or irregular fires in the dryseason. See climax.scab Any plant disease that producesdry, raised, scablike lesions due to the formationof cork layers. The disease is usuallycaused by a bacterium, e.g. commonscab of potatoes (Streptomyces scabies), ora fungus, e.g. apple scab (Venturia inaequalis).scalariform thickening A type of secondarywall formation consisting of interlacedhelical bands of thickening giving aladderlike formation. It allows for very littlefurther extension, and is found in tissuessuch as metaxylem tracheids and vessels,which do not elongate after maturation.See xylem. Compare pitted thickening; spiralthickening.Schiff’s reagent A reagent used to detectthe presence of aldehyde and ketonegroups in certain compounds. It is producedby the reduction of fuchsin by sulfurousacid, and is oxidized by aldehydesand ketones, restoring the magenta colorof the fuchsin.schizocarp A dry fruit, formed fromtwo or more carpels, that divides at maturityinto one-seeded achenelike segmentstermed mericarps. Such fruits are seen inhollyhock (Alcea).Schleiden, Matthias Jakob (1804–81)German botanist. Having abandoned a careerin law, Schleiden eventually becameprofessor of botany at the University ofJena in 1839. After years studying the microscropicstructure of plants, he proposedhis cell theory Contributions to Phytogenesis(1838), in which he stated that the variousplant structures are all composed ofindividual cells. He also recognized the significanceof the nucleus. Mistakenly, hebelieved that new cells formed by beingbudded off the nuclei. His cell theory wasalso adopted and expanded by SCHWANN,who applied it to animals also.Schultze, Max Johann Sigismund(1825–74) German zoologist. During histime as professor of anatomy at Bonn University,Schultze studied the cells of manydifferent species of animals. This led to thepublication of the famous paper (1861) inwhich he concluded that protoplasm is thephysical basis of life. He described cells asbeing composed of ‘nucleated protoplasm’and maintained that this constituent ofcells was more important than cell walls (ofplants) or membranes. He supported hisconclusion by pointing out that certain embryoniccells do not possess membranes.Schultze’s solution A solution of zincchloride, potassium iodide, and iodineused mainly for testing for cellulose andhemicellulose. Both materials stain a blue209

Schwann, Theodorcolor with the reagent, that of hemicellulosebeing weaker. See also staining.Schwann, Theodor (1810–82) Germanphysiologist. Schwann was educatedat a number of German universities beforeeventually moving to Belgium. He is bestknown for his work on different animal tissues,which he suggested were composed ofindividual cells, rather than forming directlyfrom molecules. He thus confirmedthe cell theory of Schleiden and extended itto animals. His earlier work in Germany,attempting to disprove the theory of spontaneousgeneration, led him to discoverthat yeast is involved in fermentation. Hewas so discredited for this work that he leftGermany. When Pasteur’s work on fermentationwas published in the 1850s, hisgood name was restored.scion A shoot or bud from one plantthat is joined to another plants with roots(the STOCK) by grafting or budding. Thescion supplies only aerial parts to theGRAFT.sclereid Any SCLERENCHYMA cell exceptthe long fibers. The cell wall has undergonesecondary thickening, and is often lignified.The various forms of sclereid includethe star-shaped astrosclereid, the rodshapedmacrosclereid, and the isodiametricstone cell.sclerenchyma The main supporting tissuein plants, made up of cells with heavilythickened, often lignified, walls and emptylumina. The cells usually have simple unborderedpits. Unlike collenchyma, it is notvery extensible and is thus not formed inquantity until after the young tissues havefully differentiated. Sclerenchyma is oftenfound associated with vascular tissue andexists as two distinct types of cell: the longFIBER and the shorter SCLEREID. It may developby thickening of the secondary wallsof parenchyma cells, or it may arise directlyfrom meristematic cells.purpurea) and root-rot (Sclerotinia spp),formed from a mass of hyphae. Sclerotiaare often rounded or club-shaped. They donot contain spores, and eventually produceeither a mycelium or fruiting bodies. Seealso stroma.Scrophulariaceae A family of dicotyledonousplants, mainly herbs, but includingsome trees, shrubs, climbers, and aquaticswith a wide range of leaf form and flowerstructure. A few are semiparasitic on theroots of angiosperms, especially grasses,such as eyebright (Euphrasia) and lousewort(Pedicularis). There are about about5100 species. Leaves of the foxglove (Digitalispurpurea) are the source of drugssuch as digitalin.scutellum The part of the embryo ofPoaceae (grasses) that lies next to theendosperm. It is thought to be the modifiedcotyledon.secondary cell wallSee apposition.secondary growth Plant growth derivedfrom secondary or lateral meristems,i.e. the vascular and cork cambia. It is usuallyabsent in monocotyledons, but occursin most dicotyledons and conifers, and in afew lower plants. In dicotyledons, the resultof secondary growth is termed secondarythickening since there is usually anincrease in width rather than length. Theactivity of the vascular CAMBIUM gives riseto the secondary xylem and secondaryphloem. Wood is mostly secondary xylem.The cork cambium (PHELLOGEN) gives riseto the PERIDERM, a protective layer of tissueon the outside of the stem or root, whichconsists of the cork (PHELLEM), the phellogen,and the PHELLODERM (secondarycortex). The parts of the plant formed bysecondary growth are called the secondaryplant body. Compare primary growth. Seeannual ring.secondary phloem See secondary growth.sclerotium (pl. sclerotia) The restingbody of certain fungi, e.g. ergot (Clavicepssecondary plant bodygrowth.See secondary210

self-compatibilitysecondary structure In a PROTEIN, thebasic shape of the polypeptide, i.e. the wayit folds. There are two main forms: thea-helix and the b-pleated sheet. Thea-helix is an extended spiral with one turnfor every 3.6 amino acids; the turns areheld in position by HYDROGEN BONDS betweenadjacent C=O and NH groups. Sucha secondary structure allows for flexibility,the extent of which is determined by theamount of further linking of the structureby DISULFIDE BRIDGES. In the b-pleated sheetthe polypeptides are more extended, andadjacent chains of amino acids are arrangedantiparallel to each other, linked byhydrogen bonds between the C=O and NHgroups of adjacent chains. b-pleated sheetshave high tensile strength, but are inflexible.Many proteins contain both a-helicesand b-pleated sheets in different parts ofthe molecule. See conformation; primarystructure; quaternary structure; tertiarystructure.secondary thickeninggrowth.See secondarysecondary xylem See secondary growth.secretion The process of dischargingmaterials from cells. The term is usuallyapplied to materials that have been synthesizedby the cell.seed The structure that develops fromthe OVULE following fertilization in angiospermsor gymnosperms. In floweringplants one or more seeds are containedwithin a fruit developed from the ovarywall. The individual seeds are composed ofan embryo and, in those seeds in whichfood is not stored in the embryo cotyledons,a nutritive endosperm tissue. Thisdifference enables seeds to be classified asnonendospermic or endospermic. Thewhole is surrounded by a testa developedfrom the integuments of the ovule. In gymnospermsthe seeds do not develop withina fruit but are shed ‘naked’ from the plant.Following dispersal from the parent plant,seeds may germinate immediately to form aseedling or may remain in a relatively inactivedormant state until conditions are favorablefor germination. In annual plants,seeds provide the only mechanism for survivingthe cold or dry seasons. Seeds maybe formed asexually in certain plants byAPOMIXIS, e.g. in dandelion.The development of the seed habit,which makes water unnecessary for fertilization,is one of the most significant advancesin plant evolution. It has enabledgymnosperms and angiosperms to colonizedry terrestrial habitats where lower plantsare unable to establish themselves.seed ferns Seed-bearing gymnospermsof the extinct order Pteridospermales(Cycadofilicales), represented only by fossilforms that flourished in the Carboniferousbut became extinct in the Cretaceous.The plant body resembled a fern and didnot produce flowers, the seeds developingfrom megasporangia borne on the marginsof the fronds. Their stems showed somesecondary thickening.seed plantsSee spermatophyte.segregation The separation of the twoalleles of a gene into different gametes,brought about by the separation of homologouschromosomes at anaphase 1 ofmeiosis.seismonasty (seismonastic movements)A NASTIC MOVEMENT in response to shock.An example is the collapse of leaves of thesensitive plant (Mimosa pudica) whentouched or shaken.Selaginella See Selaginellales; Lycophyta.Selaginellales An order of the Lycophyta(clubmosses) containing the singleliving genus Selaginella, sometimes calledspike-mosses, with about 700 species,found worldwide, but especially in thetropics. They range from erect, sometimestufted, forms to prostrate creeping plantsand climbers.self-compatibilityThe capacity to self-211

self-incompatibilityfertilize. Many flowers are capable of selffertilizationif pollination fails, and a fewspecies do it regularly.self-incompatibility (self-sterility) Thecondition in whereby male gametes cannotfertilize female gametes from the same individualand give rise to a viable embryo.The pollen tube may be blocked by calloseas it tries to grow down the style, be preventedfrom penetrating the stigma surface,abort on reaching the ovule or thepollen grain may fail to germinate. See incompatibility;heterostyly.selfish DNA DNA that can movearound within the genome of an organismor insert copies of itself at various siteswithout serving any apparent useful function.The prime examples of selfish DNAare the mobile genetic elements calledtransposons. Some biologists also regardINTRONS as selfish DNA. Its name derivesfrom the hypothesis that selection actswithin the genome, favoring anything thatresults in increased replication of DNA.The apparently surplus DNA does not appearto confer any advantage on the organism,so the DNA is considered to be acting‘selfishly’. Compare junk DNA; repetitiveDNA. See transposon.self-pollinationself-sterilitySee pollination.See self-incompatibility.Seliwanoff’s test A standard test for thepresence of fructose or other ketose sugarsin solution. A few drops of Seliwanoff’sreagent, resorcinol in 50% hydrochloricacid, are heated with the test solution. Ared color or red precipitate indicates fructose.semiconservative replication See replication.See osmo-semipermeable membranesis.senescence The phase of the agingprocess of an organism or part of an organismbetween maturity and naturaldeath. It is usually characterized by a reductionin capacity for self-maintenanceand repair of cells, and hence deterioration.There is often accumulation of wastemetabolic products, a decrease in dryweight (as substances are withdrawn fromthe affected part), and a rise in the rate ofrespiration (see climacteric).sensitive plant A plant (Mimosa pudica)of the family Fabaceae (pea family)whose leaves are divided into many smallleaflets or pinnae, which progressively collapseand fold when the leaf is touched.This is the result of turgor changes in cellsin swellings (see pulvinus) in the petiole atthe bases of the leaflets. See also nasticmovements.sepal One of the structures situated immediatelybelow the petals of a flower.Their collective name is the calyx. They areoften green and hairy and enclose and protectthe flower bud. Sometimes they arebrightly colored and attract insects for pollination.Sepals contain several vascularbundles, and are thought to be derivedfrom leaves.septum (pl. septa) A wall, partition, ormembrane separating two cavities. For example,the capsule of a poppy (Papaver) isdivided by septa, as are the individual compartmentsof the mycelia of many fungi.sere Any plant community in a successionin which each community itself effectschanges in the habitat that determine thenature of the following stage. The successivestages are known as seral stages. Seresresult eventually in a climax community.Stages in a secondary succession, which appearwhen the biotic components of a primarysere are destroyed, e.g. by fire, arecalled subseres; seres in microhabitats arecalled microseres. The initial (pioneer)community in a succession is termed a prisere.See also climax; halosere; hydrosere;succession.serineAn AMINO ACID synthesized from212

siblingsglycerate 3-phosphate. Serine is also aproduct of photorespiration and othermetabolic reactions. It is a component ofseveral phosphoglycerides. It is brokendown by removal of the amino group toform pyruvic acid.sessile Unstalked, for example an acorn,leaf, flower, or other organ that is attachedto the main body of the plant.seta (pl. setae) The part of the SPOROGO-NIUM that forms the stalk between the footand the capsule in bryophytes, e.g. Funariaand Pellia. It may contain vascular tissue,enabling the sporophyte capsule to drawnutrients from the parent gametophyte.Sewall Wright effectSee genetic drift.sex chromosomes Chromosomes thatcarry sex-determining genes. They are rarein the plant kingdom, and many plants arehermaphrodite. Sex chromosomes occur insome dioecious plants, e.g. Silene. In otherdioecious plants a single pair of alleles determinessex.sexual reproduction The formation ofnew individuals by fusion of two nuclei orsex cells (GAMETES) to form a diploidZYGOTE. In unicellular organisms whole individualsmay unite but in most multicellularorganisms only the gametes combine. Inorganisms showing sexuality, the gametesare of two types: male and female. Theymay be produced in special organs (e.g.carpel and anther in angiosperms, archegoniaand antheridia in lower plants andsome algae). The gametes may be derivedfrom the same parent (AUTOGAMY) or fromtwo different parents (ALLOGAMY). Individualsproducing both male and female gametesare termed hermaphrodite if in thesame flower or MONOECIOUS if in separateflowers; those in which male and femalegametes are borne on different individualsare termed DIOECIOUS.Generally meiosis occurs before gameteformation, resulting in the gametes beinghaploid (having half the normal number ofchromosomes). At fertilization, when thehaploid gametes fuse, the diploid numberof chromosomes is restored. In this waysexual reproduction permits genetic recombination,which results in greater varietyin offspring and so provides amechanism for evolution by natural selection.In organisms showing ALTERNATIONOF GENERATIONS, the gametes may be producedby mitosis in the haploid generation,or by meiosis in the diploid generation andgive rise directly to gametes or gameteproducingcells.shade plant A plant that can tolerateand thrive in low light intensity. Someshade plants are sensitive to very brightlight and cannot live in open habitats.shadowing A method of preparation ofmaterial for electron microscopy enablingsurface features to be studied. It can beused for small entire structures, subcellularorganelles, or even large molecules (e.g.DNA). The specimen is supported on aplastic or carbon film on a small grid andsprayed with vaporized metal atoms fromone side while under vacuum. The coatedspecimen appears blacker (more electronopaque)where metal accumulates, and thelengths and shapes of ‘shadows’ cast (regionsbehind the objects not coated withmetal) give structural information. It isoften used in association with freeze fracturing.See freeze fracturing; microscope.shoot The aerial photosynthetic portionof a plant that generally consists of a stemupon which leaves, buds, and flowers areborne.short-day plant (SDP) A plant that appearsto require short days (days with lessthan a critical length of daylight) in orderto flower. In fact, the plants require a minimumperiod of darkness (night length).Short-day plants are typically found intemperate latitudes. Examples includespring-flowering plants such as strawberry(Fragaria) and autumn-flowering plantssuch as Chrysanthemum. See critical daylength; photoperiodism. Compare longdayplant; day-neutral plant.213

