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BOTANY 

HIGHER SECONDARY - FIRST YEAR 

VOLUME - I 

REVISED BASED ON THE RECOMMENDATIONS OF THE 

TEXT BOOK DEVELOPMENT COMMITTEE 

A Publication Under 

Government of Tamilnadu 

Distribution of Free Textbook Programme 

(NOT FOR SALE) 

Untouchability is a sin 

Untouchability is a crime 

Untouchability is inhuman 

TAMILNADU 

TEXTBOOK CORPORATION 

College Road, Chennai - 600 006.

© Government of Tamilnadu 

First Edition - 2005 

Revised Edition - 2007 

Chairperson 

Dr. A. JAFFAR HUSSAIN 

Head of the Department of Botany (Rtd.) 

Presidency College (Autonomous) 

Chennai - 600 005. 

Authors 

Dr. MUJEERA FATHIMA 

Senior Scale Lecturer in Botany 

Government Arts College (Men) 

Nandanam, Chennai - 600 035. 

N. SHANTHA 

P.G. Teacher 

Government Higher Secondary School 

Kodambakkam, Chennai - 600 024. 

NALINI P. RAJAGOVINDAN 

Assistant Head Mistress 

Corporation Boys' 

Higher Secondary School 

Saidapet, Chennai - 600 015. 

Reviewers 

Dr. V. MURUGANANDAM 

Reader in Botany 

R.M. Vivekananda College 

Chennai - 600 004. 

T. R. A. DEVAKUMAR 

Selection Grade Lecturer in Botany 

Government Arts College (Men) 

Nandanam, Chennai - 600 035. 

VARALAKSHMI SIVALINGAM 

Selection Grade Lecturer in Botany 

Head of the Department 

Queen Mary's College 

Chennai - 600 004. 

Price : Rs. 

This book has been prepared by the Directorate of School Education on behalf of the 

Government of Tamil Nadu 

This Book has been printed on 60 GSM Paper

PREFACE 

We are passing through an "Era of Biology". Words like "Biotechnology', 

'Bioremediation", "Biochips", "Biomineralization", "Bioinformatics" etc. have 

become familiar even with "common man". Certainly there is a new unusual neverbefore-tried 

approach to address and solve many problems associated with modern 

life and to enhance the quality and standard of living by application of modern 

tools of Biology; particularly the Genetically Modified Foods (GM Foods) and 

other GM Products have revolutionized our Life. 

Application and exploitation of biological principles has become possible because 

of extensive knowledge and study of descriptive and functional aspects of living 

organisms over the years commonly studied under "Biology" which broadly 

comprises Botany and Zoology. Infact, Botany/Zoology is the 'mother science' of 

Molecular Genetics, Biochemistry, Microbiology, Molecular Cell Biology, 

Biochemical Engineering and ultimately Biotechnology, These recent applied fields 

are natrual outcome of a sound knowledge and study of basic Science, Botany 

(and of course Zoology). Without the study of structural and functional aspects of 

Green plants, Fungi, Bacteria, Viruses and their interrelationships, the"modern biology" 

is not possible. In fact, applied sciences, however 'modern', cannot replace basic 

sciences. 

The notion of 'Boring Botany' with its tasks of memorising 'technical terms', 

drawing diagrams can be dispensed with if only it is realized that application of 

the discipline BOTANY has unlimited potentialities in our complicated modern 

life through what is called Modern Biology. 

In this Book, BOTANY, a sincere attempt is made by my colleagues, on the 

basis of syllabus placed, to provide a simple and lucid account of Botany at the 

XI Standard. 

Each chapter is discussed with simplicity and clarity with Self-Evaluation at 

the end of each lesson. While preparing for the examination, students should not 

restrict themselves to the question/problems given in the Self-Evaluvation, they 

must be prepared to answer the questions and problems from the entire text. Infact, 

they are advised to refer to 'Reference Books' listed at the end to further their 

knowledge. 

DR. A. JAFFAR HUSSAIN 

Chairperson 

Text-Book Writing Committee (XI-Bio-Botany) 

iii

SYLLABUS : HIGHER SECONDARY 

- FIRST YEAR - BOTANY 

Unit 1 : Biodiversity (20 hours) 

Systematics : Two Kingdom and Five Kingdom Systems - Salient features of 

various Plant Groups (Algae, Fungi, Bryophytes, Pteridophytes and Gymnosperms) 

- Viruses - Bacteria - Algae : Spirogyra - Fungi : Mucor - Bryophyta : Riccia - 

Pteridophyta : Nephrolepsis - Gymnosperms : Cycas. 

Unit 2 : Cell Biology (20 hours) 

Cells as the basic Unit of Life - Cell Theory - Prokaryotic and Eukaryotic cells 

(Plant Cell) - Light Microscope and Electron Microscope (TEM & SEM) - Ultra 

Structure of Prokaryotic and Eukaryotic Cells - Cell Wall - Cell Membrane (Fluid 

Mosaic Model) Membrane Transport Model - Cell Organelles : Nucleus, 

Mitochondria, Plastid, Ribosomes - Cell Divisions : Amitosis, Mitosis and Meiosis 

and their significance. 

Unit 3 : Plant Morphology (10 hours) 

Structure and modifications of Root, Stem and Leaf - Structure and types of 

Inflorescences - Structure and types of flowers, fruits and seeds 

Unit 4 : Genetics (10 hours) 

Concept of Heredity and Variations - Mendel's Laws of Inheritance - Chromosomal 

basis of Inheritance - Intermediate Inheritance (incomplete Dominance) - Epistasis 

iv

Unit 5 : Plant Physiology (30 hours) 

Cell as physiological unit - Properties of Protoplasm - Water relations - Absorption 

and movement - Diffusion, Osmosis, Plasmolysis - Theories of Water Transport - 

Root pressure - Transpiration pull - Factors affecting rate of Transpiration - 

Mechanism of Stomatal opening and closing - Potassium ion theory - Factors 

affecting Stomatal movement - Functions of Minerals - Essential major elements 

and trace elements - Deficiency symptoms of elements - Theories of Translocation 

- Translocation of Solutes - Nitrogen Metabolism and Biological Nitrogen Fixation 

Movements - Geotropism - Phototropism - Turgor Growth Movements - Tropic - 

Nastic & Nutation. 

Unit 6 : Reproduction Biology (30 hours) 

Modes of Reproduction in Angiosperms - Vegetative propagation (natural and 

artificial) - Micropropagation - Sexual Reproduction - Pollination : types - Double 

fertilization - Development of male and female gametophytes - Development of 

Dicot Embryo - Parthenogenesis and Parthenocarpy - Germination of Seeds - 

Parts of Seed - Types of Germination - Abscission & Senescence. 

Unit 7 : Environmental Biology (20 hours) 

Organisms and environment as factors : Air, Water, Soil, Temperature, Light and 

Biota - Hydrophytes, Mesophytes, Xerophytes and their adaptations - Natural 

Resources Types, use and misuse - Conservation of water (RWH) - Ecosystems: 

(a) Structure & Function, (b) Energy flow, (c) Decomposition, (d) Nutrient Cycling, 

(e) Major Biomes, Forests - Grasslands, Deserts - Ecological Succession : 

Mechanism & Types (Hydrosere & Xerosere). 

v

Unit 8 : Practical Work (30 periods) 

1. Study of the following plants through specimens and slides and labelled 

sketches in the Botany Record Book 

1.1 Spirogyra 

1.2 Mucor 

1.3 Riccia 

1.4 Neprolepsis 

1.5 Cycas 

2. Study of the plant cells 

2.1 Onion peel : Observe in the microscope and draw labelled sketches 

2.2 Hydrilla leaves whole mount in pond water : observe and draw labelled 

sketches 

2.3 Squash preparation of onion root tip : Observe stages of Mitosis and 

draw labelled sketches 

3. Study of modifications of stem and root and draw labelled sketches 

3.1 Undderground root modifications : Radish, Carrot, Beet-root 

3.2 Aerial roots : Banyan Prop Roots - Climbiung Root of piper betel 

3.3 Underground stem modifications : Potato, Ginger, Onion, Yams 

4. Flower : Structure, Vertical Section, Floral Diagram and Floral Form 

of the following 

4.1 Hibiscus 

4.2 Datura 

vi

5. Physiology Experiments 

5..1 Transpiration 

(a) Transpiration pull 

(b) Ganong's Potometer 

5.2 Osmosis 

(a) Osmometer - using semipermeable plant memabrane 

(b) Potato Osmometer 

5.3 Root Pressure : Experiment to demonstrate root pressure in Dicots 

6. Germination of Seeds 

6.1 Hypogeal type 

6.2 Epigeal type (students to do project work) 

7. Hydrophytes & Xerophytes 

7.1 To study the specimens and write to note 

(a) Hydrophytes : Hydrilla / Vallisneria, Eichhornia, Pistia 

(b) Xerophytes : Opuntia, Euphorbia tirucalli, E. antiquorum, 

Aloe, Nerium. 

vii

CONTENTS 

Pages 

I. BIODIVERSITY........................................................................... 1-98 

1. Systematics .................................................................................. 1 

2. Salient Features of Various Groups ............................................ 11 

2.1 Viruses ........................................................................... 11 

2.2 Bacteria.......................................................................... 22 

2.3 Fungi .............................................................................. 31 

2.3.1 Mucor ...........................................................................39 

2.4 Algae.............................................................................. 45 

2.4.1 Spirogyra ......................................................................53 

2.5 Byrophytes ..................................................................... 59 

2.5.1 Riccia ............................................................................63 

2.6 Pteridophytes ................................................................. 71 

2.6.1 Nephrolepsis ...............................................................75 

2.7 Spermatophytes (Gymnosperms)................................... 81 

2.7.1 Cycas.............................................................................86 

II. CELL BIOLOGY .....................................................................99-144 

1. Cell as the basic unit of life ........................................................ 99 

2. Cell Theory ............................................................................... 103 

3. Prokaryotic and Eukaryotic Cell .............................................. 105 

4. Light Microscope and Electron Microscope ............................ 110 

5. Cell Wall ................................................................................... 113 

6. Cell Membrane ........................................................................ 118 

7. Cell Organelles ......................................................................... 126 

8. Cell Division ............................................................................. 136 

viii

III. PLANT MORPHOLOGY ................................................... 145-198 

1. Root, Stem and Leaf ................................................................ 145 

2. Inflorescence ........................................................................... 163 

3. Flowers, Fruits and Seeds ................................................................. 173 

IV. GENETICS ............................................................................ 199-225 

1. Heredity and Variations............................................................ 199 

2. Mendel's Laws of Inheritance ................................................. 203 

3. Chromosomal Basis of Inheritance .......................................... 214 

4. Intermediate Inheritance .......................................................... 218 

5. Epistasis.................................................................................... 220 

ix

BOTANY CHAPTERS 

I. BIODIVERSITY 1-98 

II. CELL BIOLOGY 99-144 

III. PLANT MORPHOLOGY 145-198 

IV. GENETICS 199-225 

V. PLANT PHYSIOLOGY 226-274 

VI. REPRODUCTION BIOLOGY 275-317 

VII. ENVIRONMENTAL BIOLOGY 318-367 

\\\\ 

x

Diversity in living organisms 

I. BIODIVERSITY 

1. Systematics 

There is a great diversity among living organisms found on the planet earth. 

They differ in their structure, habit, habitat, mode of nutrition, and physiology. 

The Biodiversity of the earth is enormous. Current estimates suggest that the 

earth may have anywhere from 10 to over 40 million species of organisms, but 

only about 1.7 million have actually been described including over 7,50,000 insects, 

about 2,50,000 flowering plants and 47,000 vertebrate animals. We call such a 

diversity among living organisms as Biodiversity. Even though there is such a 

variety and diversity among them, the living organisms show a lot of similarities 

and common features so that they can be arranged into many groups. In order to 

understand them and study them systematically, these living organisms, mainly 

the plants and animals are grouped under different categories. 

The branch of biology dealing with identification, naming and classifying 

the living organisms is known as Taxonomy. Taxonomy in Greek means rendering 

of order. The word Systematics means to put together. It was Carolus Linnaeus 

who used this word first in his book ‘Systema Naturae’. Systematics may be 

defined as the systematic placing of organisms into groups or taxa on the basis of 

certain relationships between organisms. 

Need for Classification 

It is not possible for any one to study all the organisms. But if they are 

grouped in some convenient way the study would become easier as the characters 

of a particular group or a family would apply to all the individuals of that group. 

Classification allows us to understand diversity better. 

History of Classification 

In the 3 rd and 4 th century BC Aristotle and others categorized organisms into 

plants and animals. They even identified a few thousands or more of living 

organisms. 

Hippocrates (460-377 BC), the Father of Medicine listed organisms with 

medicinal value. Aristotle and his student Theophrastus (370-282 BC) made 

1

the first attempt to classify organisms without stressing their medicinal value. They 

tried to classify the plants and animals on the basis of their form and habitat. It 

was followed by Pliny the Elder (23-79 AD) who introduced the first artificial 

system of classification in his book ‘Historia Naturalis’. John Ray an English 

naturalist introduced the term species for the first time for any kind of living 

things. It was then Carolus Linnaeus the Swedish naturalist of 18 th century now 

known as Father of Taxonomy developed the Binomial System of nomenclature 

which is the current scientific system of naming the species. In his famous book 

‘Species Plantarum’(1753) he described 5,900 species of plants and in “systema 

Naturae’(1758) he described 4200 species of animals. 

Taxonomy and Phylogeny 

Taxonomy is the branch of biology that deals with identification and 

nomenclature (naming) of living organisms and their classification on the basis of 

their similarities and differences. It was the Swiss-French botanist Augustin- 

Pyramus de Candolle(1778-1841) who coined the word Taxonomy, the science 

of naming and classifying of organisms. 

Species 

Species is the basic unit of Classification. It is defined as the group of individuals 

which resemble in their morphological and reproductive characters and interbreed 

among themselves and produce fertile offsprings. 

Species are then grouped into more inclusive taxa, which are grouped into 

larger taxa so that the classification is a hierarchy of a system of units that increase 

in inclusiveness from each level to the next higher level. The seven main categories 

used in any plan of classification are given below. 