sieve cellsiblings (sibs) Two or more offspringfrom the same cross. The term sibs is usuallyused in a narrower sense to meanplants derived by selfing or by crossing betweengenetically similar parents. The termsibling species is sometimes used to describespecies that are extremely similar becauseof recent common ancestry. Suchspecies may be almost impossible to distinguishin the field.sieve cellSee sieve element.sieve element A vascular cell in thePHLOEM, whose function is to transportsugars and other nutrients from the site ofproduction to the site of utilization or storage.The term includes both sieve cells andsieve tubes. Sieve cells are long, narrowcells with specialized sieve areas in the primarywall, perforated by several pores, derivedfrom pits. The pores are lined withcallose, and allow cytoplasmic strands topass from one sieve cell to the next. Theyare found in gymnosperms and lower vascularplants. The sieve tubes of angiospermsare shorter and wider, and havehighly specialized sieve areas called sieveplates, in some species (e.g. Nicotiana)with several sieve areas, usually on the endwalls of the elements. They are usuallyjoined end to end, allowing continuouspassage of materials up and down theplants. In contrast to xylem components,sieve tube elements have nonlignified cellwalls, and a living enucleate (without a nucleus)protoplast. They are usually associatedwith specialized parenchyma cellscalled COMPANION CELLS, whose protoplastsare connected with the protoplasmof the sieve tube by plasmodesmata.sieve platesieve tubeSee sieve element.See sieve element.silicon A MICRONUTRIENT found in manyanimals and plants, although not essentialfor growth in most plants. It is found inlarge quantities in the cell walls of certainalgae (e.g. desmids, diatoms) and horsetails,and in smaller amounts in the cellwalls of many higher plants, especiallygrasses, where it can be a useful taxonomicfeature.silicula A capsular fruit that is formedfrom a bicarpellary ovary. It is flattened,short, and broad and is divided into two orfour loculi (valves) by a false septum,which bears the seeds. Siliculas are typicalof the Brassicaceae, e.g. honesty (Lunariaannua). Compare siliqua.siliqua (silique) A fruit of some Brassicaceae,similar to the SILICULA but longerand thinner, for example the wallflowerfruit (Erysimum cheiri).silt Mineral particles between 0.05 and0.002 mm in diameter. A silt soil is onewith more than 80% silt and less than 12%clay, and has a smooth, soapy texture. Seesoil. Compare sand.Silurian The period, some 440–405 millionyears ago, between the Ordovicianand the Devonian periods of the Paleozoic.It was a relatively warm period in theEarth’s history, during which the first landplants appeared. It is characterized by simpleplants such as liverworts; fossils ofplants similar to vascular plants, e.g.Cooksonia, are known from the later partof the period, some of which had simplestrands of tracheids. Cooksonia had a slenderdichotomously branching stem with noleaves, terminal sporangia and spores impregnatedwith cutin. It resembles thepsilophyte Rhynia from the Rhynie chertof the Devonian period. See also geologicaltime scale.simple pitSee pit.siphonostele A stele with a central coreof pith internal to the xylem. The phloemmay lie external to the xylem (ectophloic),or both internal and external to the xylem(amphiphloic). See stele.SI units (Système International d’Unités)The internationally adopted system ofunits used for scientific purposes. It has214

soil structureseven base units (the meter, kilogram, second,kelvin, ampere, mole, and candela)and two supplementary units (the radianand steradian). Derived units are formedby multiplication and/or division of baseunits; a number have special names. Standardprefixes are used for multiples andsubmultiples of SI units. See Appendix.sliding growth A pattern of plantgrowth seen, for example, in many epidermalcells where, in order to accommodategrowth by adjoining cells, expanding cellwalls of adjacent cells slide over each other.Thus growth is achieved without disruptionof neighboring cells, although it doesresult in breakage of plasmodesmata.Compare symplastic growth.slime bacteriaSee Acrasiomycota; Myx-slime moldsomycota.See myxobacteria.smuts Plant diseases caused by basidiomycetefungi of the order Ustilaginales.Many are important parasites of cerealsand form a mass of sooty black spores inplace of the grain. In other smuts the sporesform an uncovered mass of black powder.Examples of smuts are Tilletia and Ustilago.See Ophioglos-snake’s tongue fernssales.snurpSee spliceosome.society A minor plant community withina larger community, characterized by aspecific dominant species, for example, aTrillium-dominated society in a communitysuch as an oak woodland.sodium An element found in all terrestrialplants, although it is believed not to beessential in most, with the exception ofsome salt-tolerant C 4 plants.sodium bicarbonate indicator A mixtureof the dyes cresol red and thymol bluein a sodium bicarbonate solution, whichchanges from red to orange and yellowwith a slight increase in acidity, and thusindicates change in pH.soil The accumulation of mineral particlesand organic matter that forms a superficiallayer over large parts of the Earth’ssurface. It provides support and nutrientsfor plants and is inhabited by numerousand various microorganisms and animals.A section down through the soil is termeda soil profile and this can characteristicallybe divided into three main layers or horizons.Horizon A, the topsoil, is darker thanthe lower layers due to the accumulation oforganic matter as humus. It is the most fertilelayer and contains most of the soil populationand a high proportion of plantroots. Horizon B, the subsoil, contains materialswashed down from above and maybe mottled with various colors dependingon the iron compounds present. Horizon Cis relatively unweathered parent materialfrom which the mineral components of theabove layers are derived. The depth andcontent of the horizons are used to classifysoils into various types, e.g. podsols andbrown earths. The texture, structure, andporosity of soil depends largely on the sizesof the mineral particles it contains and onthe amount of organic material present.Soils also vary depending on environmentalconditions, notably rainfall. See soilstructure.soil profileSee soil.soil structure The arrangement of themineral particles in the soil – whether theyare free or bound into aggregates by decomposingorganic matter. These unitsmay be further grouped into crumbs,blocks, plates, and so on. The structure affectsfertility, drainage, aeration, and easeof cultivation. A blocky soil will tend todry out and suffer from leaching of mineralsfrom the upper horizons. The soil structureis affected by soil texture – the sizes ofthe different mineral particles, which alsoinfluences drainage and aeration, and theretention of water by capillarity. The sizeof the particles depends in part on the par-215

soil textureent rock and how it weathers. The mainsize classes of mineral particles are graveland stones (greater than 2 mm in diameter),sand (0.05 to 2 mm in diameter), silt(0.002 to 0.05 mm in diameter), and clay(smaller than 0.002 mm in diameter). Claysoils, which have small particles, tend to bewet and sticky, with poor drainage andaeration. However, they tend to be electrostaticallycharged and attract humus, forminga clay–humus complex that can attractand retain minerals. The humus sticks theclay particles together, forming larger aggregatesand improving drainage and aeration.Clay soils can be further improved byadding lime, which promotes clumping ofparticles. Sandy soils have large particles,and therefore larger spaces between themfor drainage and aeration. They may needaddition of humus or mulches to improvewater and mineral retention and preventleaching.soil textureSee soil structure.Solanaceae A family of dicotyledonousherbs, shrubs, lianas, and small trees. Theyinclude commercial food plants such asaubergine (Solanum melongena), potato(Solanum tuberosum), sweet peppers (Capsicum),tomato (Lycopersicon esculentum),and Nicotiana (the source oftobacco).solenostelesolute potentialSee stele.See water potential.somatic Describing the cells of an organismother than germ cells. Somatic cellsdivide by mitosis producing daughter cellsidentical to the parent cell.soredium (pl. soredia)See lichens.sorus (pl. sori) 1. A reproductive structurefound in ferns comprising a collectionof sporangia borne on a cushion of tissuetermed the placenta. The placenta developsover a vein ending on the underside of afoliage leaf or on a specialized fertile leaf.The sorus is covered by a flap of tissue, theindusium.2. The reproductive area of certain algalthalli, e.g. Laminaria.3. The spore-bearing body of a rust fungus.Southern blotting A technique fortransferring DNA fragments from an electrophoreticgel to a nitrocellulose filter ornylon membrane, where they can be fixedin position and probed using DNA probes.Named after its inventor, E. M. Southern(1938– ), it is widely used in geneticanalysis. The DNA is first digested with restrictionenzymes and the resulting mixtureof fragments separated according to sizeby electrophoresis on an agarose gel. Thedouble-stranded DNA is then denatured tosingle-stranded DNA using sodium hydroxide,and a nitrocellulose filter pressedagainst the gel. This transfers, or blots, thesingle-stranded DNA fragments onto thenitrocellulose, where they are permanentlybound by heating. The DNA probe canthen be applied to locate the specific DNAfragment of interest, while preserving theelectrophoretic separation pattern. SeeDNA probe. Compare Western blotting.SP (suction pressure)spadix (pl. spadices)spatheSee inflorescence.See osmosis.See inflorescence.speciation The formation of one ormore new SPECIES from an existing species.Speciation occurs when a population separatesinto isolated subpopulations that developdistinctive characteristics as a resultof NATURAL SELECTION or random GENETICDRIFT, and cannot then reproduce with therest of the population, even if there are nogeographical or other physical reasons toprevent them from doing so. Speciation occurswhen barriers to interbreeding arise,such as geographical changes (e.g. in rivercourses), habitats becoming uninhabitable(e.g. following urbanization), changes intiming of flowering, or when populationsat the margins of the main populationexperience different selection pressures216

spiral thickeningas they colonize different environments.Another cause of rapid speciation in plantsis polyploidy. See also adaptive radiation.species A taxonomic unit comprisingone or more populations, all the membersof which are able to breed amongst themselvesand produce fertile offspring. Theyare normally isolated reproductively fromall other organisms, i.e. they cannot breedwith any other organisms. Two or more relatedspecies unable to breed because ofgeographical separation are called allopatricspecies. Related species that are notgeographically isolated, and which couldinterbreed but in practice do not because ofdifferences in behavior, breeding season,etc., are called sympatric species. Somespecies can interbreed with other relatedspecies, but the hybrids are often wholly orpartially sterile. For some species theremay be an almost complete continuum betweenclosely related species, as in certainorchids. This may also happen in disturbedareas or in regions where the range of twoor more species or even genera overlap,producing hybrid swarms. Where speciesform apomictic clones that are very difficultto distinguish from each other, theyare termed aggregate species, e.g. bramble(Rubus fruticosus agg.). In general, speciesare regarded as distinct if they remain reproductivelyisolated for most of their geographicalrange and have recognizablydistinct morphological characteristics.Within a species, there may be subgroupswith distinct morphological or (especiallyin microorganisms) physiological characteristics;these groups are termed SUBSPECIESor RACES. Groups of similar species areclassified together in genera.spectrophotometer An instrument formeasuring the amount of light of differentwavelengths absorbed by a substance. Itgives information about the identity orconcentration of the specimen and can beused to plot absorption spectra. It can alsobe used to investigate how electromagneticradiation interacts with matter, e.g. the absorptionof light in photosynthesis.spermatophyte (seed plant) Any seedbearingplant. In many older classificationsthey constituted the division (phylum)Spermatophyta, subdivided into the classesAngiospermae and Gymnospermae. Comparetracheophyte.spermatozoidS phaseSee cell cycle.See antherozoid.Sphenophyta A phylum of vascularnonseed-bearing plants that contains oneliving order, the Equisetales, comprisingone genus, Equisetum (horsetails or scouringrushes), and three extinct orders, theCalamitales, Sphenophyllales, and Pseudoborniales.Sphenophytes were particularlyabundant in the Carboniferous periodwhen the genus Calamites formed a largeproportion of the forest vegetation, sometreelike species reaching heights of 30 m.spherosome A small spherical organelleof plant cells, about 0.5–1.0 m m in diameter,bounded by a single membrane andstoring lipid.spikeSee inflorescence.spindle The spindle-shaped structureformed in the cytoplasm during MITOSISand MEIOSIS that is responsible for movingthe chromatids and chromosomes to oppositepoles of the cell. The spindle consists ofa longitudinally orientated system of proteinMICROTUBULES whose synthesis startslate in interphase. A special region of thecentromeres of each pair of sister chromatids,the kinetochore, becomes attachedto one or a bundle of spindle microtubules.During anaphase, the kinetochore itselfacts as the motor, disassembling the attachedmicrotubules and hauling the chromatidtoward the spindle pole. Later inanaphase, the unattached interpolar microtubulesactively slide past each other, elongatingthe entire spindle.spine A modified leaf reduced to asharply pointed structure. In some xerophytes,such as cacti, most of the leaves are217