1. Kingdom 

2.Phylum or Division 

3.Class 

4.Order 

5.Family 

6.Genus 

7.Species 

Phylogeny 

The evolutionary history of a particular taxon like species is called phylogeny. 

The classification based on the basis of evolution is called phylogenetic classification. 

Phylogenetic classification is not always possible since there are several gaps in 

2

the fossil records which form the basis of phylogenetic studies and also evolution is 

never unidirectional. Classification not explicitly based on evolutionary relationships 

is called artificial, for example, organisms are grouped according to usefulness 

(economic plants) size (herbs, shrubs) colour (flowers) ecological role (ground 

cover) and so-forth. Nevertheless many biologists make use of this non-systematic 

classification. 

Two Kingdom System of Classification 

Carolus Linnaeus(1758) divided all the living organisms into two kingdoms. 

1. Kingdom Plantae 

2. Kingdom Animalia 

1. Kingdom Plantae: 

This kingdom includes bacteria(Prokaryotes), photosynthetic plants and non - 

photosynthetic fungi. The characteristic features of this kingdom are: 

1. Plants have branches, asymmetrical body with green leaves. 

2. Plants are non motile and fixed in a place. 

3. During the day time plants more actively involve in photosynthesis than in 

respiration and hence take more of CO 2 

and liberate O 2 

& during night O 2 

is 

taken in and CO 2 

is liberated. 

4. They are autotrophic in their mode of nutrition since they synthesize their 

own food. 

5. Plants have growing points which have unlimited growth. 

6. Excretory system and nervous system are absent. 

7. Reserve food material is starch. 

8. Cells have a cell wall. Cells have a lager vacuole. Plant cells lack centrosome 

and they may have inorganic crystals. 

9. Reproduction takes place with help of agents such as air, water and insects. 

Asexual and vegetative method of reproduction is also not uncommon. 


This kingdom includes unicellular protozoans and multi-cellular animals or 

metazoans. They are characterized by 

1. Definite shape of the body and absence of branches. 

2. Ability to move from place to place. 

3

3. During day and night take in O 2 

and release CO 2 

i.e only respiration takes 

place and there is no photosynthesis. 

4. Holozoic mode of nutrition since no chlorophylls present and hence they 

are heterotrophs. 

5. Growth is limited in animals. Growth stops after attaining a particular size 

and age. 

6. Excretory system and nervous system are well developed. 

7. Reserve food material is glycogen. 

8. Lacks cell wall. They have small vacuoles. Centrosomes are present. 

Cells do not have inorganic crystals. 

9. Animals do not depend on any external agents for sexual reproduction. 

Regeneration of body parts and asexual reproduction is found only in lower 

organisms. 

Limitations of Two Kingdom System of Classification 

The two kingdom system 

of Classification proposed by 

Linnaeus has been in use for a 

long time. But later it proved 

to be inadequate and 

unsatisfactory in view of new 

information and discoveries 

about the lower forms of 

organisms. The following are 

the shortcomings of the two 

kingdom system of 


1. Certain organisms 

share the characteristics 

of both plants and 

animals. eg. Euglena 

Plantae 

Pteridophtes 

Spermatophytes 

Bryophytes 

and Sponges. In 

Euglena, some species 

have chlorophyll and are autotrophic like plants. However like animals 

they are dependent on an external supply of vitamins B, and B 12 

which they 

cannot synthesize themselves. A few species of Euglena lack chloroplasts 

and are therefore colourless and non-photosynthetic (heterotrophic). They 

have a saprotropic mode of nutrition, carrying out extra-cellular digestion. 

4 

j j 

j 

j 

Fungi 

j 

j 

Bacteria 

Algae 

j 

I n v e r t e b r a t e s 

Chordates 

Fig: 1.1. Diagrammatic representation of 

Two Kingdom System 

j 

j 

Animalia 

j 

j

Other colorless forms ingest small food particles and carryout intracellular 

digestion (holozoic nutrition). If green species of Euglena are kept in 

darkness they lose their chloroplasts and become colourless and survive 

saprotrophically. Chloroplasts return when the organisms are returned to 

light. Euglena is also characterized by the presence of an animal pigment 

astaxanthin in the eye spot. 

2. Fungi are a group of organisms which have features of their own. They 

lack chlorophyll. They are heterotrophic like animals. They are placed 

along with green plants. 

3. Many primitive organisms such as bacteria did not fit into either category 

and organisms like slime moulds are amoeboid but form fruiting bodies similar 

to fungi. 

4. The status of virus whether they are living or non living is a point of debate 

even to -day. 

For all these reasons the two hundred and fifty years old Linnaeus system 

of classifying organisms into two rigid groups animals and plants is considered 

highly arbitrary and artificial. 

The Five Kingdom System of Classification 

In order to suggest a better system of classification of living organisms, 

R.H. Whittaker (1969) an American Taxonomist divided all the organisms into 

5 kingdoms based on their phylogenetic relationships. This classification takes into 

account the following important criteria. 

1. Complexity of Cell structure – prokaryote to Eukaryote 

2. Mode of nutrition – autotrophs and heterotrophs 

3. Body organization -unicellular or multi-cellular 

4. Phylogenetic or evolutionary relationship 

The Five kingdoms are Monera, Protista, Fungi, Plantae and Animalia. 

1. Monera 

The Kingdom of Prokaryotes 

This kingdom includes all prokaryotic organisms i.e. mycoplasma, bacteria, 

actinomycetes(filamentous bacteria) and cyanobacteria (blue green Algae). They 

show the following characters. 

1. They are microscopic. They do not possess a true nucleus. They lack 

membrane bound organelles. 

5

2. Their mode of nutrition is autotrophic or heterotrophic. Some bacteria are 

autotrophic and are photosynthetic. i.e. they can synthesize their organic 

food in the presence of sunlight eg. Spirillum. Some bacteria are 

chemosynthetic i.e. they can synthesize their organic food by deriving energy 

from some chemical reactions. eg. Nitrosomonas and Nitrobacter. 

3. Many other bacteria like Rhizobium, Azotobacter and Clostridium can fix 

atmospheric nitrogen into ammonia. This phenomenon is called Biological 

Nitrogen Fixation . 

4. Some bacteria are parasites and others live as symbionts. 

5. Some monerans like Archaebacteria can live in extreme environmental 

conditions like absence of oxygen (anaerobic), high salt condition, high 

temperature like 80 0 c or above and highly acidic soils. 

2. Kingdom Protista 

This kingdom includes eukaryotic unicellular mostly aquatic cells. They 

show the following characters. 

1. They have a typical Eukaryotic cell organization. 

2. They often bear cilia or flagella for locomotion. Most of them are 

photosynthetic autotrophs. They form the chief producers of food in oceans 

and in fresh water. All unicellular plants are collectively called as 

phytoplanktons and unicellular animals as zooplanktons. Phytoplanktons are 

photosynthetically active and have cell wall. 

3. Zooplanktons are mostly predatory. They lack cell wall and show holozoic 

mode of nutrition as in Amoeba. 

4. Some protists are parasitic. Some are symbionts while others are 

decomposers. 

Euglena, a protozoan has two modes of nutrition. In the presence of sunlight 

it is autotrophic and in the absence of sunlight it is heterotrophic. This mode of 

nutrition is known as myxotrophic and hence they form a border line between 

plants and animals and can be classified in both. 

3. Kingdom Fungi 

This kingdom includes moulds, mushrooms, toad stools, puffballs and bracket 

fungi. They have eukaryotic cell organization. They show the following 

characteristics. 

1. They are either unicellular or multi-cellular organisms. 

6

2. Their mode of nutrition is heterotrophic since they lack the green pigment 

chlorophyll. Some fungi like Puccinia are parasites while others like 

Rhizopus are saprotrophic and feed on dead organic matter. 

3. Their body is made up of numerous filamentous structures called hyphae. 

4. Their cell wall is made up of chitin. 

4. Kingdom Plantae 

It includes all multi-cellular plants of land and water. Major groups of Algae, 

Bryophytes, Pteridophytes, Gymnosperms and Angiosperms belong to this 

kingdom. It shows the following characteristics. 

1. The cells have a rigid cell wall made up of cellulose. 

2. They show various modes of nutrition. Most of them are autotrophs since 

they have chlorophyll. Some plants are heterotrophs. For eg. Cuscuta is a 

parasite. Nepenthes and Drosera are insectivorous plants. 


This kingdom includes all multi-cellular eukaryotic organisms. They are also 

referred to as metazoans. They show the following characteristic features. 

1. All animals show heterotrophic mode of nutrition. They form the consumers 

of an ecosystem. 

2. They have contractibility of the muscle cells. 

3. They can transmit impulses due to the presence of nerve cells. 

4. Some groups of animals are parasites eg. tapeworms and roundworms. 

Merits of the Five Kindom Classification 

1. It shows the phylogenetic relationships among the organisms. 

2. It is based on the complexity of the cell structure from prokaryotic to 

eukaryotic cell organization. 

3. It is based on the complexity of body organization from unicellular to multicellular. 

4. It is based on the modes of nutrition: autotrophic or heterotrophic mode of 

nutrition. 

Demerits of Five Kingdom Classification 

1. Chlamydomonas and Chlorella are included under the kingdom Plantae. 

They should have been included under kingdom Protista since they are 

unicellular. 

7

8 

4. Yeasts, though unicellular eukaryotes, are not placed in the kingdom 

Protista. 

3. Animal protozoans are included under the kingdom Protista which include 

unicellular plants. They show different modes of nutrition. 

2. Animal protozoans are not included along with animals. 

Table : 1.1 

Major differences among five kingdoms in the Five Kingdom System of Classification: 

Property Monera Protista Fungi Plantae Animalia 

Cell type Prokaryotic Eukaryotic Eukaryotic Eukaryotic Eukaryotic 

Cell Mostly unicellular 

Mostly 

organization 

unicellular 

Cell wall Present in most Present in 

some: absent 

in others 

Nutritional Phototrophic,heterotrophic Heterotrophic 

Multicellular 

and unicellular 

Mostly 

Multicellular 

Mostly 

Multicellular 

Present Present absent 

Heterotrophic phototrophic Heterotrophic 

class or chemoautotrophic and 

phototrophic 

Mode of Absorptive 

Absorptive or Absorptive Mostly Mostly 

nutrition 

ingestive 

Absorptive ingestive 

Motility Motile or non 

Motile or Nonmotile Mostly Mostly Motile 

motile 

nonmotile 

nonmotile

Difficulties in classification 

Since living organisms exhibit great variety and diversity and also they have 

evolved through millions of years and there are many missing links between groups, 

it is very difficult to have a clear cut and well defined classification. Biological 

classification reflects the state of our knowledge. It changes as we acquire new 

information. By the 1970s molecular biologists realized that prokaryotes consist of 

two different and unrelated groups. To accommodate this new information three 

microbiologists, C.Woese, O.Kandler, and M.L Wheelis introduced a new 

classification scheme in 1990. They proposed that all organisms be divided into 

three major groups called domains: the Eucarya (containing all eukaryotes), the 

Bacteria (containing most familiar prokaryotes), and the Archaea (originally called 

archaebacteria and containing prokaryotes that live mostly in extreme environments.) 

This scheme is currently accepted by most biologists. 

Classification will undoubtedly continue to change. 

SELF EVALUATION 

One Mark 

Choose the correct answer 

1. The basic unit of classification is 

a. genus b.species c.family d.taxon 

2. Unicellular plants found floating in oceans and freshwater are called 

a. algae b.zooplanktons c.phytoplanktons d.epiphytes 

3. Carolus Linnaeus proposed the following system of classification 

a. Phylogenetic b. Two kingdoms c. Five Kingdoms d. Natural 

Fill in the blanks 

1. “Systema Naturae” is written by_____________ 

2. Father of Ayurveda is_______________ 

3. __________ introduced the term species for the first time. 

4. The author of “Species Plantarum” is ___________ 

5. __________ coined the word Taxonomy. 

9

Match the following 

Fossil records 

Whittaker 

Carolus Linnaeus 

John Ray 

Augustin de Candolle 

- Five kingdom System 

- Species 

- Taxonomy 

- Phylogenetic studies 

- Species Plantarum 

Two Marks 

1. Define biodiversity. 

2. What are the aims of classification? 

3. Define Taxonomy. 

4. Define species. 

5. Write the hierarchy of the units of classification. 

6. Define phylogeny. 

7. Give any two reasons why phylogenetic classification is not always possible? 

8. What is meant by phylogenetic classification? 

9. What is meant by artificial system of classification? Give example. 

10. What are Archaebacteria? 

11. Name the three domains according to the modern classification proposed by 

C.Woese, O.Kandler and M.C.Wheelis. 

12. Define systematics. 

Five Marks 

1. List the differences between plants and animals. 

2. How do you justify a separate kingdom status for fungi. 

3. What are the difficulties encountered in classifying Euglena? 

Ten Marks 

1. Discuss the Five kingdom system of classification. List it’s merits and demerits. 

2. Discuss the Two kingdom system of classification. List it’s merits and demerits. 

10

Introduction 

2. Salient Features of Various Groups 

2.1 Viruses 

Viruses are still biologists’ puzzle because they show both living and nonliving 

characters. Hence viruses are regarded as a separate entity. It is not 

taken into account in Whittaker’s five kingdom classification. Viruses are now 

defined as ultramicroscopic, disease causing intra cellular obligate parasites. 

Brief history of discovery 

Viruses were not known to biologists for a long time due to their 

ultramicroscopic structure though their presence was apparent by infectious diseases 

which were proved not due to bacteria. It attracted the attention of investigators 

only in the 19 th century when a virus called tobacco mosaic virus (TMV) 

caused severe damage to commercially important tobacco crop. 

Table : 1.2. Enigma of Viruses 

Living characteristics of virus Non-living characteristics of virus 

1. Ability to multiply inside a host Inability to multiply extra cellularly 

plant or animal cell 

2. Ability to cause diseases Absence of any metabolic activity 

3. Possession of nucleic acid, Absence of protoplasm 

protein, enzyme, etc. 