spirillummodified into spines, greatly reducing theleaf area for transpiration; the photosyntheticfunction of the leaves is taken overby the green stems. The dense mass ofwhitish spines also helps to reflect incomingradiation, further reducing transpiration.spiral thickening (helical thickening)The type of secondary wall formation inwhich a spiral band of lignified deposits isformed on the inner wall of the cell. It isfound in protoxylem and first metaxylemtracheids and vessels. Like annular thickening,it allows for continued elongation ofthe xylem. See annular thickening; xylem.Compare reticulate thickening; scalariformthickening.spirillumA helically shaped bacterium.spirochetes Long spirally twisted bacteriasurrounded by a flexible wall. An axialfilament, with a similar structure to a bacterialflagellum, is spirally wound aboutthe protoplast inside the cell wall. Theyswim actively by flexing the cell. They arefound in mud and water and can withstandlow oxygen concentrations. Many spirochetesare pathogens causing yaws,syphilis, and relapsing fever.Spirogyra A genus of filamentous greenalgae found in freshwater and having acharacteristic spiral chloroplast. Reproductionis by conjugation: two filamentsbecome aligned and pairs of cells, one fromeach filament, become joined by a conjugationtube. The contents of one cell passthrough the tube and fuse with the contentsof the second cell to form a zygote. SeeGamophyta.spliceosome A unit in the nucleus thatremoves the noncoding RNA (INTRONS)from the initial RNA transcript and splicesthe RNA molecule back together again. Itconsists of several small ribonucleoproteinscalled SNURPS, comprising RNA combinedwith proteins. The RNA moleculesact as RIBOZYMES, catalyzing both the splicingprocess and the assembly of the spliceosomes.See transcription.spongy mesophyllSee mesophyll.spontaneous generation The erroneousbelief that modern living organisms can beformed directly and spontaneously frominorganic material, given the right conditions(abiogenesis). This belief was disprovedby Redi and Pasteur in the 17th and19th centuries.sporangiophore A structure bearingsporangia (see sporangium). In fungi itmay be a simple hypha or erect branch of ahyphae; in lower vascular plants it may bea leaf (SPOROPHYLL) or a STROBILUS.sporangium (pl. sporangia) A reproductivebody in which asexual spores areformed. It may be unicellular, as in simplealgae and fungi, or multicellular, as inplants. The spores are liberated by ruptureof the sporangium wall.spore A unicellular or multicellularplant reproductive body. Generally theterm is applied to reproductive units producedasexually, such as the spores ofbryophytes and ferns. A prefix is oftenadded, providing information as to the natureof the spore, for example conidiosporesarise on a conidium, megasporesand microspores are the larger and smallerspores produced by heterosporous plants,etc. In plants that undergo ALTERNATION OFGENERATIONS, spores are produced bymeiosis by the sporophyte generation, soare usually haploid; they germinate to producethe haploid gametophyte generation.spore mother cell A cell that gives riseto four haploid spores by meiosis. In heterosporousspecies, a distinction is usuallymade between megaspore mother cells andmicrospore mother cells. Of the four productsof a megaspore mother cell, three usuallyabort.sporocarp A hard multicellular sporecontainingstructure found in water ferns218

starch(e.g. Marsilea and Pilularia). The sori becomeenclosed by the growing together offertile fronds, and the spores are not liberateduntil the sporocarp decays and ruptures.sporogonium The sporophyte generationin mosses and liverworts. It developsfrom the zygote and comprises the foot,seta, and capsule. The sporogonium is parasiticon the gametophyte generation.sporophore The aerial spore-producingbody of certain fungi, e.g. the mushroom ofAgaricus.sporophyll The sporangium-bearingstructure of vascular plants. In some fernssporangia develop on the normal foliageleaves but in higher plants, the sporophyllsare highly modified leaves. They may begrouped together in a strobilus or, in theangiosperms, located in the flowers. Seemegasporophyll; microsporophyll.sporophyte The diploid generation inthe life cycle of a plant, which gives riseasexually to haploid spores. It arises by fusionof haploid gametes produced by thehaploid generation (GAMETOPHYTE). In vascularplants, the sporophyte is the dominantgeneration, while in bryophytes it isparasitic on the gametophyte. See alternationof generations.staining A procedure that is designed toheighten contrast between different structures.Normally biological material is lackingin contrast, protoplasm beingtransparent, and therefore staining is essentialfor an understanding of structure atthe microscopic level. VITAL STAINS are usedto stain and examine living material. Moststains require dead or nonliving material.Staining is done after fixation and eitherduring or after dehydration. Double staininginvolves the use of two stains; the secondis called the counterstain. Acidic stainshave a colored anion, basic stains have acolored cation. Some stains are neutral.Materials can be described as acidophilicor basophilic depending on whether theyare stained by acidic or basic dyes respectively.Basic stains are suitable for nuclei,staining DNA. Stains for light microscopyare colored dyes; those for electron microscopycontain heavy metals, e.g. osmiumtetroxide.For immediate observation, temporarystains may be used. These stains may fadein time, e.g. ruthenium red. Or the stainmay damage the specimen if left, for exampleSCHULTZE’S SOLUTION, which dissolvescell walls and causes swelling. Permanentstains are used for specimens that are to bekept. They do not fade or damage specimens.Examples include fast green and thecounterstain safranin. See also negativestaining.stamen The male reproductive organ inflowering plants consisting of a fine stalk,the filament, bearing the pollen producinganther. It is equivalent to the microsporophyllpresent in the gymnosperms and heterosporousmembers of the Filicinophytaand Lycophyta. The collective term for thestamens is the androecium. See also microsporophyll.staminode A sterile stamen. It may berudimentary, consisting of only the filament,as in figwort (Scrophularia), or itmay form a conspicuous part of the flower,as in Iris.standard deviation In statistics, a measureof the dispersion of a frequency distribution:it is the average magnitude ofdeviations from the center of a normalcurve, calculated by squaring all the deviations,calculating their mean, then findingthe square root of the mean. This gives avalue s, which is the point of maximumslope either side of the center of the curve.It is more suitable for use than the meandeviation, because it avoids problems ofpositive and negatives values on either sideof the center.standing crop The total weight of allthe living organisms present in an ecosystemat a given moment, usually expressedas dry weight per unit area.219

starch sheathstarch A polysaccharide that occurs exclusivelyin plants. It is an early end productof photosynthesis, and is storedtemporarily in the chloroplasts. In the darkthis starch is broken down by enzymes tosucrose and transported to other parts ofthe plant. Starch is a storage reservoir forthe plant: it is broken down by enzymes tosimple sugars and then metabolized to supplyenergy needs. It is present in plant cellsas granules dispersed in the cytoplasm. Theshape of these granules is used in taxonomy.In most plants it is found mainly inthe root cortex, and in storage organs suchas swollen taproots, corms, etc. Starch isextracted commercially from maize,wheat, barley, rice, potatoes, sorghum,cassava, and other crops. It is also the mainstorage material in many seeds. Starch is apolymer of glucose. It is not a single molecule,but a mixture of amylose (straightchains of glucose subunits) and amylopectin(branching chains of subunits).starch sheath The innermost layer ofcells of the cortex replacing the endodermisin some stems, especially young herbaceousdicotyledonous stems. Its cells containprominent starch grains. See alsoendodermis.starch–statolith hypothesis A hypothesisconcerning the mechanism of gravityperception in plants. See gravitropism.statocyte A gravity-sensitive plant cellcontaining STATOLITHS. See gravitropism.statolith One of a number of largestarch grains found in the statocytes, plantcells that are thought to be gravity sensitive.They move through the cytoplasm tothe lowermost cell surface. See gravitropism.stearic acid (octadecanoic acid) A saturatedcarboxylic acid, which is widely distributedin nature as the glyceride ester. Itis present in most fats and oils of plant andanimal origin, particularly the so-calledhard fats, i.e. those of higher melting point.Stearic acid is formed from palmitic acid. Itis an intermediate in the synthesis of oleic,linoleic, and linolenic acids.stele The vascular tissue and associatedparenchyma tissue and (if present) the surroundingpericycle and endodermis of astem or root. The arrangement of tissues inthe stele changes from stem to root. In dicotyledons,for example, the stem has aring of discrete vascular bundles, which allowsit to resist bending, while the root hasStele: types of stele220

stomaa central core of vascular tissue, adapting itto resist pulling stresses. Stelar arrangementsvary considerably in plants from thesimple protostele, in which leaf gaps areabsent, to the complex dictyostele, inwhich there are many closely spaced LEAFGAPS. Intermediate between these is thesolenostele in which the leaf gaps are morewidely spaced vertically so that only one isseen per cross section of the stem. See polystely.stem A longitudinal axis upon which areborne the leaves, buds, and reproductiveorgans of the plant. The stem is generallyaerial and erect but various modificationsare found, for example underground stemslike RHIZOMES, BULBS, and CORMS, and horizontalstructures, such as RUNNERS. Thestem serves to conduct water and food materialsup and down the plant and, particularlyin young plants and some succulents,it may serve as a photosynthetic organ. Thestem is generally cylindrical and consists ofregularly arranged conducting (vascular),strengthening, and packing cells, the wholebeing surrounded by a protective epidermis.and lower rainfall in the east. The speciescomposition varies with the climate. Largeareas of the steppe are under cultivation forgrain production.sterigma (pl. sterigmata) A fingerlikeprojection upon which spores are formedin most basidiomycete fungi. Usually foursterigmata, each bearing one basidiospore,are borne on each basidium. The cultivatedmushroom is an exception, with only twosterigmata on each basidium.steroid Any member of a group ofcompounds having a complex basic ringstructure. Many plant steroids are alcohols(sterols); some are alkaloid-like compounds.See also sterol.sterol A steroid with long aliphatic sidechains (8–10 carbons) and at least one hydroxylgroup. They are lipid-soluble andoften occur in membranes (e.g. cholesteroland ergosterol).stigma 1. The receptive tip of the carpelof plants. It may be elevated on a stalklikeprotrusion called a style.2. See eyespot.stilt root An enlarged form of PROPROOT, seen in some mangroves and a fewpalms and other trees, that helps supportplants in unstable soils.stimulus A change in the external or internalenvironment of an organism thatelicits a response in the organism. Thestimulus does not provide the energy forthe response.Steroid: skeleton of the molecular structuresteppe Grassland dominated by droughtresistantspecies of perennial grasses, foundin regions of LOESS soil and extreme temperaturerange, often on CHERNOZEM soils,in a zone from Hungary eastwards throughthe Ukraine and southern Russia to CentralAsia and China. There are pronouncedseasons, with hot summers and cold winters,with greater temperature extremesstipe 1. In the more highly differentiatedPhaeophyta (brown algae), such as wracksand kelps, the stalk between the holdfastand the blade.2. The stalk of a mushroom or toadstool,which bears the cap or pileus.stipule A modified leaf found as an outgrowthfrom the petiole or leaf base.stockA plant onto which SCIONS (shoots221

stomatal apparatusor buds) of another plant are grafted. Thestock provides the underground parts ofthe plant, and affects its size, vigor, and thetiming of flowering and fruiting. See graft.stolon An initially erect branch that, dueto its great length, eventually bends overtoward the ground. Where a node touchesthe soil adventitious roots develop and theaxillary bud at that node grows out to forma new plant. See also layering; offset; runner.stoma (pl. stomata) One of a large numberof pores in the epidermis of plantsthrough which gaseous exchange occurs.Each stoma is surrounded by two crescentshapedGUARD CELLS, attached to eachother at their ends, which regulate theopening and closing of the pore by changesin their turgidity. The arrangement of cellulosemicrofibrils in the guard-cell wallscauses the guard cell to increase in lengthrather than width as it takes in water. Thisextra length is accommodated by the cellbulging away from the aperture, so wideningthe pore. Adjacent to the guard cellsthere are usually modified epidermal cellscalled subsidiary or accessory cells. Thestoma and guard cells are collectivelycalled the stomatal apparatus. In mostplants stomata are located mainly in thelower epidermis of the leaf, where evaporationrates are lower. In many xerophytesand in some conifers of cooler climates,where soil water may be difficult to obtainin winter, the guard cells – and hence thestoma – are sunk in pits in the epidermis,often protected by hairs from passing aircurrents. TRANSPIRATION rates are thusgreatly reduced, while allowing gaseousexchange for photosynthesis. In verticalleaves, such as those of grasses, they maybe equally distributed on both surfaces.stomatal apparatusSee stoma.stomium (pl. stomia) The site at which asporangium or pollen sac ruptures to releasethe spores or pollen.stone cell (brachysclereid)See sclereid.strain Any group of similar or identicalindividuals, such as a CLONE, mating strain,physiological RACE, or PURE LINE.stratification 1. A seed treatment thatenables seeds that require vernalization togerminate the following spring: the seedsare placed between layers of moist sand orpeat and exposed to low temperatures,usually by leaving them outside throughthe winter.2. The existence of layers of water of differentdensity in a lake or other body ofwater. The differences in density may bedue to temperature or to salinity. In manylakes in summer that is a warm low-densitylayer (EPILIMNION) lying above a colderdenser layer (the HYPOLIMNION), the zoneof rapid temperature change between thetwo layers being called the thermocline.strobilus (pl. strobili) A group of sporophylls(spore-bearing leaves) arrangedaround a central axis, found in gymnosperms,Lycophyta (clubmosses), andSphenophyta (horsetails). In the clubmossesand horsetails only one type of strobilusis formed, while in the gymnospermsfemale megastrobili and male microstrobiliboth develop. The strobili of gymnospermsand cycads are also termed cones.stroma (pl. stromata) 1. The colorlessground matter between the grana lamellaein a CHLOROPLAST. It is the site of the dark(light-independent) reactions of photosynthesis.2. A solid mass of fungal hyphae, sometimesincluding host tissue, in which fruitingbodies may be produced. An example isthe compact black fruiting body of theergot fungus (Claviceps purpurea).stromatolite A layered cushionlikemass of carbonate-rich rock formed by theactions of CYANOBACTERIA. Communitiesof these organisms secrete calcium carbonate,often building up concentric rings ofdeposits forming cushions up to a meter indiameter. Modern stromatolite-buildingcommunities are confined to salt flats orshallow salty lagoons where invertebrate222