4. Ability to undergo mutation Can be crystallized. 

Mayer demonstrated that the disease could be transmitted just by applying 

the sap of infected leaf to the leaf of healthy plant. He thought that the disease 

was due to a bacterium. It was then the Russian biologist Iwanowsky (1892) who 

demonstrated that the sap of infected leaves even after passing through bacterial 

filter remained infective, ruling out the bacterium as the causative agent. Dutch 

microbiologist Beijerinck (1898) confirmed the findings of Iwanowsky and called 

the fluid “contagium vivum fluidum” which means contagious living fluid. This 

was later on called virion (poison) and the disease causing agent as virus. 

W.M. Stanley (1935), the American biochemist, isolated virus in crystalline form 

and demonstrated that even in that state it maintained the infectivity. This marked 

the beginning of a new branch of science called virology. 

General characteristics 

Viruses are ultramicroscopic and can cause diseases in plants and animals. 

They are very simple in their structure. They are composed of nucleic acid 

11

surrounded by a protein coat. Nucleic acid can be either RNA or DNA, but 

never both. They have no cellular organization and have no machinery for any 

metabolic activity. They are obligate intracellular parasites and they multiply within 

their host cells. Once outside the host cell they are completely inactive. 

Size and Shape 

Viruses are very minute particles that they can be seen only under electron 

microscope. They are measured in millimicrons ( 1 millimicron = 1/1000micron). 

(1micron – 1/1000 millimeter). Generally they vary from 2.0 mm to 300 mm in size. 

Very small size and ability to pass through bacterial filters are classic attributes 

of viruses. The following methods are used to determine the size of the viruses. 

1. Direct observation by using electron microscope: 

2. Filtration through membranes of graded porosity: In this method viruses are 

made to pass through a series of membranes of known pore size, the 

approximate size of any virus can be measured by determining which 

membrane allows the virus to pass through and which membrane holds it 

back. 

3. Sedimentation by ultra centrifugation : The relationship between the size 

and shape of a particle and its rate of sedimentation permits determination 

of particle size. 

4. Comparative measurements: The following data is used for reference. 

a. Staphylococcus has a 

diameter of 1000 mm. 

Cubical 

b. Bacteriophage varies 

(A deno Virus) 

in size from 10-100 nm. 

Broadly speaking viruses 

occur in three main shapes: 

1. Cubic symmetry: 

polyhedral or spherical – eg. 

Adeno virus, HIV 

2. Helical symmetry: e g . 

Tobacco Mosaic virus 

(TMV), Influenza virus. 

3. Complex or atypical eg. 

Bacteriophage, Pox 

virus. 

Complex 

(Bacteriophage) 

Head 

(C apsid) 

Tail 

Core tube 

Tail Fibres 

12 

DNA 

Neck 

Collar 

Helical 

End Plate 

Spikes 

Fig. 1.2 Different shapes of Viruses 

M atrix 

Nucleo capsid 

(Protein) 

Envelope 

(Lipid) 

Spikes 

(G lyco 

protein)

Structure of a virus 

A virus is composed of two 

Nucleic 

major parts 1.Capsid (the 

protein coat) 2.Nucleic acid. 

Acid 

The capsid is the outer protein 

(RNA) 

coat. It is protective in function. 

It is often composed of many 

identical subunits called 

capsomeres. Some of the 

Capsid 

viruses have an outer covering 

(Protein) 

called envelope eg. HIV. They 

are called enveloped viruses. 

Others are called naked viruses 

or non- enveloped viruses. The 

Fig.1. 3 Basic components of a virus (TMV) 

capsid is in close contact with the 

nucleic acid and hence known as nucleocapsid. The nucleic acid forms the 

central core. Unlike any living cell a virus contains either DNA or RNA, but 

never both. The infective nature of the virus is attributed to the nucleic acid while 

host specificity is attributed to the protein coat. 

Virion 

An intact, infective virus particle which is non-replicating outside a host 

cell is called virion. 

Viroids 

A viroid is a circular molecule of ss RNA without a capsid. Viroids 

cause several economically important plant diseases, including Citrus 

exocortis. 

Prions(pronounced “preeons” ) 

They are proteinaceous infectious particles. They are the causative 

agents for about a dozen fatal degenerative disorders of the central 

nervous systems of humans and other animals. eg. Creutzfeldt-Jacob 

Disease(CJD), Bovine Spongiform Encephalopathy (BSE)-Commonly 

known as mad cow disease, etc .They are very unique among infectious 

agents because they contain no genetic material i.e DNA/RNA. Stanley 

Prusiner did most of the work on prions and was awarded Nobel Prize in 

1998. 

13

Classification of virus 

Although viruses are not classified as members of the five kingdoms, they are 

diverse enough to require their own classification scheme to aid in their study and 

identification. 

According to the type of the host they infect, viruses are classified mainly into 

the following four types. 

1. Plant viruses including algal viruses-RNA/DNA 

2.Animal viruses including human viruses-DNA/RNA 

3.Fungal viruses(Mycoviruses)-ds RNA 

4.Bacterial viruses (Bacteriophages) including cyanophages-DNA 

1.Plant viruses 

They infect plants and cause diseases. Some common plant viral diseases are: 

a. Mosaic diseases of tobacco (TMV), cucumber (CMV), cauliflower. 

b. Bunchy top of banana 

c. Leaf-roll of potato 

d. Spotted wilt of tomato 

Generally, plant viruses have RNA with the exception of some viruses such 

as cauliflower mosaic virus which has DNA. 

2. Animal viruses 

They infect animals and cause diseases. The nucleic acid is either DNA or 

RNA. some of the diseases caused by viruses in human beings are: common cold, 

measles, small pox ( now extinct) chicken pox, Jaundice, herpes, hapatitis A 

B,C,D,E,G, influenza, polio, mumps, rabies, AIDS and SARS. Viruses also cause 

diseases in cattle. eg. Foot and mouth disease. (FMD) in cattle, encephalomyelitis 

of horse, distemper of dog, rabbies etc. 

3. Viruses that cause diseases in fungi are called mycophages and viruses that 

attack blue green algae/cyanobacteria and cause diseases are called cyanophages. 

14

4. Bacteriophages 

Virus that infects bacteria is called bacteriophage or simply phage. It is 

tadpole like and the nucleic acid is DNA eg. T 2 

, T 4 

, T 6 

bacteriophages. 

Life cycle of a phage 

Phages exhibit two different types of life cycle. 

1. Virulent or lytic cycle 

2. Temperate or lysogenic cycle. 

1. Virulent or lytic cycle 

Intra cellular multiplication of the phage ends in the lysis of the host bacterium 

and the release of progeny virions. Replication of a virulent phage takes place in 

the following stages. 

1. Adsorption 2.Penetration 3.Synthesis of phage components 4.Assembly 

5.Maturation 6.Release of progeny phage particles 

Transcription & 

Translation 

Lytic cycle 

Phage 

DNA 

Induction 

Bacterial 

Chromosome 

Prophage 

Lysogeny 

Cycle 

Integration of 

phage DNA 

C ell replication 

Viral DNA 

Replication 

Lysis of 

Bacterial cell 

Assembly & 

Release of phage 

Fig.1.4 Lytic and Lysogenic cycle of a phage 

15

1. Adsorption 

The attachment of the phage to the surface of a susceptible bacterium by 

means of its tail is called adsorption. Host specificity of the phage is determined 

in the adsorption stage of the cycle itself. Artificial injection by direct injection 

of phage DNA can be achieved even in strains of bacteria that are not 

susceptible to the phage. The infection of a bacterium by the naked phage 

nucleic acid is known as transfection. 

2. Penetration 

The process of penetration resembles injection through a syringe. The phage 

DNA is injected into the bacterial cell through the hollow core. After 

penetration the empty head and the tail of the phage remain outside the 

bacterium as the shell. 

3.Synthesis of phage components 

During this stage synthesis of bacterial protein, DNA, and RNA ceases. On 

the other hand, phage DNA, head protein and tail protein are synthesized 

separately in the bacterial cell. The DNA is compactly ‘packaged’ inside the 

polyhedron head and finally the tail structures are added. 

4. The assembly of phage components into mature infectious phage particle is 

known as (5) Maturation. 

6. Release of phages 

Release of phages typically takes place by the lysis of the bacterial cell. During 

the replication of phages, the bacterial cell wall is weakened and it assumes a 

spherical shape and finally burst or lyse. Mature daughter phages are released. 

Lysogenic cycle 

The temperate phages enter into a symbiotic relationship with the host cells. 

There is no death or lysis of the host cells. Once inside the host cell the temperate 

phage nucleic acid becomes integrated with the bacterial genome. Now the 

integrated phage nucleic acid is called a prophage. 

The prophage behaves like a segment of the host chromosome and replicates 

along with it. This phenomenon is called lysogeny. The bacterium that caries a 

prophage within its genome is called lysogenic bacterium. 

The prophage confers certain new properties on the bacterium. This is called 

lysogenic conversion or phage conversion. An example is toxin production by 

16

the Diptheria bacillus which is determined by the presence of prophage beta. 

The elimination of prophage abolishes the toxigenicity of the bacillus. 

Plant viral disease 

Bunchy top of banana 

Banana bunchy top virus causes this disease. The infected plant shows 

extremely stunted growth. Leaves become short and narrow. Affected leaves 

are crowded in a rosette like fashion (bunch of leaves) at the top of the plant. 

Chlorosis and curling of the leaves also occur. Diseased plants should immediately 

be uprooted and burnt to avoid further infection. 

Emerging viral infections( in human beings) 

Recent examples of emerging viral infections in different regions of the world 

include ebola virus, HIV, dengue, hemorrhagic fever, lassa fever, Rift valley fever, 

SARS. 

AIDS: (Acquired Immuno Deficiency Syndrome) is a recently discovered 

sexually transmitted virus disease. It is caused by Human Immuno Deficiency 

Virus ( HIV ). 

HIV belongs to a group of viruses called retroviruses. It infects the T 4 

lymphocytes known as helper cells which form the main line of body immune 

system. HIV kills the T 4 

lymphocytes and the resulting depletion of T 4 

cell 

population creates an immune deficiency. This paves way for many opportunistic 

pathogens to attack. AIDS by itself is not a killer disease. It is only the other 

opportunistic pathogens which kill the infected persons. 

Symptoms 

HIV infection causes fever, loss of body weight, persistent generalized lymph 

node enlargement and opportunistic infections like T.B . etc. The AIDS patients 

may also have headache, fatigue, persistent diarrhoea, dry cough, lymphomas and 

damage of the central nervous system. Often there is appearance of thrush in the 

mouth and throat and night sweats. Changes in behaviour and mental illness may 

also occur. 

Mode of infection 

Primarily HIV is sexually transmitted. It is predominant among homosexuals. 

Persons with veneral diseases, persons who have many sexual partners and 

prostitutes will have more chances of HIV infection. The commonest method of 

transmission is through sexual intercourse with many persons. 

17

The other methods of 

transmission are during 

blood transfusion, tissue or 

organ donation of HIV 

infected persons to healthy 

persons, injections with 

unsterilized syringes and 

needles and shared needles 

by drug addicts. AIDS can 

spread from infected 

mother to the child during 

pregnancy or through 

breast feeding. 

Prevention 

Since there is no cure for AIDS the best approach to control AIDS is 

prevention. Reduction of sexual promiscuity and adoptions of prophylactic measures 

(such as the use of condoms) can reduce transmission through sexual intercourse. 

Transmission through the shared needles by drug addicts may be reduced by proper 

education. The transmission through blood transfusion may be eliminated by proper 

serological screening of donated blood for the presence of HIV antibodies. 

Transmission from infected mother to child can be reduced by preventing or 

terminating pregnancy. Drugs like AZT (azidothymidine) only help to increase the 

life span of the victim by few a months and do not offer complete cure for the 

disease. 

Viruses and cancer 

Cancer is an uncontrollable and unorganized growth of cells causing malignant 

tumour. The cells of this tumours have the capacity to spread indiscriminately 

anywhere in the body. In recent years, there has been increasing evidence to 

prove that the cancer is caused by the DNA virus called Simian virus (SV-40) 

and a group of RNA viruses called retroviruses. The cancer causing viruses are 

also called oncogenic viruses. It is now 

believed that some viruses are involved 

in leukemia, sarcoma and some kind of 

breast cancer also. 

"Corona" 

A new disease called SARS 

Severe Acute respiratory 

Syndrome (SARS) is a respiratory illness 

that has recently been reported in South 

East Asia, North America and Europe. 

Fig : 1.5 Human Immuno Deficiency Virus 

18 

Fig: 1.6 Human CoronaVirus 

Virion 

Envelope 

Capsid 

Nucleo 

Capsid 

Double 

Stranded 

RNA

It has created panic among the people all over the world and has resulted in great 

economic loss for many countries like China, Singapore etc. 

Symptoms 

It begins with high fever. Other symptoms include headache, discomfort and 

body aches. Patient may develop dry cough and have trouble in breathing. 

How SARS spread 

It appears to spread by person to person contact especially with infectious 

material (for example respiratory secretions.) 

The viruses that cause SARS are constantly changing their form which will 

make developing a vaccine difficult. SARS is caused by a group of viruses called 

corona viruses which are enveloped viruses. Their genome is single stranded 

RNA. The nucleocapsid is helical. These viruses have petal shaped surface 

projections arranged in a fringe like a solar corona. 

Viral vaccines 

The purpose of viral vaccine is to utilize the immune response of the host to 

prevent viral diseases. Vaccination is the most cost effective method of prevention 

of serious viral infection. 

Interferons (IFN s 

) 

They are the host coded proteins of cytokine family that inhibit viral replication. 

They are produced by intact animal or cultured cells in response to viral infection 

or other inducers. They are believed to be the part of body’s first line of defense 

against viral infection. 

Significance of Viruses 

1. Viruses are a kind of biological puzzle to biologists since they are at the 

threshhold of living and non-living things showing the characteristics of both. 

2. Viruses are very much used as biological research tools due to their simplicity 

of structure and rapid multiplication. They are widely used in research 

especially in the field of molecular biology, genetic engineering, 

medicine etc. 

3. Viruses are used in eradicating harmful pests like insects. Thus they are 

used in Biological Control Programmes. 

19

4. Plant Viruses cause great concern to agriculturists by their pathogenic nature. 

Bacteriophages attack the N 2 

fixing bacteria of soil and are responsible for 

reducing the fertility of soil. 

5. In industry, viruses are used in preparation of sera and vaccines. 


One Mark 


1. T.M.V has the following symmetry. 

a. Cubical b. helical c.atypical d.square 

2. The infective nature of virus is due to 

a. protein coat b. nucleic acid c. envelope d.tail fibres. 

3. Developing a vaccine for SARS is difficult because 

a. it spreads by infectious materials b. it is an enveloped virus 

c. it is constantly changing it’s form d. it has ssRNA 

Fill in the banks 

1. ________ isolated first virus in crystalline form. 

2. The two important components of viruses are ___________ and 

___________. 

3. All ________viruses have ds.RNA. 

4. _________ is a plant virus which has DNA 

5. _________ virus causes AIDS. 

Match 

Cyanophage 

Mycophage 

SARS 

AIDS 

Phage 

- Corona virus 

- HIV 

- Blue green algae 

- Bacteria 

- Fungi 

20

Two Marks 

1. Justify: Viruses are biologists’ puzzle. 

2. Define: virus 

3. List any two living characteristics of virus. 

4. List any two non-living characteristics of virus. 

5. Viruses can undergo mutation. What does this signify? 

6. Viruses can be crystallized. What does this signify? 

7. What are the three main symmetry of viruses? 

8. What is the principle used in sedimentation by ultra centrifugation method of 

measuring the size of a virus? 

9. What are enveloped viruses? 

10. Define nucleocapsid. 

11. Name any two plant diseases / animal diseases/human diseases caused by 

viruses? 

12. Define virion/ viroid/ prion 

13. What are oncogenic viruses? 

14. What are interferons? 

Five Marks 

1. Discuss the methods that are used to measure the size of a virus? 

2. What is meant by biological control? Illustrate your answer with suitable 

examples. 

3. Write a note on: Significance of viruses. 

Ten Marks 

1. Distinquish lytic cycle of a phage from lysogenic cycle . 

2. Write an essay on the cause, symptoms and prevention of AIDS /SARS 

21

Introduction 

2.2 Bacteria 

In 1676 Anton Van Leeuwenhoek discovered the microbial world by his 

simple microscope. It was only after the invention of compound microscope by 

Hooke in 1820, that bacteria came to lime light. These very minute creatures 

were designated as “small microscopic species” or “ Infusorial animalcules”. 

Louis pasteur(1822-95 ) made a detailed study of bacteria and proposed germ 

theory of disease. Robert Koch, a german microbiologist, was the first scientist 

to prove the cause and effect relationship between microbes and animal diseases. 

Ehrenberg(1829) was the first to use the term bacterium. The branch of study 

that deals with bacteria is called Bacteriology. Bacteria are unicellular organisms 

and they are prokaryotic. i.e they do not have a membrane bound nucleus and 

membrane bound organelles . 

Occurrence 

Bacteria are omnipresent. They are found in all environments, where organic 

matter is present. They are found in air, water, soil and also in or on the bodies of 

plants and animals. Some of the bacteria live as commensals (eg. Escherichia 

coli in the human intestine) and some live as symbionts (eg. Rhizobium) in the 

root nodules of leguminous plants. Several of them cause diseases in plants, animals 

and human beings. 

Size 

Bacteria are very small, most being approximately 0.5 to 1 micron in diameter 

and about 3 to 5 microns in length. 

Classification of bacteria based on the shape and arrangement 

The rigid bacterial cell wall determines the shape of a cell. Typical bacterial 

cells are spherical (Cocci), straight rods (Bacilli) or rods that are helically curved 

(spirilla), some bacterial cells are pleomorphic ie they can exhibit a variety of 

shapes eg. Arthrobacter 

Cocci bacteria appear in several characteristic arrangements depending on 

their plane of division. 

A. Diplococci: Cells divide in one plane and remain attached in pairs. 

B. Streptococci: cells divide in one plane and remain attached to form chains. 

22

C. Tetracocci: Cells 

divide in two 

planes and form 

group of four cells. 

D. Staphylococci: 

Cocci 

Staphylococci Bacilli 

Streptobacillus 

cells divide in three 

planes, in an irregular 

Streptococcus Diplococci Spirillum 

pattern, producing 

bunches of cocci. 

Fig : 1.8 Different shapes of bacteria 

E. Sarcinae: cells 

divide in three planes, in 

a regular pattern, producing a cuboidal arrangement of cells. 

Bacilli forms occur singly or in pairs (diplobacilli) or form chains 

(streptobacilli). In Corynebacterium diphtheriae which is a bacillus species, 

the cells are arranged side by side like match sticks (palisade arrangement) 

Flagellation in Bacteria 

All spirilla, about half of the 

bacilli and a small number of cocci 

Monotrichous 

Lophotrichous 

are flagellated. Flagella vary both 

in number and arrangement 

Amphitrichous 

Peritrichous 

according to two general patterns. 

1. In a polar arrangement, the 

flagella are attached at one or both 

the ends of the cell. Three sub 

Atrichous 

types of this pattern are: 

Fig : 1.9 Flagellar arrangement in Bacteria 

a. monotrichous – with a single 

flagellum 

b. lophotrichous – with small bunches or tufts of flagella emerging from one 

end 

c. amphitrichous – with flagella at both poles of the cell 

2. In a peritrichous arrangement flagella are dispersed randomly over the surface 

of the cell. 

3. Atrichous bacteria lack flagellum. 

Flagellar Functions 

They can detect and move in response to chemical signals – a type of behaviour 

called chemotaxis. Positive chemotaxis is movement of cell in the direction of a 

23

favourable chemical stimulus (usually a nutrient). Negative chemotaxis is movement 

away from a repellant (potentially harmful) compound. 

Nutrition in Bacteria 

Autotrophic Bacteria 

Some bacteria can synthesize their food and hence they are autotrophic in 

their mode of nutrition. They may be photo autotrophs (eg. Spirillum) or 

chemoautotrophs eg. Nitrosomonas or Nitrobacter. 

Photoautotrophic bacteria 

They use sunlight as their source of energy to synthesize food. But unlike 

photosynthetic eukaryotic cells they do not split water to obtain reducing power. 

So Oxygen is not evolved during bacterial photosynthesis. Depending upon the 

nature of the hydrogen donor these bacteria may be 

1. Photolithotrops 

In this the hydrogen donor is an inorganic substance. In green sulphur 

bacteria(eg. Chlorobium) hydrogen sulphide (H 2 

s) is the hydrogen donor. The 

chlorophyll is bacterioviridin 

In purple sulphur bacteria (eg. Chromatium) thiosulphate acts as hydrogen 

donor. The chlorophyll is bacteriochlorophyll. 

2. Photo-organolithotrophs 

In this the hydrogen donor is an organic acid or alcohol eg. Purple non sulphur 

bacteria (eg. Rhodospirillum) 

Chemoautotrphic bacteria 

They do no have photosynthetic pigments and hence they cannot use sunlight 

energy. Instead they obtain energy in the form of ATP by oxidising inorganic or 

organic compounds. The energy thus obtained is used to reduce CO 2 

to organic 

matter. Based on the type of substance oxidized they may be 

1. Chemolithotrophs: Inorganic compound is oxidized to release energy. 

eg. Sulphur bacteria (eg. Thiobacillus) 

Iron bacteria (eg. Ferrobacillus), Hydrogen bacteria eg. 

Hyderogenomonas and Nitrifying bacteria (eg Nitrosomonas and 

Nitrobacter) 

2. Chemo – organotrophs: In this type it is an organic compound that is 

oxidized to release energy. eg. Methane bacteria (Methanococcus). 

24

Acetobacteria and Lactobacillus are also examples for chemoorganotrophs. 

Heterotrophic Bacteria 

They depend upon other organisms (living/dead) for their food since they cannot 

synthesize their own food. They may be saprotrophic e.g (Bacillus subtilis), 

parasitic e.g. Plant parasite-(Xanthomonas citrii) animal parasite e.g.(Bacillus 

anthracis) , Human parasite e.g (Vibrio cholerae) or symbiotic in association 

with roots of the family Leguminosae. e.g. (Rhizobium) 

Respiration in Bacteria 

Aerobic Bacteria: These bacteria require oxygen as terminal acceptor of 

electrons and will not grow under anaerobic conditions(i.e in the absence of O 2 

) 

Some micrococcus species are obligate aerobes (i.e they must have oxygen to 

survive) 

Anaerobic bacteria : These bacteria do not use oxygen for growth and 

metabolism but obtain their energy from fermentation reaction. eg. Clostridium 

species. 

Capnophilic bacteria are those that require CO 2 

for growth. 

Facultative anaerobes: Bacteria can grow either oxidatively using oxygen 

as a terminal electron acceptor or anaerobically using fermentation reaction to 

obtain energy. Bacteria that are facultative anaerobes are often termed “aerobes”. 

When a facultative anaerobe such as E. Coli is present at a site of an infection like 

an abdominal abscess it can rapidly consume all available O 2 

and change to 

anaerobic metabolism, producing an anaerobic environment and thus, allow the 

anaerobic bacteria that are present to grow and cause disease. 

Endospores are structures formed in bacillus bacteria during unfavourable 

conditions. Fortunately most pathogenic bacteria (except tetanus and anthrax 

bacteria) do not form endospores. 

Reproduction: Reproduction by binary fission is very common. It is the method 

by which many bacteria multiply very rapidly explaining the cause of spoilage of 

food stuffs, turning of milk into curd etc. 

Sexual Reproduction 

Typical sexual reproduction involving the formation and fusion of gametes is 

absent in bacteria. However, gene recombination can occur in bacteria by three 

different methods. They are 1. Conjugation 2. Transduction 3. Transformation 

25

Donor Bacterium 

(w ith Plasm id) 

Recipient Bacterium 

(w ithout Plasm id) 

Pilus 

Nick at the 

plasmid strand 

Mating Pairs 

Single strand transfer 

Replication of Donor strand 

Replication of Recipient strand 

Separation of 

mating pair 

Donor Bacterium 

Recipient Bacterium 

(with double stranded Plasmid) (with double stranded Plasmid) 

Fig : 1.10 Conjugation in Bacteria 

1. Conjugation: In this method of gene transfer, the donor cell gets attached 

to the recipient cell with the help of pili. The pilus grows in size and forms 

the conjugation tube. The plasmid of donor cell which has the F+ (fertility 

factor) undergoes replication. Only one strand of DNA is transferred to the 

recipient cell through conjugation tube. The recipient completes the structure 

of double stranded DNA by synthesizing the strand that compliments the 

strand acquired from the donor. 

26

2. Transduction : Donor DNA is carried in a phage coat and is transferred 

into the recipient by the mechanism used for phage infection. 

3. Transformation : The direct uptake of donor DNA by the recipient cell may 

be natural or forced. Relatively few bacterial species are naturally competent 

for transformation. These species assimilate donor DNA in linear form. 

Forced transformation is induced in the laboratory, where after treatment 

with high salt and temperature shock many bacteria are rendered competent 

for the assimilation of extra-cellular plasmids. The capacity to force 

bacteria to incorporate extra-cellular plasmids by transformation is 

fundamental to genetic Engineering. 

Economic Importance of Bacteria 

Bacteria play an important role in day to day activities of human beings. Some 

of them have harmful effects and others are useful to man kind. 

Harmful activities 

1. Diseases caused by bacteria in plants : 

Name of the host Name of the disease Name of the pathogen 

Citrus Citrus Canker Xanthomonas citrii 

Rice Bacterial blight Xanthomonas oryzae 

Cotton Angular leaf spot Xanthomonas malvacearum 

Pears Fire blight Pseudomonas solanacearum 

Carrot Soft rot Erwiinia caratovora 

2. Diseases caused by bacteria in animals : 

Name of the host Name of the disease Name of the pathogen 

Sheep Anthrax Bacillus anthracis 

Cattle Brucellosis Brucella abortus 

Sheep,goat Brucellosis Brucella melitensis 

3. Diseases caused by bacteria in human beings: 

Name of the disease 

Name of the pathogen 

Cholera 

Vibrio cholerae 

Typhoid 

Salmonella Ttyphi 

Tuberculosis 

Mycobacterium tuberculosis 

27

Beneficial Activities of Bacteria 

1. Sewage disposal : Organic matter of the sewage is decomposed by 

saprotrophic bacteria. 

2. Decomposition of plant and animal remains: Saprotrophic bacteria 

cause decay and decomposition of dead bodies of plants and animals. They 

release gases and salts to atmosphere and soil. Hence these bacteria are 

known as nature’s scavengers. 

3. Soil fertility : 

1. The ammonifying bacteria like Bacillus ramosus and B. mycoides convert 

complex proteins in the dead bodies of plants and animals into ammonia 

which is later converted into ammonium salts. 

2. The nitrifying bacteria such as Nitrobacter, Nitrosomonas convert 

ammonium salts into nitrites and nitrates. 

3. Nitrogen fixing bacteria such as Azotobacter and Clostridium and 

Rhizobium (a symbiotic bacterium) are capable of converting atmospheric 

nitrogen into organic nitrogen. All these activities of bacteria increase soil 

fertility. 

Recycling of matter 

Bacteria play a major role in cycling of elements like carbon, oxygen, Nitrogen 

and sulphur. Thus they help in maintaining environmental balance. As biological 

scavengers they oxidize the organic compounds and set free the locked up carbon 

as CO 2 

. The nitrogenous organic compounds are decomposed to form ammonia 

which is oxidized to nitrite and nitrate ions by the action of nitrifying bacteria. 

These ions are used by higher plants to synthesize nitrogenous organic compounds. 

The nitrogenous compounds are also oxidized to nitrogen by denitrifying bacteria. 