Sudan stainspredators cannot survive, but the fossilrecord demonstrates a much more widespreaddistribution in the Precambrianperiod. Some stromatolites date backnearly 4000 million years, making themthe oldest known fossils.style The stalklike portion of a carpel,joining the ovary and the stigma. The stylemay be elongated in plants relying onwind, insect, or animal pollination so thatthe stigma has a greater chance of cominginto contact with the pollinating agent.subclimaxSee climax.suberin A fatty acid polyester producedin the walls of the endodermis and in bark,where it renders the tissue impermeable towater and resistant to decay. The Casparianstrip in roots and some stems containssuberin, lignin, or similar substances. Bypreventing the passage of water throughthe cell walls of the endodermis, the Casparianstrip forces it to pass through the cytoplasm,thus allowing selective uptake orexclusion of dissolved substances. Seecutin.sublittoral 1. (neritic) The marine zoneextending from low tide to a depth ofabout 200 m, usually to the edge of thecontinental shelf. Light penetrates to theseabed, and the water is well oxygenated.Large algae (e.g. kelps) are found in shallowerwaters while certain Rhodophyta(red algae) may be found in deeper water.Compare littoral; benthic.2. The zone in a lake or pond between theLITTORAL and PROFUNDAL zones, extendingfrom the edge of the area occupied byrooted plants to a depth of about six to tenmeters, where the water temperature declines.Its depth is limited by the compensationlevel – the depth at which the rate ofphotosynthesis is equalled by the rate ofrespiration, and below which plants cannotlive (but some phytoplankton can).subsereSee sere.subsidiary cell (accessory cell)One of anumber of specialized epidermal cells of aplant that are found adjacent to the guardcells, and may help in opening and closingthe stomata. The number, arrangementand character of subsidiary cells is often ofvalue in taxonomy.subspecies The taxonomic group belowthe SPECIES level. Crosses can generally bemade between subspecies of a given speciesbut this may be prevented in the wild byvarious isolating mechanisms, e.g. geographicalisolation or different floweringtimes. Subspecies usually differ from eachother in morphology, and occupy distincthabitats or regional locations.substrate 1. The molecule or moleculesupon which an enzyme acts.2. The nonliving material upon which anorganism lives or grows.succession A progressive series ofchanges in vegetation and animal life of anarea over time from initial colonization tothe final stage, or CLIMAX. The climax is adynamic equilibrium because, although thesuccession can progress no further underthe environmental factors present at thetime, the populations present change, e.g.trees die, creating gaps for other species tocolonize. In addition, the climate is seldomcompletely stable – at best it is cyclical,with variations from year to year. See sere.succinic acid (butanedioic acid) A dicarboxylicacid formed by fermentation ofsugars. It is an intermediate in the KREBSCYCLE, and is formed from succinyl CoA ina reaction linked to the formation of GTP.succulent A fleshy plant. Succulents,such as cacti, store water in large parenchymacells in swollen stems and leaves.sucker An adventitious undergroundshoot that at some stage emerges above thesoil surface and gives rise to a new plant,which initially is nourished by the parentplant until it becomes established.223

sulfur An essential element in livingtissues, being contained in the aminoacids cysteine and methionine and hence innearly all proteins. Sulfur atoms are alsofound bound with iron in ferredoxin,one of the components of the electrontransportchain in photosynthesis. It isa component of coenzyme A. Plants takeup sulfur from the soil as the sulfate ionSO 2– 4 . The sulfides released by decay of organicmatter are oxidized to sulfur by SUL-FUR BACTERIA of the genera Chromatiumand Chlorobium, and further oxidized tosulfates by bacteria of the genus Thiobacilsugarsucrase An enzyme that breaks downsucrose into glucose and fructose.sucrose A SUGAR that occurs in manyplants. Sucrose is a disaccharide formedfrom a glucose unit and a fructose unit.Sucrose is the main transport sugar inhigher plants. It is formed in the chloroplastsfrom fructose 6-phosphate and glucose,and transported in the phloem to therest of the plant. It is hydrolyzed to a mixtureof fructose and glucose by the enzymeinvertase. Since this mixture has a differentoptical rotation (levorotatory) from theoriginal sucrose, the mixture is called invertsugar. Invert sugar is found in manyfruits.suction pressure (SP)See osmosis.Sudan stains Any of various stainsbased on aniline dyes, and used to colorfats and waxes, and hence cutinized (e.g.cuticle) and suberized (e.g. Casparian strip)tissues. See staining.sugar (saccharide) One of a class of carbohydratesthat are soluble in water. Sugarmolecules consist of linked carbon atomswith –OH groups attached, and either analdehyde or ketone group. The simplestsugars are the monosaccharides, such asglucose and fructose, which cannot be hydrolyzedto sugars with fewer carbonatoms. They can exist in a chain form or ina ring formed by reaction of the ketone oraldehyde group with an –OH group on oneof the carbons at the other end of the chain.It is possible to have a six-membered (pyranose)ring or a five-membered (furanose)ring. Monosaccharides are classified accordingto the number of carbon atoms: apentose has five carbon atoms and a hexosesix. Monosaccharides with aldehydegroups are aldoses; those with ketonegroups are ketoses. Thus, an aldohexose isa hexose that has an aldehyde group; aketopentose is a pentose with a ketonegroup, etc.Two or more monosaccharide units canbe linked in disaccharides (e.g. sucrose),trisaccharides, etc. See also fructose; glucose;polysaccharide; sucrose.sugar acid An acid formed from amonosaccharide by oxidation. Oxidationof the aldehyde group (CHO) of the aldosemonosaccharides to a carboxyl group(COOH) gives an aldonic acid; oxidationof the primary alcohol group (CH 2 OH) toCOOH yields uronic acid; oxidation ofboth the primary alcohol and carboxylgroups gives an aldaric acid. The uronicacids are biologically important, beingcomponents of many polysaccharides, forexample glucuronic acid (from glucose) is amajor component of gums and cell walls,while galacturonic acid (from galactose)makes up pectin. Ascorbic acid or vitaminC is an important aldonic acid found universallyin plant tissues, particularly in citrusfruits. Another aldonic acid, gluconicacid, is an intermediate in the pentosephosphate pathway.sugar alcohol (alditol) An alcohol derivedfrom a monosaccharide by reductionof its carbonyl group (CO) so that eachcarbon atom of the sugar has an alcoholgroup (OH). For example, glucose yieldssorbitol, common in fruits, and mannoseyields MANNITOL. GLYCEROL is important inlipid metabolism, and INOSITOL is an intermediatein the synthesis of cell wall polysaccharides.sugar phosphate A phosphate derivativeof a MONOSACCHARIDE.224

syncarpylus. There is thus a cycling of sulfur in nature.sulfur bacteria Filamentous autotrophicchemosynthetic bacteria that derive energyby oxidizing sulfides to elemental sulfurand build up carbohydrates from carbondioxide. They use sulfides instead of wateras a source of electrons in photosynthesis,releasing sulfur instead of oxygen. An exampleis Beggiatoa. They are found mainlyin sulfur-rich muds and springs, includinghydrothermal vents. There are two maingroups: the green sulfur bacteria (Chlorobia)and the purple sulfur bacteria (certainmembers of the Proteobacteria). A few Archaea,e.g. Sulfolobus, can oxidize elementalsulfur. As well as sulfides, some bacteriaoxidize thiosulfates, polythionates, andsulfites. Sulfur bacteria play an importantrole in the cycling of sulfur in the ecosystem.supergene A collection of closely linkedgenes that determine a particular trait orseries of inter-related traits and behave as asingle unit because crossing over betweenthem is very rare. An example is the S/sgene, which determines pin (s) and thrum(S) style length in certain species of Primula.superior In plant science, the term isused with reference to the position of theovary in relation to the other parts of theflower. When the ovary is superior, thepetals, sepals, and stamens are inserted atthe base of the ovary where it joins theflowerstalk, as in buttercup (Ranunculus)flowers. In a floral formula a superiorovary is denoted by a line below the carpelnumber. Compare inferior. See also hypogyny;perigyny.suspension culture A method of growingfree-living single cells or small clumpsof cells in a liquid medium; the liquidmedium is agitated to keep the cells in suspension.Microorganisms or cells of plantcallus tissue may be grown in this way.suspensor A temporary stalklike row ofcells, found in angiosperms, that differentiatesfrom the proembryo by mitosis andpushes the embryo into the nutritive endospermafter fertilization. In certain Lycophyta(e.g. Selaginella), a few ferns (e.g.Botrychium), and in gymnosperms itpushes the embryo into the female gametophytetissue. It also serves as a passage fornutrients to pass from the parent tissue tothe embryo.swamp An area of vegetation dominatedby trees that develops on ground thatis normally waterlogged or covered bywater all year round, such as the margin ofa lake, a river floodplain, or an area ofwater-retentive clay in an arid region.Swamp vegetation represents the earlystages of a HYDROSERE or HALOSERE. Thepresence of trees distinguishes it from amarsh. Coastal and estuarine swamps inwarm climates are dominated by MAN-GROVES. The swamp vegetation slows theflow of water, and dead plant matter buildsup.syconium (syconus) A composite inflorescenceor fruit in which the small individualflowers are borne on the inside of ahollow receptacle, as in figs (Ficus). Thesedevelop into achenes (the ‘pips’ of thefruit). See pseudocarp.symbiosis (pl. symbioses) Any close associationbetween two or more differentorganisms, as seen in parasitism, mutualism,and commensalism. Often one or bothorganisms is dependent on the other. Theterm is usually used more narrowly tomean MUTUALISM, but many mutualistic associationsmay have once been parasitic ormay become so at some stage in their lifecycle, e.g. plants and their mycorrhizalfungi.sympatric speciesSee species.symplast The living system of interconnectedprotoplasts extending through aplant body. Cytoplasmic connections betweencells are made possible by the plasmodesmata.The symplast pathway is animportant transport route through theplant. Compare apoplast.225

synchronous culturesymplastic growth A form of plantgrowth in which neighboring cell wallsstay in contact and grow at the same rate.sympodial Describing the system ofbranching in plants in which the terminalbud of the main stem axis stops growingand growth is taken over by lateral buds.These in turn lose their dominance and lateralbuds take over their role. Comparemonopodial.symport A process involving a membraneprotein that transports two kinds ofsubstance (substrates) across a membrane,one of which is usually an ion. Compareantiport; proton pump.synapsis (pairing) The association ofhomologous chromosomes during theprophase stage of meiosis that leads to theproduction of a haploid number of bivalents.syncarpousSee carpel.syncarpy The condition in which anovary is made up of fused carpels, as in theprimrose (Primula vulgaris). Compareapocarpy.synchronous culture A culture of cellsin which all the individuals are at approximatelythe same point in the cell cycle.Cells can be synchronized by a variety ofmeans, e.g. temperature, shock, or drugs.226