Role of Bacteria in Industry 

1. Dairy Industry 

Lactic acid bacteria e.g.(Streptococcus lactis) are employed to convert milk 

sugar lactose into lactic acid. 

C 12 

H 22 

O 11 

+H 2 

O 4C 3 

H 6 

O 3 

+Energy 

Lactose 

Lactic Acid 

Different strains of lactic acid bacteria are used to convert milk into curd, 

yoghurt(Lactobacillus bulgaricus) and cheese(Lactobacillus 

acidophobus). 

28

2. Vinegar 

Vinegar (Acetic acid) is obtained by the activity of acetic acid bacteria 

(Acetobactor aceti). This bacterium oxidizes ethyl alcohol obtained from molasses 

by fermentation to acetic acid or vinegar. 

3. Alcohols and Acetone 

Butyl alcohol, methyl alcohol and acetone are prepared from molasses by the 

fermentation activity of the anaerobic bacterium Clostridium acetobutylicum. 

Curing of tobacco,tea and coffee 

The leaves of tea, tobacco and beans of coffee are fermented by the activity 

of certain bacteria to impart the characteristic flavour. This is called curing of 

tea, tobacco and coffee. 

Retting of fibres 

The fibres from the fibre yielding plants are separated by the action of bacteria 

like Clostridium species. This is called retting of fibres. 

Role of bacteria in medicine 

1. Antibiotics: Antibiotics such as bacitracin (Bacillus subtilis), 

polymyxin(Bacillus polymyxa), Streptomycin(Streptomyces griseus) are 

obtained from bacterial sources. 

2. Vitamins: Escherichia coli living in the intestine of human beings produce 

large quantities of vitamin K and vitamin B complex. Vitamin B 2 

is prepared 

by the fermentation of sugar by the action of clostridium species. 

Role of bacteria in genetic engineering 

Most of our knowledge in genetics and molecular biology during 20 th century 

has been due to research work on micro-organisms, especially bacteria such as 

E.coli. One success has been the transfer of human insulin genes into bacteria and 

commercial production of insulin has already commenced. 

Role of bacteria in biological control 

Certain Bacillus species such as B.thuringiensis infect and kill the caterpillars 

of some butterflies and related insects. Since the bacteria do not affect other animals 

or plants they provide an ideal means of controlling many serious crop pests. 

29


One Mark 


1. The chlorophyll pigment found in green sulphur bacteria is 

a.bacteriochlorophyll b.bacterioviridin c.phycocyanin d.phycoerythrin 

2. Cell which keeps changing it’s shape is called 

a. Spirilla b.Pleomorphic c. Symbiont d. Gram – negative 


1. __________ proposed germ theory of disease . 

2. __________ bacteria require CO 2 

for growth . 

3. __________ is a type of movement of cells in response to chemical signals. 

4. __________ is not evolved during bacterial photosynthesis. 

Two Marks 

1. What are commensals? 

2. What are Gram-Positive bacteria? 

3. What are Gram-Negative bacteria? 

4. What are Chemoautotrophs? 

5. What is transduction/ transformation. 

6. Name any four plant diseases / human diseases caused by bacteria? 

7. Give reason: Bacteria are also known as nature’s scavengers. 

8. Name some antibiotics obtained from bacteria 

Five Marks 

1. Describe the various steps in Gram’s staining procedure? 

2. What are the different shapes found in bacteria? Give examples. 

3. Describe the various types of flagellation found in bacteria. 

4. Discuss the role of bacteria in industry? 

5. Discuss the role of bacteria in soil fertility. 

Ten Marks 

1. Write an essay on sexual reproduction in bacteria. 

2. Discuss the economic importance of bacteria. 

3. Write an essay on nutrition in bactera. 

30

2.3 Fungi 

Conventionally Fungi have been included in plant kingdom. But in pursuance 

of Whittaker’s five kingdom classification Fungi and Plants (Algae, Bryophytes, 

Pteridophytes Gymmosperms and Angiosperms) are described here as two separate 

kingdoms. Angiosperms are not described in detail here. 

Salient Features 

Fungi are non chlorophyllous, eukaryotic, organisms. They are a large and 

successful group. They are universal in their distribution. They resemble plants 

in that they have cell walls. But they lack chlorophyll which is the most important 

attribute of plants. They are ubiquitous in habitat which ranges from aquatic to 

terrestrial. They grow in dark and moist habitat and on the substratum containing 

dead organic matter. Mushrooms, moulds and yeasts are the common fungi. They 

are of major importance for the essential role they play in the biosphere and for 

the way in which they have been exploited by man for economic and medical 

purposes. The study of fungi is known as Mycology. It constitutes a branch of 

microbiology because many of the handling techniques used, such as sterilizing 

and culturing procedures are the same as those used with bacteria. 

Distinguishing Features of Fungi 

1. They have definite cell wall made up of chitin – a biopolymer made up of 

n- acetyl glucosamine units. 

2. They are without chlorophyll, hence they exhibit heterotrophic mode of 

nutrition. They may be saprotrophic in their mode of nutrition or parasitic or 

symbiotic. 

3. They are usually non – motile except the subdivision Mastigomycotina. 

4. Their storage product is not starch but glycogen and oil. 

5. They reproduce mostly by spore formation. However sexual reproduction 

also takes place. 

Structure 

The body structure of fungi is unique. The somatic body of the fungus is 

unicelllular or multi-cellular or coenocytic. When multi-cellular it is composed 

of profusely branched interwoven, delicate, thread like structures called hyphae, 

the whole mass collectively called mycelium. The hyphae are not divided into true 

31

cells. Instead the 

Aseptate hypha 

Septate hypha 

protoplasm is either 

continuous or is interrupted 

Nuclei 

Septum 

at intervals by cross walls (Coenocytic) 

Fig : 1.11. Coenocytic hypha and septate hypha 

called septa which divide 

the hyphae into 

compartments similar to cells. Thus hyphae may be aseptate(hyphae without 

cross walls) or septate (hyphae with cross walls).When aseptate they are 

coenocytic containing many nuclei. Each hypha has a thin rigid wall, whose chief 

component is chitin, a nitrogen containing polysaccharide also found in the 

exoskeleton of arthropods. Within the cytoplasm the usual eukaryotic organelles 

are found such as mitochondria, golgi-apparatus, endoplasmic reticulum, ribosomes 

and vacuoles. In the older parts, vacuoles are large and cytoplasm is confined to a 

thin peripheral layer. 

Nutrition 

Fungi are heterotrophic in their mode of nutrition that is they require an organic 

source of carbon. In addition they require a source of nitrogen, usually organic 

substances such as amino acids. The nutrition of fungi can be described as 

absorptive because they absorb nutrients directly from outside their bodies. This 

is in contrast to animals which normally ingest food and then digest it within their 

bodies before absorption takes place. With fungi, digestion is external using extracellular 

enzymes. Fungi obtain their nutrients as saprotrophs,parasites or symbionts. 

Saprotrophs 

A saprotroph is an organism that obtains its food from dead and decaying 

matter. It secretes enzymes on to the organic matter so that digestion is outside 

the organism. Soluble products of digestion are absorbed and assimilated within 

the body of the saprotroph. 

Saprotrophic fungi and bacteria constitute the decomposers and are essential 

in bringing about decay and recycling of nutrients. They produce humus from 

animal and plant remains. Humus, a part of soil, is a layer of decayed organic 

matter containing many nutrients. Some important fungi are the few species that 

secrete the enzyme cellulase which breaks down cellulose. Cellulose being an 

important structural component of plant cell walls, rotting of wood and other 

plant remains is achieved by these decomposers secreting cellulases. 

Parasites 

A parasite is an organism that lives in or on another organism, the host from 

which it obtains its food and shelter. The host usually belongs to a different 

32

species and suffers harm from the parasite. Parasites which cause diseases are 

called pathogens. Some parasites can survive and grow only in living cells and 

are called biotrophs or Obligate Parasites. Others can infect their host and 

bring about it’s death and then live saprotrophically on the remains, they are called 

necrotrophs or facultative parasites. Fungal parasites may be facultative or 

obligate and more commonly attack plants than animals. The hyphae penetrate 

through stomata, or enter directly through the cuticle or epidermis or through wounds 

of plants. Inside the plant hosts, the hyphae branch profusely between cells, 

sometimes producing pectinases(enzymes) which digest the middle lamellae of the 

cell walls. Subsequently the cells may be killed with the aid of toxins and cellulases 

which digest the cell walls. Cell constituents may be absorbed directly or digested 

by the secretion of further fungal enzymes. 

Obligate parasites often possess specialized penetration and absorption devices 

called haustoria. Each haustorium is a modified hyphal outgrowth with a large 

surface area which pushes into living cells without breaking their plasma membrane 

and without killing them. Haustoria are rarely produced by facultative parasites. 

Symbiosis : 

Two important types of symbiotic union are made by fungi. 

1. Lichens and 2. Mycorrhizae. 

Lichens 

They are symbiotic association found between algae and fungi. The alga is 

usually a green alga or blue green alga. The fungus is an ascomycete or 

basidiomycete. It is believed that the alga contributes organic food from 

photosynthesis and the fungus is able to absorb water and mineral salts. The 

fungus can also conserve water and this enables some lichens to grow in extreme 

dry conditions where no other plants can exist. 

Mycorrhizae 

These are symbiotic associations between a fungus partner and roots of higher 

plants. Most land plants enter into this kind of relationship with soil fungi. The 

fungus may form a sheath around the center of the root (an ectotrophic 

mycorrhiza) or may penetrate the host tissue (an endotrophic mycorrhiza). 

The former type is found in many forest trees such as conifers,beech and oak and 

involve the fungi of the division basidiomycetes. The fungus receives 

carbohydrates and vitamins from the tree and in return breaks down proteins of 

the soil humus to amino acids which can be absorbed and utilized by the plant. In 

addition the fungus provides a greater surface area for absorption of ions such as 

phosphates. 

Classification of Fungi 

33

Fungi 

Myxomycota Eumycota 

Plasmodio 

phoromycetes 

Myxomycetes 

Traditionally fungi have been regarded as 

plants. At one time fungi were given the status of 

a class and together with the class algae formed 

the division Thallophyta of the Plant Kingdom. 

The thallophyta were those plants whose bodies 

could be described as thalli. A thallus is a body, 

often flat, which is not differentiated into true roots, 

stem and leaves and lack a true vascular system. 

A modification of the scheme of classification of 

fungi proposed by Ainsworth(1973) and adopted 

by Webster(1980) is outlined below. 

Division Myxomycota: They lack cell wall and 

are quite unusual organisms. Possess either a 

plasmodium, a mass of naked, multinucleate 

protoplasm, which feeds by ingesting particulate 

matter and shows amoeboid movement, or 

pseudoplasmodium, an aggregation of separate 

amoeboid cells. Both are of a slimy consistency, 

hence they are also called “Slime moulds”. It 

includes three classes. 

Division Eumycota: True fungi, all with cell 

wall. It is customary to recognize five subdivisions 

under this division. 

A. Mastigomycotina: These are zoosporic 

fungi, many are solely aquatic. Three classes are 

included in this, each characterized by their 

distinctive type of zoospores. 

B. Zygomycotina: Vegetative body 

haplophase. Asexual spores are non-motile spores. 

Sexual reproduction takes place by the complete 

fusion of two multi-nucleate gametangia producing 

a zygospore. Because of this the fungi of the class 

zygomycetes are also known as conjugation fungi. Cell wall is made up of 

chitin and chitosan. It includes two classes. The common black, bread 

moulds Rhizopus and Mucor belong to this group. Rhizopus is a very 

common saprotroph similar in appearance to Mucor, but more widespread. 

Acrasiomycetes 

Deuteromycotina 

Basidiomycotina 

Ascomycotina 

Zygomycotina 

Mastigomycotina 

Fig. 1.12. Classification of Fungi by Ainsworth 

34

Mucor 

Rhizopus 

Sporangium 

Spores 

Sporangiophore 

Rhizoids 

Fig.1.13 Some Zygomycetes 

C. Ascomycotina: Hyphae are septate, vegetative body is haplophase. It has 

five classes.This subdivision includes forms such as yeasts, brown moulds, 

green moulds, pink moulds, cup fungi, and edible morels. In this group of 

fungi asexual reproduction takes place by various types of non-motile spores 

such as oidia, chlamydospores and conidia. Sexual reproduction takes place 

by means of gametangial copulation (yeasts). gametangial contact 

(Penicillium) and by somatogamy (Morchella). The ascomycetes or sac 

fungi are characterized by the development of spores called ascospores. 

These ascospores are enclosed in a sac like structure, the ascus. In primitive 

ascomycetes the asci occur singly. In advanced ascomycetes, groups of 

asci get aggregated to form campact fruiting bodies called the ascocarps. 

The ascocarps are of three types. 

1.Cleistothecium: 

These are closed and spherical ascocarps. eg. Eurotium 

2. Perithecium: 

These are flask shaped ascocarps. eg. Neurospora. 

3. Apothecium : 

These are cup shaped ascocarps. eg.Peziza. 

35

D.Basidiomycotina: It 

includes three classes. hyphae 

are septate, vegetative body is 

dikaryophase. It includes the 

highly evolved fungi. This 

group got its name from the 

basidium, the club shaped 

structure formed at the tip of 

the reproductive hypha. Each 

basidium bears four 

basidiospores at its tip. Large 

reproductive structures or 

fruiting bodies called 

basidiocarps are produced in 

this group of fungi. Common 

examples for basidiomycetes 

include mushrooms, toadstools, 

puffballs and bracket fungi . 

The mycelia of this group are of two types. Primary and secondary. Primary 

mycelium multiplies by oidia, conidia like spores and pycnidiospores. Distinct sex 

organs are absent. Fusion occurs between two basidiospores or between two hyphal 

cells of primary mycelia. Advanced forms of basidiomycetes produce fruiting 

bodies called basidiocarps. Fruiting bodies vary in size from small microscopic to 

large ones. 

E.Deuteromycotina: 

Three classes are included 

under this. They are the socalled 

“Fungi Imperfecti”. It 

is a group of fungi known only 

from their asexual (imperfect 

or anamorphic) or mycelial 

state. Their sexual (perfect or 

teleomorphic) states are either 

unknown or may possibly be 

lacking altogether. 

Economic importance of 

Fungi 

Fungi are useful to 

mankind in many ways. These 

Section of Ascocarp 

(Peziza) 

Fruiting Bodies in Section 

Cleistothecium 

Perithecium 

Toadstool 

36 

Ascus 

Ascospore 

Paraphysis 

Penicillium 

Conidia 

Conidiophore 

Fig : 1.14. Som e A scom ycetes and their Ascocarps 

Bracket Fungus 

Apothecium 

Tree trunk 

Basidiospore 

Basidium 

Section of Basidiocarp 

Fig : 1.15. Some Basidiomycetes and their Basidiocarps

organisms play an important role in medicine, agriculture and industry. They have 

harmful effects also. 

Useful aspects of fungi 

The antibiotic Penicillin was discovered in 1928 by Alexander Fleming of 

Britain from the fungus Penicillium notatum, which in 1940s emerged as a 

‘wonder drug’ for the treatment of bacterial diseases. It gave another important 

‘niche’ to fungi in the realm of biological sciences as producers of antibiotics. 

Many other important antibiotics are produced by moulds 

Many fungi such as yeast, mushrooms, truffles, morels etc., are edible. Edible 

mushrooms contain proteins and vitamins. Certain species of Agaricus such as 

A. Bisporus, A. arvensis are edible. Volvariella volvacea and V. dispora are 

also edible mushrooms cultivated commercially. 

Brewing and baking industries rely heavily on the uses of yeast 

(saccharomyces).Yeasts ferment sugar solution into alcohol and carbon-di oxide. 

Alcohol is used in brewing industry and CO 2 

in baking industry. 

The ‘biochemical genetics’ which later developed into the fasinating 

‘molecular biology’ was founded by studies with Neurospora crassa, a fungus 

which even dethroned Drosophila from the Kingdom of genetics as this fungus 

was especially suited for genetical analysis. Fungi like Neurospora and Aspergillus 

continue to be important organisms studied in genetics. 

“Without fungi even death will be incomplete” said Pasteur. The dead cellulosic 

vegetation is decomposed into carbon and minerals by the saprotrophic fungi and 

these elements are returned to the same environment from where they were 

obtained. Thus fungi maintain the carbon and mineral cycles in nature. 

Harmful aspects of Fungi 

Fungi are great nuisance. They grow on every thing from jam to leather and 

spoil them. LSD (d- lysergic acid diethylamide) produced from the fungus ergot 

Table : 1.3. Some fungal diseases 

Common fungal diseases of plants 

Causal organisms 

1. Wilt of cotton Fusarium oxysporum F.sp.vasinfectum 

f 

2. Tikka disease (Leaf spot) of ground nut Cercospora personata 

3. Red rot of sugarcane Colletotrichum falcatum 

Fungal diseases of human beings 

Causal organisms 

1. Ring worm (tinea) Epidermophyton spp. 

2. Ring worm(tinea) Trichophyton spp. 

3. Candidiasis Candida albicans 

37

(Claviceps purpurea) produces hallucinations. Hence this fungus is called “ 

hallucinogenic fungus” and has caused greatest damage to the frustrated youth 

by giving an unreal, extraordinary lightness and hovering sensation. 

The association of fungi with several plant diseases has now come to light. 

The devastating disease called ‘late blight of potato’ caused by the fungus 

Phytophthora infestans in Ireland in the year 1845 has resulted in such a disaster 

that about one million people died of starvation and over 1.5 million people fled to 

other countries since potato was the staple food of Ireland. Since then ‘Plant 

pathology’ a new science started which deals with diseases of plants caused not 

only by fungi but also by bacteria, viruses etc. 


One Mark 


1. The study of Fungi is called 

a. phycology b. plant pathology c. systematics d.mycology 

2. The fungal cell wall is made up of 

a. chitin b. cellulose c. pectin d. peptidoglycan 


1. The storage products of fungi are __________ and __________ 

2. Haustoria are rarely produced by_________ parasites. 

Two Marks 

1. What is a coenocytic mycelium? 

2. What is meant by septate hypha? 

3. Distinguish obligate parasite from facultative parasites. 

4. What are haustoria? 

5. What are mycorrhizae? 

6. Name some fungal diseases of plants. 

7. Name some edible fungi. 

8. Justify the statement by Pasteur: “Without fungi even death will be incomplete” 

9. Which fungus is called hallucinogenic fungus and why? 

Five Marks 

1. Write about the symbiotic mode of nutrition as seen in fungi. 

2. Give the salient features of the subdivision Ascomycotina / Basidiomycotina / 

Zygomycotina 

Ten Marks 

1. Write an essay on the mode of nutrition in fungi. 

2. Give a concise account on the economic importance of fungi. 

38

2.3.1 Mucor 

Division : Eumycota 

Sub Division : Zygomycotina 

Class : Zygomycetes 

Order : Mucorales 

Family : Mucoraceae 

Genus : Mucor 

Occurrence : Mucor is a saprotrophic fungus. The appearance of the 

sporangiophores and sporangia resembles a collection of pins and hence it is 

commonly known as “pin mould”. As the matured sporangia are black in colour, 

it is also called “black bread mold”. There are more than 50 species that have 

been reported in this genus. It grows on dung (eg : Mucor mucedo), wet shoes, 

stale moist bread, rotton fruit, decaying vegetables and other stale organic media. 

It can be grown easily in the laboratory on a piece of moist bread or on horse dung 

kept under a bell jar in a warm place for three or four days. 

Somatic structure : The body of 

Mucor is composed of a mass of white, 

delicate, cottony threads collectively 

known as mycelium. It is always much 

branched and coenocytic i.e. aseptate 

and multinucleate. Each thread of the 

mycelium is known as hypha which is 

aseptate. The mycelium spreads in all 

directions over the substratum. Some 

of the hyphal branches penetrate down 

into the substratum to absorb water and 

nutrients. 

Reproduction : This takes place 

vegetatively, asexually and sexually. 

The asexual reproduction by formation 

of spores is very common in Mucor. 

Vegetative reproduction : Fragmentation of the mycelium results in the 

multiplication of the fungus by developing fresh stock of mycelia. 

Asexual reproduction : Asexual reproduction is by spores or 

chlamydospores or sometimes by oidia. 

Sporangiospores or spores : Mucor readily reproduces asexually by forming 

sporangiospores or spores (Fig.). Here the spores are produced in sporangia. Under 

favourable conditions of moisture and temperature, numerous slender, erect hyphae 

39 

Fig.1.16 Mucor - Mycelium with 

sporongia and spores

called sporangiophores arise from the mycelium. These sporangiophores are 

hyaline and do not produce rhizoids at their base. The apex of the sporangiophore 

enlarges to form a vesicle which may be globose or spherical, multispored and 

columellate structure - the sporangium. As the hypha begins to swell, the 

protoplasmic contents migrate to its tip and accumulate there. The protoplasm 

differentiates into a central vacuolated region and an outer fertile dense peripheral 

region containing large number of nuclei. The central vacuolated region is separated 

from the peripheral 

region by a cleft or 

furrow and this 

furrow later 

develops into a wall. 

This central sterile 

region is called 

columella. The 

peripheral fertile 

region forms spores 

as follows : The 

peripheral 

protoplasm now 

gives rise to a 

number of small, 

multi-nucleate, 

angular masses by 

cleavage. Each 

multinucleate mass 

becomes rounded 

off and is covered by 

a wall forming a 

spore. The spore 

wall thickens and 

darkens. The wall of 

the sporangium is 

thin and brittle. The 

columella swells due 

to the accumulation 

of a quantity of fluid 

and exerts a 

considerable 

pressure on the wall 

of the sporangium. 

40

As a consequence, the sporangium bursts, setting free the spores. The spores are 

blown by wind. The columella persists for some time after the bursting of the 

sporangium. Being very minute, light and dry the spores float about in the air, and 

under favourable conditions, they germinate in a suitable medium to produce a 

new thallus of Mucor. 

Chlamydospores. It is sometimes seen that under 

unfavourable conditions, the mycelium of Mucor becomes 

segmented into a short chain of cells. These cells swell up and 

become thick-walled and large. These are called 

chlamydospores. These spores are resting spores. They 

germinate under favourable conditions to give rise to somatic 

structure of the fungus. 

Sometimes the hyphae produce yeast-like budding cells called 

‘oidia’. 

Sexual reproduction : The mycelium of most of the species 

of Mucor is unisexual and hence unable to produce both the male 

and female gametangia. Sexual reproduction takes place when 

Fig.1.18 Yeast 

like oidia 

hyphae derived from different spores come in contact with one another. There is 

no morphological distinction between the two sexual hyphae although physiologically 

they are dissimilar. Since physiologically dissimilar thalli (hyphae) are involved in 

sexual reproduction, this phenomenon is called heterothallism. The two sexual 

hyphae are called (+) and minus (-) strains. Here (+) strain and (-) strain hyphae 

readily mate and hence are called mating types. Whereas (+) and (+) or (-) and (- 

) strains do not mate. The growth of the (+) strain (female) is more vigorous than 

that of (-) strain (male). In heterothallic species (eg. M. mucedo), sexual 

reproduction begins when (+) and (-) strain hyphae intermix. In some species (eg. 

M. hiemalis) any two hyphae from the same mycelium derived from a single 

spore may initiate sexual reproduction. This phenomenon is called homothallism. 

When a (+) strain and (-) strain hyphae of heterothallic species (eg. 

M. mucedo) come in contact with one another, the tips of both hyphae 

swell to form progametangia. The two progame-tangia meet and adhere together 

at their tips. Each progametangium gets transformed into a terminal gametangium 

and a basal suspensor (Fig.). Each multinucleate gametangium is regarded as a 

coenogamete. The wall between the two gametangia dissolves and the protoplasm 

of both gametangia fuse to form a black thick-walled zygospore. As the thickwalled 

zygospore has reserve food material, it is able to tide over unfavourable 

41

Fig.1.19 Sexual reproduction in Mucor 

conditions. During favourable conditions, the zygospore germinates and undergoes 

meiosis to produce a sporangiophore and a sporangium at its tip (Fig.). Such a 

sporangium lacks columella and produces numerous haploid spores. The spores 

are released due to dehiscence of sporangium. Upon germination, the spores 

produce (+) or (-) strain hyphae. 

In unusual cases, the gametangia fail to fuse and develop into thick-walled 

structures called azygospores. 

42

Fig.1.20 Life-Cycle of Mucor 


One mark 

1. Mucor is commonly called 

a) Black bread mould b) Red bread mould 

c) White bread mold d) Brown bread mold 

2. The mycelium of Mucor has this condition 

(a) Uninucleate (b) Binucleate (c) Multi nucleate (d) None of the above 

3. The erect hypha that bears sporangium is known as 

(a) Zoospore (b) Sporangiophore (c) Zygospore (d) Azygospore 


1. The multinucleate gametes of Mucor are called_____________. 

2. ______is formed by the fusion of two gametes of different strains in Mucor. 

3. The basal part of the terminal gametangium is known as__________ in Mucor. 

43

4. The dome shaped sterile portion of the sporangium is known as________ . 

5. Each thread of the mycelium of Mucor is known as____________. 

Two Marks 

1. Describe the mycelium of Mucor. 

2. What is coenocytic condition? 

3. What is columella? Where it can be seen? 

4. How azygospores are produced? 

Five Marks 

1. Briefly describe the process of production of zygospore. 

2. Give an account of asexual reproduction in Mucor. 

3. Briefly write about Chlamydospores. 

4. Describe the somatic structure of Mucor. 

Ten Marks 

1. Write briefly about the sexual reproduction in Mucor. 

2. Trace the life cycle of Mucor with properly labelled diagrams. 

3. Describe the process of homothallism and heterothallism. 

44

2.4 Algae 

Salient Features 

Algae are autotrophic organisms and they have chlorophyll. They are O 2 

producing photosynthetic organisms that have evolved in and have exploited an 

aquatic environment. The study of Algae is known as Algology or phycology. 

In Algae the plant body shows no differentiation into root, stem or leaf or true 

tissues. Such a plant body is called thallus. They do not have vascular tissues. 

The sex organs of this group of kingdom plantae are not surrounded by a layer of 

sterile cells. 

Occurrence and Distribution 

Most of the algae are aquatic either fresh water or marine. Very few are 

terrestrial. A few genera grow even in extreme condition like thermal springs, 

glaciers and snow. 

The free floating and free swimming minute algae are known as 

phytoplanktons. Species that are found attached to the bottom of shallow water 

along the edges of seas and lakes are called Benthic. Some of the algae exhibit 

symbiotic association with the higher plants. Some species of algae and fungi are 

found in association with each other and they are called Lichens. A few species 

of algae are epiphytes (i.e they live on another plant or another alga) and some of 

them are lithophytes (i.e they grow attached to rocks) 

Thallus organization 

The thalli of algae exhibit a great range of variation in structure and organization. 

It ranges from microscopic unicellular forms to giant seaweeds like Macrocystis 

which measures up to 100 meters long. Some of them form colonies, or filaments. 

The unicellular form may be motile as in Chlamydomonas or non-motile as in 

Chlorella. Most algae have filamentous thallus. eg. Spirogyra. The filaments 

may be branched. These filamentous form may be free floating or attached to a 

substratum. Attachment of the filament is usually effected through a simple 

modification of the basal cell into a holdfast. Some of the Algae are macroscopic. 

eg. Caulerpa, Sargassum, Laminaria, Fucus etc.where the plant body is large. 

In Macrocystis it is differentiated into root, stem and leaf like structures. 

The chlroplasts of algae present a varied structure. For eg. they are cup 

shapedin Chlamydomonas, ribbon-like in Spirogyra and star shaped in Zygnema. 

45

Green Algae 

Chlamydomonas Spirogyra Volvox Spirogyra 

Gelidium 

Red Algae 

Chondrus 

Macrocystis 

Brown Algae 

Sargassum 

Cell Structure & Pigmentation 

Fig : 1.21. Thallus Organization in Algae 

With the exception of blue green algae which are treated as Cyanobacteria, 

all algae have eukaryotic cell organization. The cell wall is made up of cellulose 

and pectin. There is a well defined nucleus and membrane bound organelles are 

found. 