Ttactic movementtaigaSee forest.tandem DNASee taxis.See repetitive DNA.tangential Orientated at a tangent to acircle, the circle being, for example, thesurface of a plant stem.tannin One of a mixed group of substancesthat, as defined by industry, precipitatethe gelatin of animal hides to formleather. Tannins are also used in dyeingand ink manufacture. They are polymersderived either from carbohydrates andphenolic acids by condensation reactions,or from flavonoids. Many plants accumulatetannins, particularly in leaves, fruits,seed coats, bark, and heartwood. Their astringenttaste may deter animals from eatingthe plant and they may discourageinfection. Species that produce largeamounts of tannin include tea (Camelliasinensis) and the bark of oak (Quercus).tapetum (pl. tapeta) A food-rich layersurrounding the spore mother cells in theanthers of vascular plants. These cells usuallydisintegrate, liberating food substancesthat are subsequently absorbed by thespore mother cells and the developingspores.taproot A persistent primary root thatgrows vertically downward. Swollen taprootsused for food storage are common inmany biennial plants, such as carrot (Daucuscarota).Taxaceae A family of evergreen coniferoustrees and shrubs, the yews, withneedlelike or linear leaves.taxis (pl. taxes) (tactic movement) Movementof an entire cell or organism (i.e.locomotion) in response to an externalstimulus, in which the direction of movementis dictated by the direction of thestimulus. Movement toward the stimulus ispositive taxis and away from the stimulusis negative taxis. It is achieved by protoplasmicstreaming, extrusion of cell substances,or by locomotory appendages,such as flagella and undulipodia. See aerotaxis;chemotaxis; phototaxis. Comparenastic movements; tropism.taxol A diterpene originally isolatedfrom the bark of the Pacific yew (Taxusbrevifolia), that has proved an effectivetreatment for solid tumors, especiallybreast cancer. It blocks tumor cells by stabilizingand polymerizing microtubules.taxon (pl. taxa) A group of any rank intaxonomy. Ranunculaceae (a family) andTriticum (a genus) are examples.taxonomy The area of systematics thatcovers the principles and procedures ofclassification, specifically the classificationof variation in living organisms. See classification;systematics.TCA cycleSee Krebs cycle.telophase The final stage in mitosis andmeiosis, in which the separated chromatids(mitosis) or homologous chromosomes ofbivalents (meiosis) collect at the poles ofthe spindle and the nuclei of the daughtercells are formed. During this stage in mitosisand the second division of meiosis thechromosomes uncoil and disperse, the nuclearspindle degenerates, nucleoli reappear,and a new nuclear membrane forms.227

temperate phageThe cytoplasm may also divide during thisphase.temperate phage A DNA bacteriophagethat becomes integrated into thebacterial DNA and multiplies with it,rather than replicating independently andcausing lysis of the bacterium. A bacteriumthus infected is said to be lysogenic. Seelysogeny. Compare virulent phage.temporary stainSee staining.tendril A slender structure found inplants, used for twining and support. Itmay be a modified terminal bud, as inthe grapevine (Vitis); a modified lateralbranch, as in the passion flower (Passiflora);a modified leaf, as in Clematis montana;a modified leaflet, as seen in many ofthe pea family (Fabaceae); or a modified inflorescence,as in Virginia creeper (Parthenocissus).Tendrils may be branched, orunbranched, and may have terminal adhesivedisks, as in the Virginia creeper.terminal budSee bud.terminalization The movement of chiasmatato the end of the bivalent arms, aprocess that may occur during lateprophase I of meiosis. The chiasmata canslip off the ends of the bivalents, and thuschiasma frequency may be reduced by terminalization.terpene One of a complex group oflipids based on the hydrocarbon skeletonC 5 H 8 (isoprene). Monoterpenes are builtfrom two C 5 residues (C 10 H 16 ), diterpenesfrom four, etc. The C 10 to C 20 terpenes arepresent in essential oils, giving the characteristicscent of some plants (e.g. mint,lemon). Some terpenoid substances arephysiologically active, e.g. vitamins A, E,and K.Tertiary The larger and older period ofthe Cenozoic, being composed of the Paleocene,Eocene, Oligocene, Miocene, andPliocene epochs (65–2 million years ago).Literally the ‘third age’, it is characterizedby the rapid evolution and expansion ofangiosperms and the emergence of mammals.The ferns underwent adaptive radiation,and the angiosperms underwent aneven greater explosion of adaptive radiation,in which flowers became more specialized.This went hand in hand with theevolution of specialist insect pollinators,especially bees, butterflies, and moths. Theclimate was warm and wet at the start ofthe period, but became colder and drierlater, and this was accompanied by the expansionin range of grasses. Many moderngenera of conifers arose in the Tertiary, butother gymnosperms declined. See also geologicaltime scale.tertiary structure The mode of foldingof a polypeptide (a-helix or b-pleatedsheet) in a PROTEIN. The shape may be stabilizedby HYDROGEN BONDS, disulfidebridges, and hydrophobic interactions,whereby hydrophobic parts of thepolypeptide are protected from the surroundingaqueous solution by hydrophilicparts. Such folding determines the shape ofthe active site of enzymes. See conformation;primary structure; quaternary structure;secondary structure.testa The hard dry protective covering ofa seed, formed from the integuments of theovule. After fertilization the layers of theinteguments fuse and become thickenedand pigmented.test crossSee backcross.tetrad 1. A group of four cells formed asa result of meiosis in a spore mother cell.The four cells may develop into spores, asin the formation of pollen grains.2. In meiosis, the association of four homologouschromatids seen during thepachytene stage of prophase.tetraploid A cell or organism containingfour times the haploid number of chromosomes.Tetraploid organisms may arise bythe fusion of two diploid gametes that haveresulted from the nondisjunction of chromosomesat meiosis, or by nondisjunction228

thornof the chromatids during the mitotic divisionof a zygote. Many stable tetraploidsarise initially from hybridization. See alsopolyploid; allotetraploid; autotetraploid.thallus (pl. thalli) A simple plant bodyshowing no differentiation into root, leaf,and stem and lacking a true vascular system.It may be uni- or multicellular, and isfound in the algae, lichens, bryophytes,and the gametophyte generation of the Filicinophyta,Lycophyta, and Sphenophyta.Theophrastus of Eresus (372–287 BC)Ancient Greek botanist and philosopher.Theophrastus was a pupil of Plato and assistedAristotle at the Lyceum in Athensafter Plato’s death. He became head of theschool when Aristotle died. He is oftenconsidered to be the father of scientificbotany because of his work on classifyingand naming plants, which he recorded inhis prolific writings. His pupils were encouragedto observe plants growing neartheir homes, which were often far awayfrom Athens, and possibly helped him tomake the connection between plants andthe environment, especially their adaptationsto soil and climate. His Enquiry intoPlants (nine volumes) is important eventoday. It is evident from his writings thathe had distinguished between monocotyledonsand dicotyledons and had discoveredthat seeds and fruits were derived fromflowers.thermoclineSee stratification.thermonasty (thermonastic movements)A NASTIC MOVEMENT in response to changein temperature. For example, Crocus flowersopen rapidly if the temperature increasesby 5 to 10°C, as the inner side ofthe petals grows faster than the outer side.thermoperiodism The phenomenonshown by certain plants, e.g. Chrysanthemumand tomato (Lycopersicon esculentum)in which there is a response to dailyalternations of low and high temperature.Such plants flower earlier and more profuselyif subjected to low night and highday temperatures. See also photoperiodism;vernalization.thermophilic Describing microorganismsthat require high temperatures (around60°C) for growth. It is exhibited by certainbacteria that grow in hot springs or compostand manure. Compare mesophilic;psychrophilic.therophyte A plant that survives part ofthe year as a seed and completes its lifecycle during the remainder of the year. Seealso Raunkiaer’s plant classification.thiamine (vitamin B 1 ) One of the watersolubleB-group of VITAMINS. It is a pyrimidinecompound that is synthesized byplants but not by some microorganismsand most vertebrates. It is therefore requiredin the diet of mammals. Goodsources of thiamine include unrefined cerealgrains.thigmotropism (haptotropism) A tropismin which the stimulus is touch. Thetendrils of climbing plants are thigmotropic.The part touching the substratebends toward the stimulated side, so bringingmore of the tendril into contact withthe substrate, until it encircles it. See tropism.thin-layer chromatography A chromatographicmethod in which a glass plateis covered with a thin layer of inert absorbentmaterial (e.g. cellulose or silica gel)and the materials to be analyzed are spottednear the lower edge of the plate. Thebase of the plate is then placed in a solvent,which rises up the plate by capillary action,separating the constituents of the mixtures.The principles involved are similar to thoseof paper chromatography and, like paperchromatography, two-dimensional methodscan also be employed. See paper chromatography.thorn A stiff sharply pointed woodyprocess that may be found on the stems ofvascular plants. It is a modified branch andis supplied with vascular tissue, e.g.229

tracheid An elongated xylem conductingelement with oblique end walls. Tracheidshave heavily lignified walls and theonly connection between adjacent tracheidsis through paired pits, which aremainly concentrated in the end walls.Many tracheids lack protoplasm at maturityand have lignified secondary cell walls.Tracheids form the only xylem conductingtissue of vascular plants other than anthornforesthawthorn (Crataegus monogyna). Compareprickle; spine.thorn forestSee forest.threonine An AMINO ACID derived fromaspartic acid. It is broken down to formglycine and acetyl CoA. Isoleucine can besynthesized from threonine.thylakoid An elongated flattened fluidfilledsac that forms the basic unit of thephotosynthetic membrane system in chloroplastsand photosynthetic bacteria. Seechloroplast.thymidine The nucleoside formed whenthymine is linked to D-ribose by a b-glycosidicbond. See nucleoside.thymine A nitrogenous base found inDNA, but not in RNA. It has a PYRIMIDINEring structure. It base pairs with adenine inthe complementary strand of DNA. Seebase pairing; thymidine.tissue In a multicellular organism, agroup of cells that is specialized for a particularfunction, e.g. palisade mesophyll.Several different tissues may be incorporatedinto an organ, e.g. a leaf. Compareorgan.tissue culture The growth of cells, tissues,or organs in suitable media in vitro.Such media must normally be sterile, correctlypH balanced, and contain all the necessarymicro and macronutrients andhormones for growth. Studies of such cultureshave shed light on physiologicalprocesses that would be difficult to followin the living organism. For example, the cytokininswere discovered through work ontobacco pith tissue culture.tocopherol (vitamin E) A terpenoid-likesubstance found especially in certain seeds,such as cereals, where it probably preventsoxidation of lipids, thus prolonging seedviability.tonoplast The membrane that surroundsthe large central vacuole of plantcells.torus (pl. tori) A disk-shaped structureformed from lignified primary cell-wallmaterial on the middle lamella of a borderedpit. The structure is found mainly inthe conifers. It is thought to act like a valve,sealing the pit when the pressure on thetwo sides is unequal, e.g. when an adjacenttracheid is damaged.totipotency The ability shown by manyliving cells to form all the types of tissuesthat constitute the mature organism. Thismay be achieved, even if the cells havecompletely differentiated, provided thatthe appropriate balance of nutrients andhormones is given. The best example ofthis phenomenon is the formation of adventitiousembryos in carrot tissue cultures.toxin A chemical produced by apathogen (e.g. bacteria, fungi) that causesdamage to a host cell in very low concentrations.The toxin may be formed withinthe pathogen and released when it dies (endotoxin),or secreted through its cell wall(exotoxin). Toxins are often similar to theenzymes of the host and interfere with theappropriate enzyme systems.trabecula (pl. trabeculae) An elongatedcell wall, cell, or line of cells across a cavity.In plants, trabeculae may be found inSelaginella stems, where they suspend thesteles in large air spaces.trace elementSee micronutrient.230

transfer RNAgiosperms, some Gnetales, and a few fernssuch as bracken. Compare vessel.tracheophyte (Tracheophyta) Any plantwith a differentiated vascular system; i.e.all plants except the liverworts, mosses,and hornworts.trama The inner tissue of the gills in basidiomycetefungi that is made up ofloosely packed hyphae.transamination The transfer of anamino group from an AMINO ACID to an a-keto acid, producing a new a-keto acid anda new amino acid. This is catalyzed by atransaminase enzyme in conjunction withthe coenzyme pyridoxal phosphate. Theamino group becomes attached to thecoenzyme to form pyridoxamine phosphate,and is then transferred to the a-ketoacid, which is usually pyruvic acid, oxaloaceticacid, or a-ketoglutaric acid.Transamination is a key step in the biosynthesisand breakdown of most amino acids.transcription The process in living cellswhereby RNA is synthesized according tothe template embodied in the base sequenceof DNA, thereby converting thecell’s genetic information into a coded message(messenger RNA, mRNA) for the assemblyof proteins, or into the RNAcomponents required for protein synthesis(ribosomal RNA and transfer RNA). Theterm is also applied to the formation of single-strandedDNA from an RNA template,as performed by the enzyme reverse transcriptase,for example in retrovirus infections.Details of DNA transcription differbetween prokaryote and eukaryote cells,but essentially it involves the followingsteps. Firstly, with the aid of a helicase enzyme,the double helix of the DNA moleculeis unwound in the region of the sitemarking the start of transcription for a particulargene. The enzyme RNA polymerasemoves along one of the DNA strands, thetranscribed strand (or anticoding strand,since the code is carried by the complementarybase sequence of the RNA), andnucleotides are assembled to form a complementaryRNA molecule. The polymeraseenzyme proceeds until reachinga stop signal, when formation of theRNA strand is terminated. Behind the enzyme,the DNA double helix re-forms,stripping off the newly synthesized RNAstrand. In eukaryotes, transcription is initiatedand regulated by a host of proteinscalled transcription factors; in prokaryotesan accessory sigma factor is essential fortranscription. See also translation.transduction The transfer of part of theDNA of one bacterium to another by atemperate bacteriophage. The process doesoccur naturally but is mainly known as atechnique in RECOMBINANT DNA TECHNOL-OGY, and has been used in mapping thebacterial chromosome.transect A line or belt designed to studychanges in species composition across aparticular area. The transect samplingtechnique is most often used in plant ecologyto study changes in the composition ofvegetation. See also quadrat.transferase An enzyme that catalyzesreactions in which entire groups or radicalsare transferred from one molecule to another.For example, hexokinase catalyzesthe transfer of a high-energy terminalphosphate group from ATP to glucose togive glucose 6-phosphate and ADP.transfer cell A specialized type of plantcell in which the cell wall forms protuberancesinto the cell, thus increasing the surfacearea of the wall and plasmamembrane. Transfer cells are active cellscontaining many mitochondria, and areconcerned with short-distance transport ofsolutes. They are common in many situations,for example as gland cells and epidermalcells, and in xylem and phloemparenchyma, where they are concernedwith active loading and unloading of vesselsand sieve tubes.transfer RNA (tRNA) A type of RNAthat participates in protein synthesis in livingcells. It attaches to a particular amino231

transformationacid and imports this to the site of polypeptideassembly at the ribosome when the appropriatecodon on the messenger RNA isreached. Each tRNA molecule consists ofroughly 80 nucleotides; some regions ofthe molecule undergo base pairing andform a double helix, while in others thetwo strands separate to form loops. Whenflattened out the tRNA molecule has acharacteristic ‘cloverleaf’ shape with threeloops; the base of the ‘leaf’ carries theamino acid binding site, and the middleloop contains the ANTICODON, whose basetriplet pairs with the complementarycodon in the mRNA molecule. Hence, becausethere are 64 possible codons in thegenetic code, of which about 60 or so codefor amino acids, there may be up to 60 orso different tRNAs in a cell, each with adifferent anticodon, although some ofthem will bind the same amino acid. Thecorrect amino acid is attached to a tRNAmolecule by an enzyme called an aminoacyl-tRNAtransferase. There are 20 ofthese, one for each type of amino acid. Thisattachment also involves the transfer of ahigh-energy bond from ATP to the aminoacid, which provides the energy for peptidebond formation during TRANSLATION.transformation A permanent geneticrecombination in a bacterial cell, in whicha DNA fragment is incorporated into theDNA of the cell. This may be demonstratedby growing bacteria in the presence of deadcells, culture filtrates, or extracts of relatedstrains. The bacteria acquire genetic charactersof these strains.transgenic Describing organisms, especiallyeukaryotes, containing foreign geneticmaterial. Genetic engineering hascreated a wide range of transgenic animals,plants, and other organisms for both experimentaland commercial purposes. Examplesinclude herbicide-resistant cropplants and plants that secrete pharmaceuticals.See recombinant DNA.transition zone The zone in a vascularplant where the root and shoot structuresmerge and where arrangement of the vasculartissue is intermediate between that ofthe root and shoot. See hypocotyl.translation The process whereby thegenetic code of messenger RNA (mRNA) isdeciphered by the machinery of a cell tomake proteins. Molecules of mRNA are ineffect coded messages based on informationin the cell’s genes and created by theprocess of TRANSCRIPTION. They relay thisinformation to the sites of protein synthesis,the RIBOSOMES. In eukaryotes these arelocated in the cytoplasm, so the mRNAmust migrate from the nucleus. The firststage in translation is initiation, in whichthe two subunits of the ribosome assembleand attach to the mRNA molecule near theinitiation codon, which signals the beginningof the message. This also involves variousproteins called initiation factors, andthe initiator TRANSFER RNA (tRNA), whichalways carries the amino acid N-formylmethionine. The next stage is elongation,in which the peptide chain is built up fromits component amino acids. This involvesthe participation of several proteins calledelongation factors, and tRNA moleculesthat successively occupy two sites on thelarger ribosome subunit in a sequence determinedby consecutive codons on themRNA. As each pair of tRNAs occupiesthe ribosomal sites, their amino acids arejoined together by a peptide bond. As theribosome moves along the mRNA to thenext codon, the next tRNA enters the firstribosomal site, and so on, leading to elongationof the peptide chain. Having deliveredits amino acid, the depleted tRNA isreleased from the second ribosomal site,which is then occupied by the tRNA withthe growing chain. This process continuesuntil the ribosome encounters a terminationcodon. A protein called a release factorbinds directly to the stop codon,causing the addition of a water moleculeinstead of an amino acid to the polypeptidechain. The polypeptide chain is thenreleased and the ribosome complex dissociates,marking the termination of translation.Following its release, the polypeptidemay undergo various changes, such as theremoval or addition of chemical groups, or232

trichomeeven cleavage into two parts. This posttranslationalmodification produces thefully functional protein. Folding of the proteinis assisted by a class of moleculescalled chaperones.translocation 1. The movement of mineralnutrients, elaborated food materials,and hormones through the plant. In vascularplants, the xylem and phloem serve totranslocate such substances.2. See chromosome mutation.transpiration The loss of water vaporfrom the surface of a plant. Most is lostthrough stomata when they are open forgaseous exchange. It has been shown thattranspiration through the stomataamounts to 90% of the transpiration thatoccurs from a water surface of the samearea as a the leaf. Typically, about 5% islost directly from epidermal cells throughthe cuticle (cuticular transpiration) and aminute proportion through lenticels. Acontinuous flow of water, the transpirationstream, is thus maintained through theplant from the soil via root hairs, root cortex,xylem, and tissues such as leaf mesophyllserved by xylem. Evaporation ofwater from the leaf surface sets up a waterpotential gradient that extends to theroots, helping to draw up water; waterflow is enhanced by the narrow diameter ofthe xylem vessels, which favor capillary action.ROOT PRESSURE may be a contributingfactor. Water evaporates from wet cellwalls into intercellular spaces and diffusesout through stomata. Transpiration maybe useful in maintaining a flow of solutesthrough the plant and in helping to coolleaves through evaporation, but is oftendetrimental under conditions of watershortage, when wilting may occur. It is favoredby low humidity, high temperatures,and moving air. A major control is the degreeof opening of the stomata. Compareguttation. See antitranspirant; cohesiontheory.transposon (transposable geneticelement; jumping gene) A segment of anorganism’s DNA that can insert at varioussites in the genome, either by physicallymoving from place to place, or by producinga copy that inserts elsewhere. The simplesttypes are called insertion sequences;these comprise about 700–1500 base pairs.More complex ones, called compositetransposons, have a central portion, whichmay contain functional genes, flanked byinsertion sequences. Transposons can affectboth the genotype and phenotype ofthe organism, e.g. by disrupting gene expression,or causing deletions or inversions.They are common in bacteria, wherethey usually contain only one or two genes.In eukaryotes, transposons account formuch of the repetitive DNA in the genome.Compare plasmid. See repetitive DNA.tree fernsSee Filicinophyta.triacylglyceride (triglyceride) An esterof glycerol in which all the –OH groups areesterified; the acyl groups may be the sameor different. Plant triacylglycerols are usuallyunsaturated. They are synthesizedfrom glycerol phosphate and fatty acylCoAs, usually in the microsomes. Lipasescatalyze the breakdown of triacylglycerolsto glycerol and free fatty acids. Triacylglycerolsare important food reserves, especiallyin seeds such as rape (Brassicanapus), linseed (Linum usitatissimum),castor bean (Ricinus communis) and coconut(Cocos nucifera). They are commercialsources of fats (solid triacylglycerols)and oils (liquid triacylglycerols). See alsocarboxylic acid; glyceride; lipid; microsome.Triassic The oldest period of the Mesozoicera, 250–215 million years ago. Duringthe Triassic, the climate graduallychanged from arid to more temperate.Conditions for preservation in the drierperiod were poor, so there are few fossils.There was a diversification of the gymnosperms,and fossil groups such as theBennettitales and Caytoniales evolved duringthis period. See also geological timescale.tribe A taxonomic rank consisting of agroup of closely related genera. A numberof tribes comprise a subfamily. The tribe is233

triglycerideintroduced only in classifications of verylarge families such as the grasses (Poaceae).tricarboxylic acid cyclecycle.trichocysttrichometriglycerideSee Dinomastigota.See hair.See triacylglyceride.See Krebstriplet A sequence of three nucleotidebases on a DNA or messenger RNA molecule.See genetic code.triploid A cell, tissue or organism containingthree times the haploid number ofchromosomes. In flowering plants the endospermtissue is usually triploid, resultingfrom the fusion of one of the pollen nucleiwith the two polar nuclei. Triploids areusually sterile.trisomySee aneuploidy.trophic level In complex natural communities,organisms whose food is obtainedfrom plants by the same number ofsteps are said to belong to the same trophicor energy level. The first and lowest trophiclevel contains the producers, green plantsthat convert solar energy to food by photosynthesis.Herbivores occupy the secondtrophic level and are primary consumers:they eat the members of the first trophiclevel. At the third level carnivores eat theherbivores (the secondary consumer level),and at the fourth level secondary carnivoreseat the primary carnivores (the tertiaryconsumer level). These are generalcategories, as many organisms feed on severaltrophic levels, for example omnivoreseat both plants and animals. Decomposersor transformers occupy a separate trophiclevel, which consists of organisms such asfungi and bacteria, which break downdead organic matter into nutrients usableby the producers. See food chain.tropism (tropic movement) A directionalgrowth movement of part of a plantin response to an external stimulus. Tropismsare named according to the stimulus.The organ is said to exhibit a positive ornegative tropic response, depending onwhether it grows toward or away from thestimulus respectively, e.g. shoots are positivelyphototropic but negatively gravitropic.Growth straight toward or awayfrom the stimulus (0°and 180°orientationrespectively) is called orthotropism. Primaryroots and shoots are orthotropic tolight and gravity. By contrast, growth atany other angle to the direction of the stimulusas by branches or lateral roots is calledplagiotropism. The mechanism involved inthe latter is poorly understood. In diatropismthe plant part is orientated at rightangles to the direction of the stimulus. Forexample, rhizomes grow horizontally in responseto gravity (diagravitropism), andleaves often grow at right angles to the incidentlight. Since the receptor for the stimulusis often separate from the region ofgrowth, tropic movements are often mediatedby hormones. See phototropism; gravitropism.Compare nastic movements;taxis.tryptophanSee amino acids.tuber A swollen underground stem orroot that contains stored food and acts asan organ of perennation and vegetativepropagation. Stem tubers, e.g. potato(Solanum tuberosum) develop at the end ofunderground stems by swelling of nodesand internodes. There is an increase in pithtissue to form a round tuber that bearsbuds in the axils of greatly reduced scaleleaves. The stem connecting the tuber tothe parent plant then severs. Root tubersmay develop in the same way from adventitiousroots, as in Dahlia. A root tuber canbe distinguished for a stem tuber by the absenceof buds or ‘eyes’. See perennatingorgan.tubulinSee microtubule.tundra A region or type of vegetation athigh latitudes and high altitude. Tundra iswidespread in the Arctic north of the tree234

tyrosinelines of North America and Eurasia, butoccurs in only scattered localities onAntarctic islands, where it consists mainlyof lichens and mosses.tunica–corpus theory A theory of apicalorganization and development that distinguishestwo separate tissue zones, thetunica and the corpus, in the apex of aflowering plant. The tunica is made up ofone or more peripheral layers in which celldivision is mostly anticlinal. The corpus isthe inner area of tissue in which cellarrangement and division is irregular. Theepidermis originates in the tunica regionand the other stem tissues arise from eitherthe tunica or the corpus, depending on thespecies. Compare histogen theory.turgor The state, in a plant or prokaryotecell, in which the protoplast is exertinga pressure on the cell wall owing to the intakeof water by osmosis. Being slightlyelastic, the cell wall bulges, but is rigidenough to prevent water entering to thepoint of bursting. The cell is then said to beturgid. Turgidity is the main means of supportof herbaceous plants and other unlignifiedtissues. See osmosis; plasmolysis;turgor pressure.turgor pressure (pressure potential) Thehydrostatic pressure exerted by the contentsof a cell against the cell wall. It resultsfrom the uptake of water into the cell byosmosis. It may be alternatively expressedas pressure potential – the hydrostatic pressureto which water in a liquid phase issubjected.turion 1. A swollen detached winter budthat contains stored food and is protectedby an outer layer of leaf scales and mucilage.It is an organ of perennation or vegetativepropagation, and is characteristic ofvarious water plants (e.g. Sagittaria). Turionsare usually released from the plant andlie dormant on the bed of the pond or riveruntil conditions become favorable forgrowth.2. Any vegetative shoot or sucker.tylose A bladderlike ingrowth from aparenchyma cell into an adjacent tracheidor vessel through a paired pit. Tyloses areoften found in injured tissue, older wood,and below an abscission layer, and cancompletely block the conducting vessel.They often become filled with tannins,resins or pigments, giving the heartwoodits dark color, and helping to preserve andstrengthen it. Some of these pigments areused commercially as dyes, e.g. hematoxylin.type 1. The material used to define aspecies. It is usually a dried specimenstored in a herbarium but may also be adrawing. Type specimens of microorganismsare often living cultures.2. The representative species of a genus, therepresentative genus of a family, etc. For235