Three types of Photosynthetic pigments are seen in algae. They are 1. 

Chlorphylls 2. Carotenoids 3. Biliproteins. While chlorophyll a is universal 

in all algal classes, chlorophyll b,c,d,e are restricted to some classes of algae. 

46

The yellow, orange or red coloured pigments are called carotenoids. It includes 

the caroteins and the Xanthophylls. The water soluble biliproteins called 

phycoerythrin (red) and phycocyanin (blue) occur generally in the Rhodophyceae 

and Cyanophyceae and the latter is now called cyanobacteria. These pigments 

absorb sunlight at different wavelengths mainly in blue and red range and help in 

photosynthesis. Pigmentation in algae is an important criterion for classification. 

The colour of the algae is mainly due to the dominance of some of the pigments. 

For example in red algae(class Rhodophyceae) the red pigment phycoerythrin is 

dominant over the others. The pigments are located in the membranes of chloroplasts. 

In each chloroplast one or few spherical bodies called pyrenoids are present. 

They are the centres of starch formation. 

Nutrition and reserve food materials in Algae 

Algae are autotrophic in their mode of nutrition. The carbohydrate reserves 

of algae are various forms of starch in different classes of Algae. For example, in 

Chlorophyceae, the reserve food is starch and in Rhodophyceae it is Floridean 

starch, in Phaeophyceae it is laminarian starch while in Euglenophyceae it is 

paramylon. Members of Phaeophyceae store mannitol in addition to carbohydrate. 

Members of Xanthophyceae and Bacillariophyceae store fats, oils and lipids. 

The nature of reserve food material is also another important criterion used in 


Arranagement of Flagella 

Flagella or cilia( sing.flagellum / cilium) are organs of locomotion that occur in 

a majority of algal classes. There are two types of flagella 

namely whiplash (Acronematic) and tinsel 

(pantonematic). 

Tinsel 

The whiplash flagellum has a smooth surface while 

the tinsel flagellum has fine minute hairs along the axis. 

The number, insertion, pattern and kind of flagella appear 

to be consistent in each class of algae and it is an important 

Whiplash 

criterion for classification of algae. Motile cells of the 

Algae are typically biflagellate. When both flagella are 

Fig : 1.22. Types of Flagella 

of equal length and appearance, they are described as isokont. Heterokont 

forms have dissimilar flagella with reference to their length and types. Red 

algae(Rhodophyta) and Blue green algae(Cyanophyta) lack flagella. Each 

flagellum consists of two central microtubles surrounded by a peripheral layer of 

nine doublet microtubles. This is called 9+2 pattern of arrangement which is a 

characteristic feature of eukaryotic flagellum. The entire group of microtubles is 

surrounded by a membrane. 

47

Reproduction 

Three common methods of reproduction found in Algae are 1. Vegetative 

2. Asexual and 3. Sexual 

Vegetative reproduction 

It lakes place by fragmentation or by the formation of adventitious branches. 

Asexual reproduction: 

It takes place by means of different kinds of spores like Zoospores, 

Aplanospores and Akinetes. Zoospores are naked, flagellated and motile. 

eg.(Chlamydomonas) Aplanospores are thin walled and non motile (eg Chlorella) 

Akineties are thick walled and non motile spores (eg Pithophora) 


Sexual reproduction involves fusion of two gametes. If fusing gametes belong 

to the same thallus it is called homothallic and if they belong to different thalli it is 

heterothallic. Fusing gametes may be isogametes or heterogametes. 

Isogamy 

It is the fusion of two morphologically and physiologically similar gametes.eg. 

Spirogyra and some species of Chlamydomonas . 

Heterogamy 

This refers to the fusion of dissimilar gametes. It is of two types 1. Anisogamy 

2. Oogamy 

1. Anisogamy is the fusion of two gametes which are morphologically 

dissimilar but physiologically similar (both motile or both non-motile) 

2. Oogamy refers to the fusion of gametes which are both morphologically 

and physiologically dissimilar. In this type of fusion the male gamete is 

usually referred to as antherozoid which is usually motile and smaller in 

size and the female gamete which is usually nonmotile and bigger in size is 

referred to as egg. The sex organ which produces the antherozoids is called 

antheridium and the egg is produced in oogonium. The fusion product of 

antherozoid and egg is called Zygote. The zygote may germinate directly 

after meiosis or may produce meiospores which in turn will germinate. 

Classification 

F.E. Fritsch (1944-45) classified algae into 11 classes in his book “Structure 

and Reproduction of Algae” based on the following characteristics. 

48

1. Pigmentation 2. Reserve food 3. Flagellar arrangement 4. Thallus 

organization 5. Reproduction. 

The 11 classes of algae are: 

1. Chlorophyceae 2. Xanthophyceae 3. Chrysophyceae 

4. Bacillariophyceae 5. Cryptophyceae 6. Dinophyceae 7. 

Chlromonodineae 8.Euglenophyceae 9. Phaeophyceae 10. Rhodophyceae 

and 11. Myxophyceae 

Some major groups of Algae and their characteristics are summarized in Table 1.4. 

Economic Importance of Algae 

Recent estimates show that nearly half the world’s productivity that is carbon 

fixation, comes from the oceans. This is contributed by the algae, the only vegetation 

in the sea. Algae are vital as primary producers being at the start of most of 

aquatic food chains. 

Algae as Food: Algae are important as a source of food for human beings, 

domestic animals and fishes. Species of Porphyra are eaten in Japan, England 

and USA. Ulva, Laminaria, Sargassum and Chlorella are also used as food in 

several countries. Sea weeds (Laminaria, Fucus, Ascophyllum) are used as 

fodder for domestic animals. 

Algae in Agriculture: Various blue green algae such as Oscillatoria, 

Anabaena, Nostoc, Aulosira increase the soil fertility by fixing the atmospheric 

nitrogen. In view of the increasing energy demands and rising costs of chemically 

making nitrogenous fertilizers, much attention is now being given to nitrogen 

fixing bacteria and blue green algae. Many species of sea weeds are used as 

fertilizers in China and Japan. 

Algae in Industry 

a. Agar – agar : This substance is used as a culture medium while growing 

bacteria and fungi in the laboratory. It is also used in the preparations of 

some medicines and cosmetics. It is obtained from the red algae Gelidium 

and Gracilaria. 

b. A phycocolloid Alginic acid is obtained from brown algae. Algin is used as 

emulisifier in ice creams, tooth pastes and cosmetics. 

c. Idodine: It is obtained from kelps (brown algae) especially from speicies of 

Laminaria. 

d. Diatomite : It is a rock-like deposit formed on the siliceous walls of 

diatoms(algae of Chrysophyceae). When they die they sediment, so that 

on the seabed and lake bottom extensive deposits can be built up over long 

periods of time. The resulting ‘diatomaceous earth’ has a high proportion 

of silica. Diatomite is used as a fire proof material and also as an absorbent. 

49

Table : 1.4. Characteristics of Major Groups of Algae 

Class Pigments Flagella 

Chlorophyceae 

(green algae) 

Xanthophyceae 

Chrysophyceae 

(diatoms, golden 

algae) 

Bacillariophyceae 

Cryptophyceae 

Dinophyceae 

(Dinoflagellates) 

Chloromonodineae 

Euglenophyceae 

(Euglenoids) 

Phaeophyceae 

(brown algae 

Rhodophyceae 

(Red algae) 

Myxophyceae 

Chlorophyll-a,b 

Carotene 

XanthophyII 

Chlorophyll-a, b 

Carotene 

XanthophyII 

ChlorophyII-a, b 

Carotenoids 

Chlorophyll-a, c 

Carotenes 

Chlorophyll-a, c 

Carotenes and 

xanthophylls 

ChlorophyII-a, c 

Carotenoids 

Xanthophyll 

ChlorophyII-a, b 

Carotenes 

Xanthophyll 

Chlorophyll-a, b 

ChlorophyII-a 

Xanthophyll 

Chlorophyll-a 

Phycocyanin 

Phycoerythrin 

Chlorophyll-a, 

carotene, 

phycocyanin, 

phycoerythrin 

Two identical 

flagella per cell 

Heterokont type, 

one whiplash 

type and other 

tinsel 

One,two or more 

unequal flagella 

Very rare 

Heterokont typeone 

tinsel and 

other whiplash 

Two unequal 

lateral flagella in 

different plane. 

Isokont type 

One,two or three 

anterior flagella. 

Two dissimilar 

lateral flagella 

Non-motile 

Non-motile 

Reserve 

food 

Starch 

Fats and 

Leucosin 

Oils and 

Leucosin 

Leucosin 

and fats 

Starch 

Starch 

and oil 

Oil 

Fats and 

paramylon 

Laminarin, 

fats 

Starch 

Cyanophyce 

an starch 

50

It is used in sound and fire proof rooms. It is also used in packing of 

corrosive materials and also in the manufacture of dynamite. 

Algae in space travel: Chlorella pyrenoidosa is used in space travel to get 

rid of Co 2 

and other body wastes. The algae multiplies rapidly and utilizes the Co 2 

and liberate 0 2 

during photosynthesis. It decomposes human urine and faeces to 

get N 2 

for protein synthesis. 

Single cell protein (SCP): Chlorella and Spirullina which are unicellular 

algae are rich in protein and they are used as protein source. Besides, Chlorella 

is a source of vitamin also. The rich protein and aminoacid content of chlorella 

and Spirulina make them ideal for single cell protein production. An antibiotic 

Chlorellin is extracted from Chlorella. 

Sewage Disposal:Algae like Chlorella are grown in large shallow tanks, 

containing sewage. These algae produce abundant oxygen by rapid photosynthesis. 

Microorganisms like aerobic bacteria use these oxygen and decompose the organic 

matter and thus the sewage gets purified. 

Harmful effects of Algae 

Under certain conditions algae produce ‘blooms’, that is dense masses of 

material. This is especially true in relatively warm conditions when there is high 

nutrient availability, which sometimes is induced by man as and when sewage is 

added to water or inorganic fertilizers run off from agricultural land into rivers and 

lakes. As a result of this a sudden and explosive growth of these primary producers 

(algae) occurs. They are produced in such a huge quantity that they die before 

being eaten. The process of decomposition is carried out by aerobic bacteria 

which in turn multiply rapidly and deplete the water of oxygen. The lack of oxygen 

leads to the death of fish and other animals and plants in the lakes. The increase of 

nutrients which starts off the entire process is called eutrophication and if rapid 

it constitutes a major problem of pollution. The toxins produced by algal bloom can 

also lead to mortality. This can be a serious problem in lakes and oceans. Sometimes 

the toxins may be stored by shellfish feeding on the algae and be passed on to man 

causing the disease called paralytic shellfish poisoning. Algae also cause problems 

in water storage reservoirs where they may taint the water and block the beds of 

sand used as filters. 

Self Evaluation 

One Mark 


1. Phycology is the study of 

a. plants b.virus c.Algae d.bacteria 

51


1. _________ is the red colour pigment found in algae. 

2. __________ is the blue colour pigment found in algae. 

3. __________ algae lack motile cells. 

Match 

Macroscopic - Attached to the bottom of shallow water 

Epiphyte - Laminaria 

Benthic - Spirogyra 

Lithophyte - Growing on another plant 

Filamentous - Grow attached to the rocks. 

Two Marks 

1. Define: thallus 

2. What is a Lichen? 

3. Name the three types of photosynthetic pigments found in algae? 

4. Differentiate a whiplash flagellum from a tinsel flagellum. 

5. What are pyrenoids? 

6. Differentiate isokont from heterokont type of flagella? 

7. Define isogamy / heterogamy/ anisogamy/ oogamy. 

8. What is agar-agar? 

9. What is diatomite? 

10. Write any two uses of diatomite. 

11. How are the algae used in space travel? 

12. What is SCP? 

13. How are algae used in sewage disposal? 

14. What is algal bloom. How does it affect the lakes? 

15. Algae are not associated with diseases unlike many fungi and bacteria. What 

is the reason for this? 

Five Marks 

1. What is eutrophication? What is it’s significance? 

2. Write notes on: Nutrition and reserved food materials in algae. 

3. Write about the pigmentation in algae. 

Ten Marks 

1. Write an essay on the economic importance of algae. 

2. Write an essay on reproduction in algae. 

52

2.4.1 Spirogyra 

Class : Chlorophyceae 

Order : Conjugales 

Family : Zygnemaceae 

Genus : Spirogyra (Spiro-coiled; gyra-curved) 

Occurrence 

Spirogyra is a very common free floating fresh water alga found in fresh water 

pools, ponds, lakes etc., in great abundance. It is also known as “water silk” or 

“pond silk". The filaments are slimy in nature because of the presence of a 

mucilaginous substance around them. Spirogyra adnata is an attached form and found 

in flowing waters. In later stage this species also becomes free floating. Some of the 

species develop rhizoid like haptera, occasionally at the basal ends of the filaments. 

Spirogyra is a chlorophyllous alga. S. columbiana is reported from South India and 

S. jogensis has been reported from Jog falls, Mysore. 

Structure 

The filaments of Spirogyra are unbranched with many cells placed end to end. 

All the cells are similar in structure. The cells are cylindrical in shape and some time 

several times longer than their breadth. The cell wall of the filament is usually two- 

neighbouring 

cell 

single 

large 

vacuole 

spiral 

chloroplast 

in 

peripheral 

cytoplasm 

pyrenoid 

Fig.1.23 Structure of Spirogyra cell - Diagram of Side View 

53

layered. The outer most layer consists of pectic substances 

and the layer just outside the protoplast consists of cellulose, 

the outer most portion of pectic substances dissolve in water 

to form slimy sheath. This is some times referred to as the 

third layer of the cell wall. The cells are uninucleate, the 

nucleus is usually situated in the centre of the cell and 

connected by cytoplasmic strands to the dense cytoplasm of 

the peripheral region. There is a big central vacuole the 

cytoplasmic strands are also called as primordial utricle. In 

Spirogyra the chlorololats are spiral and ribbon like, they may 

be serrated or smooth at the margins. The number of 

chloroplasts ranges 1-14 in different species. Many pyrynoids 

are found in each ribbon like chloroplast. Some times the 

filaments of some species of Spirogyra exhibit gliding 

movements. 