Uubiquinone See coenzyme Q.ultracentrifuge A high-speed centrifuge,operating at up to a million revolutionsper second that is used to sedimentprotein and nucleic acid molecules. Ultracentrifugesoperate under refrigeration in avacuum chamber and forces 50 milliontimes that of gravity may be reached. Therate of sedimentation depends on the molecularweight of the molecule and thus theultracentrifuge can be used to separate amixture of large molecules, such as largecolloidal proteins, and estimate particlesizes.ultramicrotomeSee microtome.ultrastructure (fine structure) The detailedstructure of biological material as itcan be revealed, for example, by electronmicroscopy, but not by light microscopy.umbelSee inflorescence.undulipodium (pl. undulipodia) Awhiplike organelle that protrudes from aeukaryotic cell and is used chiefly for locomotion(e.g. motile gametes) or feeding(e.g. ciliate protoctists). Undulipodia includeall eukaryotic cilia and flagella,which share the same essential structure,and differ markedly from bacterial flagella(see flagellum). They are complex structures,containing over 500 different proteins.The shaft comprises a cylindricalarray of nine doublet microtubules surroundinga central core of two single microtubules.The outer wall of the shaft is anextension of the cell membrane. Its structureis similar to that of a centriole. Strictly,the term flagella should be used only in relationto bacteria. Long undulipodia (over10 m m), formerly called flagella, occursingly or in pairs and produce successivewaves of bending that are propagated tothe tip of the shaft, e.g. Chlamydomonas.unicellular Describing organisms thatexist as a single cell. Such a state is characteristicof most protoctists and bacteriaand is also found in many fungi. Comparemulticellular; acellular.unisexual Describing organisms thathave either male or female sex organs, butnot both. Unisexual plants are said to bedioecious. Compare hermaphrodite; monoecious.universal indicatorSee indicator.uracil A nitrogenous base that is foundin RNA but not in DNA. During TRANSCRIP-TION, the thymine of the DNA template issubstituted by a uracil in the RNA copy.Uracil has a PYRIMIDINE ring structure, andis derived from sugars and amino acids.Undulipodiumuridine The NUCLEOSIDE formed whenuracil is linked to D-ribose by a b-glycosidicbond.236

Vvacuole A fluid-filled organelle of variablesize found in the cytoplasm and separatedfrom it by a single membrane, theTONOPLAST. Many mature plant cells havea single large central vacuole that confinesthe cytoplasm to a thin peripheral layer.Newly formed plant cells often have anumber of small vacuoles derived from theendoplasmic reticulum. As they fill withsap, they enlarge and coalesce to form asingle vacuole. The vacuole is filled withcell sap, which contains substances in solution,e.g. sugars, salts, and organic acids,often in high concentrations resulting in ahigh osmotic pressure. Water thereforemoves into the vacuole by osmosis makingthe cell turgid. Vacuoles may also containstorage products such as starch grains, oildroplets, crystals, and waste substancesthat are byproducts of the cell’s metabolism.valine An AMINO ACID synthesized frompyruvate. It is broken down to yield succinylCoA.variation The extent to which the characteristicsof the individuals of a speciescan vary. Variation can be caused by environmentaland genetic factors. Environmentalvariation (phenotypic plasticity)results in differences in the appearance ofindividuals of a species because of differencesin nutrition, disease, light intensity,etc. Genetic variation is caused by recombinationand occasionally by MUTATION.These differences may be favored or discriminatedagainst by NATURAL SELECTION.See discontinuous variation; continuousvariation; recombination.variegation The occurrence of patchesof different colors on leaves or flowers. Itmay be caused by infection with a pathogen,mineral deficiency, or genetic or physiologicaldifferences between patches ofcells. It occurs naturally in some species,such as the dumb canes (Dieffenbachia).variety The taxonomic group below thesubspecies level. The term is often looselyused to describe breeds of livestock or variouscultivated forms of agricultural andhorticultural species. Varieties are morphologicalvariants, which may differ incolor or growth habit. See also cultivar.vascular bundle A strand of conductingtissue found in vascular plants. In plantsshowing secondary thickening (e.g. manydicotyledons) the vascular bundles join togetherto form a ring and contain meristematiccambium tissue between the xylemand phloem, but this is generally lacking inplants without secondary thickening (e.g.most monocotyledons). See also vascularplants.vascular cambium The ring of CAM-BIUM formed by the activity of the interfascicularcambium and the intrafascicularcambium that gives rise to secondaryxylem and secondary phloem. It is a lateralmeristem and contains fusiform initialsgiving rise to secondary xylem and phloemand ray initials giving rise to medullaryrays. See also secondary growth.vascular plants Plants containing differentiatedcells forming conducting tissue(vascular tissue, also known as fasciculartissue), which comprises the xylem andphloem. Vascular tissue transports waterand nutrients through the plant and alsoprovides strength and support. It consistsprincipally of XYLEM and PHLOEM (waterandfood-conducting tissues respectively),and also contains strengthening tissue237

vascular system(SCLERENCHYMA) and packing tissue(PARENCHYMA). The arrangement of vasculartissue (the vascular system) in the stemis very varied, giving a number of differenttypes of STELE. Primary vascular tissue,which is found in all vascular plants, isformed from the procambium. Secondaryvascular tissue, found only in plants withsecondary thickening, develops from thevascular cambium. The vascular cambiumextends to form a complete ring of meristematictissue around the stem, the separatevascular bundles being linked byinterfascicular cambium. Vascular plantsare able to achieve considerable verticalgrowth both upwards and into the soil.They have thus been able to colonize thedrier habitats that are inaccessible to thenonvascular bryophytes. The vascularplants may be grouped into a single division,the Tracheophyta.vascular system (vascular tissue)vascular plants.vascular tissueSee vascular plants.Seevector 1. An agent that carries a diseasecausingorganism to a healthy plant oranimal, causing the latter to become infected;.In its widest sense, the term includesagents such as wind, rain, etc., but itis usually restricted to vectors that are animalsor microorganisms. Insects such asaphids carry viruses from plant to plant asthey feed, and mites and nematodes canalso carry disease. Humans, including gardenersand scientists, may carry fungal andbacterial spores on their clothing or skin.2. (cloning vector) An agent used as a vehiclefor introducing foreign DNA, for examplea new gene, into host cells. Severaltypes of vector are used in GENE CLONING,notably bacterial plasmids and bacteriophages.The segment of DNA is firstspliced into the DNA of the vector, such asa plasmid, then the vector is transferred tothe host cell (e.g. the bacterium, E. coli),where it replicates along with the host cell.The result is a clone of cells, all of whichcontain the foreign gene, which is expressedby the host cell machinery. A commonvector used to transfer cloned DNA toplant cells is the bacterium Agrobacteriumtumefasciens. The foreign DNA is splicedinto a plasmid, which is taken up by thebacterium. The bacterium then infects aplant cell, and the plasmid DNA may becomeintegrated into the plant chromosome.vegetative cellSee vegetative nucleus.vegetative growth (vegetative propagation;vegetative reproduction) Growth ofany parts of the plant other than flowers.This involves both upwards growth andoutwards. Sometimes outward growth involvesSTOLONS or RHIZOMES or the buddingof BULBS or CORMS. These parts mayget detached from the parent and becomeindependent, although they are identicalgenetically. Some plants have specializedorgans such as GEMMAE or BULBILS for suchspread. See also budding; cutting; graft;layering; perennating organ.vegetative nucleus (tube nucleus; vegetativecell) One of the two or three nuclei ina young pollen grain that are formed afterdivision of the haploid nucleus. After thepollen grain germinates on the surface ofthe stigma, the vegetative nucleus migratesto the tip of the pollen tube. It is thought toregulate the growth and development ofthe pollen tube. The tube nucleus disintegratesas the pollen tube penetrates the nucellus.vegetative parts All parts of a plant exceptfor those involved in sexual reproduction.vein One of the vascular bundles in aleaf. The pattern in which the veins arearranged in a leaf is called the VENATION.velamen A layer surrounding the aerialroots of epiphytic plants (e.g. orchids)which, due to the spongy nature of thecells, is able to soak up surface water. It ismade up of several layers of dead emptycells situated on the epidermis. The cellsare spirally thickened, and are translucent238

vesicleto allow light through to the photosynthetictissue beneath.velumSee annulus.venation The distribution of veins (vascularstrands) in a leaf. Dicotyledons usuallyshow a netlike arrangement whereasmonocotyledons generally show a paralleldistribution of veins. Venation is a charactersometimes used in taxonomy.venter The swollen base of an archegoniumthat contains the egg cell (oosphere).ventral 1. In thallose plants, such assome liverworts, describing the lower surfaceclosest to the substrate.2. Designating the upper or adaxial surfaceof the lateral organs of plants, such asleaves. The term is not in common use inthis context. Compare dorsal.vernalization The cold treatment ofungerminated or partially germinatedseeds. Certain plants will germinate andother flower only if exposed to low temperatures(1–2% C) at an early period ofgrowth, i.e. they have a chilling requirement.Thus winter varieties of cereals willonly flower in summer if sown the previousautumn. Spring-sown winter varieties remainvegetative throughout the season unlessthey have been vernalized.vernation The arrangement of leaves inrelation to each other in the bud, or thearrangement of perianth segments in aflower bud. See illustration overleaf. Seealso ptyxis.versatile Describing an anther that is attachedto the tip of the filament abouthalfway along its length in such a way thatit can turn freely in the wind, thus aidingpollen dispersal. Compare basifixed; dorsifixed.vesicle A small vacuole of variable originand shape. It may contain secretoryproducts, e.g. a vesicle budded off from theGolgi apparatus, or particles, e.g. endocy-Venation239

virus An extremely small infectiousagent that causes a variety of diseases inplants, such as tobacco mosaic disease.Viruses can only reproduce in living tissues;outside the living cell they exist as inactiveparticles consisting of a core of DNAor RNA surrounded by a protein coat (capsid).Most plant viruses are single-strandedRNA viruses. The inert extracellular formof the virus, termed a VIRION, penetratesthe host membrane and liberates the viralnucleic acid into the cell. Usually, the nucleicacid is translated by the host cell ribosomesto produce enzymes necessary forthe reproduction of the virus and the forvesselorder Gnetales, and a few other plants suchas bracken (Pteridium aquilinum).vestigial organ An organ that is functionlessand generally reduced in size butbears some resemblance to the correspondingfully functioning organs found in relatedorganisms. Examples include thescale leaves of parasitic flowering plants orthe reduced stamens in females of dioeciousspecies.virion The extracellular inert phase of avirus. A virion consists of a protein coatsurrounding one or more strands of DNAor RNA. Virions may be polyhedral orhelical and vary greatly in size.viroid A tiny infectious agent found inplants that is similar to a virus but lacks acapsid, consisting simply of a circle ofRNA, 300–400 nucleotides long. Viroidsreplicate within the plant cell and causecharacteristic disease symptoms; examplesare the potato spindle tuber viroid and thehop stunt viroid.virulence The relative pathogenicity ofan organism: its ability to cause disease.Virulence may vary between strains of thesame organism.Vernation: types of vernationtotic vesicle. The membrane envelope isolatesthe contents from the surrounding cytoplasm.vessel A form of xylem conducting tissuecomposed of vertically arranged vesselelements (the individual cells that make upa xylem vessel) joined end to end, generallybroader than TRACHEIDS. The end walls ofthe vessel elements have broken down toleave perforation plates. These are usuallyhorizontal rather than the slanting endwalls of tracheids. Vessel elements have lignifiedsecondary cell walls and at maturitylack a living protoplast. Vessels are foundin most angiosperms, the gymnospermvirulent phage A bacteriophage that infectsa bacterial cell and immediately replicates,causing lysis of the host cell.Compare temperate phage.240

Volvocalesmation of daughter virions. The virions arereleased by lysis of the host cell. Otherviruses remain dormant in the host cell beforereproduction and lysis, their nucleicacid becoming integrated with that of thehost. Viruses are transmitted by VECTORSsuch as aphids and nematodes. Some producevisible symptoms, such as leaf mosaicsor deformed growth. Others have lessobvious effects, but can significantly reducecrop yields. A virus that infects a bacteriumis termed a BACTERIOPHAGE (phage).vital stains Nontoxic coloring materialsthat can be used in dilute concentrations tostain living material without damaging it.See also staining.vitamins Organic chemical compoundsthat are essential in small quantities for metabolismbut are not synthesized by animals,which therefore need to obtain themfrom plant or microorganisms. The vitaminshave no energy value; most of themseem to act as coenzymes for essentialchemical changes in the body, each one influencinga number of vital processes.Vitamins A, D, E, and K are fat-soluble vitamins,while vitamins B and C are watersolublevitamins.vivipary The germination of seeds orspores that are still attached to the parentplant. An example is mangroves, in whichthe seeds germinate into good-sizedseedlings before they are released from theplant, enabling them to plunge far enoughinto the mud to establish themselves.VolvocalesAn order of the Chloro-241

WWallace, Alfred Russel (1823–1913)British naturalist. After a basic educationand a short career as a surveyor, Wallacemet the entomologist Henry Bates and togetherthey embarked on a scientific expeditionas naturalists to the Amazon.Wallace described this expedition in ANarrative of Travels on the Amazon andthe River Negro (1853). His second expedition,to the Malay Archipelago, tookeight years during which he collected over125 000 specimens. He noticed the markeddifferences between the Australian andAsian flora and fauna and drew a line onthe map separating the regions, still referredto as Wallace’s line. His observationsof the marsupials in Australia led himto conclude that Australian animals weremore primitive than Asian ones. He believedthat these animals had been cut offfrom the Asian animals when the land massof Australia had broken away from Asia.This resulted in less competition for theAustralian animals than those on the largerEurasian land mass. These conclusions ledhim to formulate a theory of evolution bynatural selection, which he communicatedto DARWIN; as a result the two men presentedjoint papers at the famous LinnaeanSociety meeting of 1858. Unlike Darwin,Wallace never believed that the theory ofevolution could be applied to humans.Wallace continued to gather evidence forthe theory and published his results in GeographicalDistribution of Animals (1876)and Island Life (1880).wall pressurewater culturewater fernsSee turgor pressure.See hydroponics.See Marsileaceae.water moldswater poreSee Saprolegniales.See aquaporin.water potential Symbol: y A measureof the energy available in an aqueous solutionto cause water to move across a selectivelypermeable membrane duringosmosis. Water potential may be defined asthe chemical potential of water in a biologicalsystem compared to the chemicalpotential of pure water at the same temperatureand pressure. It is measured inkilopascals (kPa), and pure water has thevalue 0 kPa; solutions with increasing concentrationsof solute have more negativevalues of y, since the solute molecules interferewith the water molecules. This effectis termed the solute potential (orosmotic potential), denoted by y s ; it is measuredin kPa and always has a negativevalue, with increasing concentrations ofsolute having increasingly negative valuesof y s . In turgid plant cells there is also apressure exerted by the walls of the cell;this is called the pressure potential; it is definedby y p , and has a positive value (althoughin xylem cells it may be negativedue to water movement in the transpirationstream). Water potential is alsoaffected by capillary forces, especially inthe narrow tubes of the xylem, and in theinterstices of cell walls, and by imbibitionalforces. These forces are particularly importantin cell walls and intercellular spaces,increasing in wilting plants.wax One of a group of water-insolublesubstances with a very high molecularweight; they are esters of long-chain alcoholswith fatty acids. Waxes form protectivecoverings to leaves, stems, fruits, seeds,242

WWFanimal fur, and the cuticles of insects, servingprincipally as waterproofing.WCMC See World Conservation MonitoringCentre.WCUSee World Conservation Union.Western blotting A technique analogousto SOUTHERN BLOTTING used to separateand identify proteins instead of nucleicacids. The protein mixture is separated byelectrophoresis and blotted onto a nitrocellulosefilter. Antibodies specific to the proteinof interest are applied and bind to theirtarget proteins. A second antibody, specificto the first antibody, is then applied. Thiscarries a radioactive label, so enabling it tobe located by autoradiography wherever itbinds to the antibody–protein complex.wild-type The most commonly foundform of a given gene in wild populations.Wild-type alleles, often designated +, areusually dominant and produce the ‘normal’phenotype.wilt A plant disease characterized bywilting, usually due to pathogenic fungi orbacteria, or to parasites. It is often a rootdisease that eventually prevents the planttaking up water. Other wilts occur whenthe vascular tissues become blocked, e.g.Dutch elm disease, caused by the fungusCeratocystis ulmi, which causes the plantto block off its own infected tissues.wilting The loss of turgor due to lack ofwater. The tissues become limp and leavesand flowers droop. Wilting may be due tothe rate of transpiration exceeding the rateat which the roots can take up water in thesoil during drought periods, or to infection(see wilt). The permanent wilting point isreached when the plant has wilted to apoint where it will not recover, even ifmore water is added, because the cells haveplasmolyzed. See turgor.wood The hard fibrous structure foundin woody perennials such as trees andshrubs. It is normally formed from the secondaryxylem and thus found in plants thatshow secondary thickening, namely theconifers, Gnetum and dicotyledons. Waterand nutrients are only transported in theoutermost youngest wood, termed the SAP-WOOD. The nonfunctional compactedwood of previous seasons’ growth is calledthe HEARTWOOD and it is this that is importantcommercially.woody perennialSee perennial.World Conservation Monitoring Centre(WCMC) An information service setup by the World Conservation Union, theWorldwide Fund for Nature and theUnited Nations Environment Program tocompile information on conservation andthe sustainable use of the world’s naturalresources. The WCMC collects data onthreatened and endangered species andhabitats, national parks and nature reserves,international agreements, and conservationand environment programs. SeeRed Data Book.World Conservation Union (WCU)Formerly the International Union for theConservation of Nature (IUCN), foundedin 1948, an organization that brings togetherstates, government agencies, and arange of nongovernmental organizations topromote conservation and the sustainableuse of natural resources. Over 10 000 expertvolunteers collect information onparticular species and biodiversity conservationprojects, providing a base of expertisethat enables the WCU to advisecountries on national conservation strategies.Worldwide Fund (for Nature) (WWF)The largest independent conservation organizationin the world, comprising aglobal network of national organizations,associates, and program offices. It providesconservation services based on global policy,fieldwork, and scientific information.Its goal is to prevent genetic, species, andecosystem diversity, to ensure the sustainableuse of resources, and to promote actionsto reduce pollution.WWF See Worldwide Fund (for Nature).243

XYZxanthophyll One of a class of yellow toorange pigments derived from carotene,the commonest being lutein. In mostplants, xanthophylls function as accessorypigments in photosynthesis: they absorblight at wavelengths where chlorophyll haslow absorption and pass the absorbed energyto chlorophyll. In the Phaeophyta(brown algae) the xanthophylls fucoxanthinand peridinin act as the primary lightabsorbingpigments. See carotenoids;photosynthetic pigments.xeromorphic Structurally adapted towithstand dry conditions. See xerophyte.xerophyte Any plant adapted to growingin dry conditions or in a physiologicallydry habitat, such as an acid bog or a saltmarsh, or an exposed, very windy situation,by storing available water, reducingwater loss, or possessing deep root systems.Succulents, such as cacti and agaves(family Agavaceae), have thick fleshy stemsor leaves that store water. Features associatedwith reducing water loss include:shedding or dieback of leaves, e.g. ocotillo(Fouquieria splendens); waxy leaf coatingscoupled with closure or plugging of stomata,e.g. Kalanchoe; sunken or protectedstomata e.g. marram grass (Ammophilaarenaria); folding, rolling or repositioningof leaves to reduce sunlight absorption, e.g.marram grass; a dense covering of white reflectivespines, e.g. cacti; and the developmentof a dense hairy leaf covering, e.g.Espeletia. Compare hydrophyte; mesophyte.See also halophyte.xylem The water-conducting tissue invascular plants. It consists of dead hollowcells (the TRACHEIDS and VESSELS), whichare the conducting elements. It also containsadditional supporting tissue in theform of fibers and sclereids and some livingparenchyma. The proto- and metaxylem,which together constitute the primaryxylem, are formed from the procambiumof the apical meristems, while the secondaryxylem is differentiated from thecambium or lateral meristem. See secondarygrowth.yeastsSee Saccharomycetales.zinc A micronutrient that is needed forthe activity of the enzyme alcohol dehydrogenase,which catalyzes the conversion ofacetaldehyde to ethanol in anaerobic respiration.See micronutrient.zoosporangium (pl. zoosporangia)sporangium that produces ZOOSPORES.zoospore An asexual motile spore producedby a zoosporangium. It has one ormore undulipodia. These motile sporesmay encyst in adverse conditions or may bethe means by which a fungus or parasiticprotoctist penetrates a new host.zwitterion An ion with both a positiveand a negative charge. Amino acids canform zwitterions: the amino group has theform –NH + 3 and the acid group is ionizedas –COO – .zygomorphySee bilateral symmetry.zygomycete Any fungus belonging tothe phylum Zygomycota.Zygomycota A phylum of fungi whosemembers have haploid, nonseptate hyphae(i.e. hyphae that lack cross walls). Theircell walls contain chitin or chitosan. TheyA244

zygotenedo not produce motile spores at any stageof the life cycle. Zygomycetes are mostlysaprophytic, absorbing nutrients from decayingvegetation and other organic matter;the bread molds Rhizopus and Mucorare examples.zygospore A resistant sexual sporeformed when a zygote develops a thickwall. Zygospores are characteristic of theZYGOMYCOTA, especially the Mucorales(which include such fungi as Mucor andRhizopus) and the GAMOPHYTA (conjugatinggreen algae, e.g. Spirogyra and Zygnema).zygote The diploid cell resulting fromthe fusion of two haploid gametes. A zygoteusually undergoes mitosis immediately.See sexual reproduction.zygoteneSee prophase.245

APPENDIXES

Appendix ISI UnitsBASE AND DIMENSIONLESS SI UNITSPhysical quantity Name of SI unit Symbol for SI unitlength meter mmass kilogram(me) kgtime second selectric current ampere Athermodynamic temperature kelvin Kluminous intensity candela cdamount of substance mole mol*plane angle radian rad*solid angle steradian sr*supplementary unitsDERIVED SI UNITS WITH SPECIAL NAMESPhysical quantity Name of SI unit Symbol for SI unitfrequency hertz Hzenergy joule Jforce newton Npower watt Wpressure pascal Paelectric charge coulomb Celectric potential difference volt Velectric resistance ohm Welectric conductance siemens Selectric capacitance farad Fmagnetic flux weber Wbinductance henry Hmagnetic flux density tesla Tluminous flux lumen lmilluminance (illumination) lux lxabsorbed dose gray Gyactivity becquerel Bqdose equivalent sievert SvDECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITSSubmultiple Prefix Symbol Multiple Prefix Symbol10 –1 deci- d 10 1 deca- da10 –2 centi- c 10 2 hecto- h10 –3 milli- m 10 3 kilo- k10 –6 micro- m 10 6 mega- M10 –9 nano- n 10 9 giga- G10 –12 pico- p 10 12 tera- T10 –15 femto- f 10 15 peta- P10 –18 atto- a 10 18 exa- E10 –21 zepto- z 10 21 zetta- Z10 –24 yocto- y 10 24 yotta- Y248

The following all have useful information:Appendix IIWebpagesAmerican Society of Plant BiologistsBotanical Society of AmericaEuropean Initiative for BiotechnologyEducationInternet Directory for BotanyMissouri Botanical GardenNational Museum of Natural HistoryNew York Botanical GardenPhotosynthesis and the WebRoyal Botanic Gardens, KewUniversity of California Botanical Gardenwww.aspb.org/education/www.botany.orgwww.rdg.ac.uk/EIBE/www.botany.net/IDB/info.htmlwww.mobot.org/nmnhwww.si.edu/departments/botany.htmlwww.nybg.orgphotoscience.la.asu.edu/photosyn/photoweb/default.htmlwww.rbgkew.org.uk/www.mip.berkeley.edu/garden/249

BibliographyAlberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. MolecularBiology of the Cell. 4th ed., New York: Garland Science, 2002Ayala, Francisco J., Fitch, Walter M., & Clegg, Michael T. (eds). Variation andEvolution in Plants and Microorganisms. Washington: National Academy Press,2000Bell, Arnold D. Plant Form: An Illustrated Guide to Flowering Plant Morphology.Oxford, U.K.: Oxford University Press, 1991Bell, Peter R. & Hemsley, Alan R. Green Plants: Their Origin and Diversity. 2nd ed.Cambridge, U.K.: Cambridge University Press, 2000Campbell, Neil A. & Reece, Jane B. Biology. 6th ed. New York: Benjamin Cummings,2002Hopkins, W. G. Introduction to Plant Physiology. 2nd ed. New York: John Wiley andSons, 1995Lack, A. J. & Evans, D. E. Instant Notes: Plant Biology. New York: Springer-Verlag,2001Mabberley, D. J. The Plant-Book: A portable dictionary of the vascular plants. 2nd ed.Cambridge, U.K.: Cambridge University Press, 1997Margulis, L. & Schwartz, K. V. Five Kingdoms: An Illustrated Guide to the Phyla ofLife on Earth. 3rd ed. New York: W. H. Freeman, 1998Niklas, Karl J. The Evolutionary Biology of Plants. Chicago: University of ChicagoPress, 1997Purves, William K., Sadava, David, Orians, Gordon H., & Heller, H. Craig. Life: TheScience of Biology. 6th ed. New York: W. H. Freeman, 2001Raven, P. H., Evert, R. F. & Eichorn, S. E. Biology of Plants. 6th ed. New York: W.H.Freeman, 1998Ridge, Irene (ed). Plants. New York: Oxford University Press, 2002Stern, K. R. Introductory Plant Biology. 8th ed. New York: Sinauer Associates, 1998Westhoff, P., Jeske, H., Jurgens, G., Kloppstech, K., & Link, G. Molecular PlantDevelopment - from Gene to Plant. New York: Oxford University Press, 1998250

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