(a) 

(b) 

Reproduction 

Fig.1.24 (a) Spirogyra habit 

It takes place by the following methods. 

(b) Aknite 

(1) Vegetative Reproduction 

By (a) parthenospores, (b) akinetes and (c) aplanospores 

(2) Sexual reproduction by conjugation 

(1) Vegetative reproduction 

The vegetative filaments break accidentally into many small fragments. Each 

such fragment developes into a new filament. 

(a) By parthenospore 

Some times the contents of a cell recede from the cell wall and lie in the middle. 

A thick wall surrounds the whole content which is called a parthenospore. It directly 

forms a new plant. 

Parthenogenesis 

The formation of parthenospores or azygospores, has been observed in many 

species. Here the conjugation does not take place and the contents of the cells become 

rounded. The walls are serrated around these protoplasts and they are called 

parthenospores (or) azygospores. Azygospore formation occurs in S. greenlandica. 

The process of formation of parthenospores is known as parthenogenesis. 

(b) By akinetes 

In S. farlowi thick walled akinetes are formed. The entire cell content of the 

akinete gives rise to a new plant. 

54

(c) By aplanospores 

During unfavourable conditions, aplanospores are formed in Spirogyra. The 

contents recede from the cell wall and the whole structure comes to rest. It is nonmotile. 

On the return of favurable conditions the old cell wall of the parent is cast off 

and new wall develops. They directly give rise to a new plant. 


In Spirogyra the sexual reproduction takes place by special gametes called 

aplanogametes and the process is aplanogamy. The motile gametes are always 

lacking. Aplanogammy takes place by conjugation, which may be scalariform or lateral. 

In each cell a single aplanogamete is produced which moves into the other cell through 

a conjugation tube in amoeboidal fashion. The species may be homothallic or heterothallic 

Scalariform conjugation is the process of 

reproduction of heterothallic species. Lateral 

conjugation, takes place in homothallic species. 

In scalariform conjugation the aplano gametes of 

two filaments opposite to one another, unite where 

as in lateral conjugation the aplanogametes of the 

two adjacent cells of the same filament unite. 

Scalariform conjugation 

This type of reproduction is found in 

majority of the species of Spirogyra. The 

filaments taking part in conjugation lie side by side. 

Very soon the out growths are given out from the 

lateral walls of the opposite cells of the filaments. 

The out growths of opposite cells touch each other 

very soon the wall of contact dissolves and a 

tubular passage is formed between the two 

opposite cells of the two filaments lying side by 

side. This tubular passage is called conjugation 

tube. Simultaneously the contents of the cells 

retract and aplanogametes are developed. A 

single aplanogamete is developed in each cell. 

The aplanogametes formed in the cells of one 

filament pass into the opposite cells of the other 

filament through conjugation tubes in amoeboid 

FEMALE 

GAMETANGIUM 

Fig. 1.25 Scalariform 

conjugation in Spirogyra 

55

fashion. The plasmogamy is followed by karyogamy. The transferring aplanogametes 

are considered to be male gametes while the receiving aplanogametes are female 

gametes. Just after the fusion the walls are formed around the zygote and they are 

called zygospores. A single zygospore develops in each cell of the female filament. 

The wall of the female decays and the zygospores are set free in the water. Each 

zygospore germinate into a new 

plant after a resting period. 

Lateral conjugation 

Conjugation tube 

This type of reproduction 

occurs in homothallic species. 

Here the aplanogametes of the 

adjacent cells of the same filament 

unite. At the septum a tube like 

structure develops and through 

this opening the content of one 

cell passes into the other. Then 

both a manosomefes fuse to form 

Male gametangium 

Female gametangium 

Zygospore 

a zygote. The empty cells are Fig. 1.26 Lateral conjugation in Spirogyra 

considered as male gametangia 

and the cells with zygotes are female gametangia. Very soon a thick wall is secreted 

around each zygote and dark coloured zygospores develop. They undergo a period of 

rest and germinate under favourable conditions. 

Germination of zygospore 

Prior to germination, the diploid nucleus 

divides meiotically and, four haploid nuclei are 

formed. Three of the four haploid nuclei 

disintegrate and only one remains functional. 

The zygospore wall breaks and the germling 

comes out which soon develops into a new 

filament. 

Fig. 1.27 Germination of 

Zygospore 

56

Fig.1.28 Schematic representation of Life Cycle of a Spirogyra 


One Mark 


1. Spirogyra occurs in 

a) Running salt water b) Running fresh water c) Marine water d) Stagnant water 

57

2. On germination each zygospore of Spirogyra gives rise to 

a) Four plants b) Two plants c) Three plants d) One plant 

3. In Spirogyra pyrenoid is found in 

a) Nucleus b) Cell wall c) Chloroplast d) Cytoplasm 

4. The slimy sheath that is seen in Spirogyra is considered sometime as 

a) Third layer b) Second layer c) First layer d) Fourth layer 

5. In Spirogyra cells the chloroplasts are 

(a) Spherical in shape (b) Conical in shape 

(c) Spiral in shape (d) Rectangular in shape 

6. Scalariform conjugation is the process of reproduction in 

(a) Homothallic species (b) Parthenocarpic species 

(c) Symbiotic species (d) Heterothallic species 


1. ........................................... is the common name of Spirogyra. 

2. ........................................... are produced as a result of Parthenogenesis. 

3. Vegetative reproduction takes place by..................... 

Two Marks 

1. Define conjugation. 

2. What are common names of Spirogyra. 

3. Briefly explain the nature of cell wall in Spirogyra. 

4. Define aplanogametes. 

Five Marks 

1. Differentiate zygospore from azygospore. 

2. What is parthnegenesis? 

3. Give a brief out line of scalariform conjugation with proper example. 

4. Describe the external structure of Spirogyra. 

Ten Marks 

1. Describe the two types of sexual reproduction seen in Spirogyra. 

2. Give an account of the habitat and structure of Spirogyra. 

3. Describe the Life Cycle of Spirogyra. 

58

2.5 Bryophytes 

There are fossil records of blue green algae (Cyanobacteria) living 3000 million 

years ago and many eukaryotic organisms have existed for more than 1000 million 

years. However the first organisms to colonize the land, primitive plants did so 

only 420 millions years ago. The greatest simple problem to overcome in making 

the transition from water to land is that of desiccation. Any plant not protected in 

some way, for example, by a waxy cuticle, will tend to dry out and die very soon. 

Salient features of Bryophyta 

Bryophyta are the simplest group of land plants. They are relatively poorly 

adapted to life on land, so they are mainly confined to damp,shady places. These 

are terrestrial non-vascular plants(no vascular tissue namely xylem and phloem) 

which still require moist environment to complete their life-cycle. Hence these are 

called amphibians of plant kingdom. They are more advanced than algae in that 

they develop special organs. The male sex organ is called antheridium and the 

female sex organ is called archegonium. Bryophytes show distinct alternation of 

generation in their life cycles. Bryophytes include mosses, liverworts and hornworts. 

Distinguishing features of Bryophytes 

1. They are small terrestrial plants. 

2. They are without a distinct root system but are attached to the substratum 

by means of thin, filamentous outgrowth of the thallus called rhizoids. 

3. Water and mineral salts can be absorbed by the whole surface of the plant 

body, including the rhizoids. So the main function of rhizoids is anchorage, 

unlike true roots (true roots also possess vascular tissues, as do true stems 

and leaves). Thus the “stems” and “leaves” found in some Bryophytes are 

not homologous with stems and leaves of vascular plants. The plant body is 

called thallus. 

4. They do not possess true vascular tissues. 

5. Male sex organ is called antheridium and female sex organ is called 

archegonium. 

6. Sex organs are multi-cellular and they have a protective jacket layer of 

sterile cells. 

7. Sexual reproduction is of oogamous type. 

59

8. Bryophytes show distinct alternation of gametophytic generation with 

sporophytic generation. 

9. Gametophyte generation is dominant and independent. 

10. Sporophyte generation is very small, microscopic and dependent on the 

gametophyte phase. 

Alternation of Generations 

In common with all land plants and some advanced algae such as Laminaria, 

bryophytes exhibit alternation of generations. Two types of organism, a haploid 

gametophyte generation and a diploid sporophyte generation, alternate in the 

life cycle. The cycle is summarized in the fig below. 

The haploid 

generation is called the 

gametophyte because it 

undergoes sexual 

reproduction to produce 

gametes. Production of 

gametes involves 

mitosis, so the gametes 

are also haploid. The 

gametes fuse to form a 

diploid zygote which 

grows into the next 

generation, the diploid 

sporophyte generation. 

It is called sporophyte 

because it undergoes 

asexual reproduction to 

produce spores. 

Production of spores 

involves meiosis, so 

that there is a return to 

the haploid condition. The haploid spores give rise to the gametophyte generation. 

One of the two generations is always more conspicuous and occupies a greater 

proportion of the life cycle. This generation is called as the dominant generation. 

In all Bryophytes the gametophyte generation is dominant. In all other 

land plants the sporophyte generation is dominant. It is customary to place 

60

the dominant generation in the top half of the life cycle diagram. The figure given 

above summarises the life cycle of a typical Bryophyte. One point that must be 

remembered here is that gamete production involves mitosis and not meiosis as in 

animals. Meiosis occurs before the production of spores. 

BRYOPHYTA 

1. Hepaticae 2. Anthocerotae 3. Musci 

Liverworts hornworts Mosses 

Eg. Riccia eg. Anthoceros eg.Funaria 

Classification 

Bryophyta is divided into three major classes. 

Marchantia 

Columella 

Riccia 

Branch 

(Fem ale) 

Sporophyte 

Thallus 

Thallus 

Rhizoids 

Fig : 1.30. Some liverworts 

Fig: 1.31. Hornwort-Anthoceros 

Involucre 

Thallus 

1. Class Hepaticae 

These are lower forms of Bryophytes. They are more simple in structure 

than mosses and more confined to damp and shady habitats. They have an 

undifferentiated thallus. Protonemal stage is absent. Sporophyte is very simple 

and short lived . In some forms sporophyte is differentiated into foot, seta and 

capsule. Eg. Marchantia. In some the foot and seta are absent. Eg. Riccia. 

2. Class Anthocerotae 

Gametophyte is undifferentiated thallus. Rhizoids are unicellular and 

unbranched. Protonemal stage is absent. Sporophyte is differentiated into foot 

and capsule and no seta. Eg. Anthoceros. 

3. Class Musci 

61

They have a more differentiated structure than 

liverworts. They often form dense cushions. These are 

higher forms in which the gametophyte is differentiated 

into ‘stem’ like and ‘leaf’ like parts and the former 

showing radial symmetry. Rhizoids are multi-cellular 

and branched. Protonemal stage is present. Sporophyte 

is differentiated into foot, seta and capsule Eg. Funaria. 

Economic Importance 

1. Bryophytes form dense mat over the soil and 

prevent soil erosion. 

2. Sphagnum can absorb large amount of water. 

It is extensively used by gardeners in nursery to 

keep seedlings and cut plant parts moist during 

propagation. 

3. Peat is a valuable fuel like coal. Mosses like 

Sphagnum which got compacted and fossilized 

over the past thousands of years have become 

peat. 

4. Mosses are good sources of animal food in rocky 

areas. 


One Mark 


1. Production of gametes in Bryophytes involve 

Rhizoids 

Capsule 

Branch 

(Female) 

Fig : 1.32. Moss - Funaria 

Calyptra 

a. Meiosis b. Mitosis c. fertilization d. reduction division 


1. In all bryophytes __________ generation is dominant. 

2. In all land plants other than bryophytes __________ generation is dominant. 

Two Marks 

1. Give reasons: Bryophytes are called the amphibians of plant kingdom. 

2. Name the three main classes of Bryophyta. 

3. What is peat? 

4. How is Sphagnum used in nursery? 

Ten Marks 

1. Discuss the problems associated with the transition from an aquatic environment 

to terrestrial habitat. 

2. Discuss the classification of Bryophyta. 

Seta 

62

2.5.1 Riccia 

Class : Hepaticae 

Order : Marchantiales 

Family : Ricciaceae 

Genus : Riccia 

Riccia, which belongs to the family Ricciaceae is the single genus with 130 species. 

Riccia is more confined to damp and shady 

habitats. It can grow in water, plains and in hilly 

regions. It has an undifferentiated thallus and 

simple in structure. 

The following are some of the important 

species of Riccia, R. fluitans, R. gangetica, 

R. cruciata, R. kashyapii, R. discolor (this 

rosette habit 

is also called as R. himalayensis). Of these, 

R. gangetica and R. fluitans are native of 

India. Excepting R. fluitans, the rest are land 

Fig.1.33 Riccia thallus - habit 

plants. R. fluitans is aquatic in nature and is found floating in stagnant waker 

reservoir. 

Riccia thallus grow in large number in close association, covering the surface 

like a mat. They are relatively poorly adapted to life on land. So they are mainly 

confined to damp and shady places. The species of Riccia are terrestrial, 

nonvascular plants commonly called amphibians of plant kingdom. 

The structure of mature gametophyte 

Riccia has two generations namely, gametophytic and sporophytic. 

Gametophytic generation is dominant and independent. Sporophyte is dependent 

upon the gametotophyte. 

The adult terrestrial gametophyte is prostrate, rosette-like dichotomously 

branched dorsiventral and deep green coloured plant found on suitable substratum 

of dampsoil. The dichotomous branching of the thallus is found quiet close to each 

other and thus a rosette-like appearance is attained. Each and every branch of 

the thallus is more of less spoon shaped or rectangular in outline. A distinct 

longitudinal groove is found on the dorsal side of each branch of the thallus. The 

growing point is situated in the notch found between dichotomous branched thallus. 

Numerous scales and rhizoids are seen on the ventral surface of the thallus. The 

ventral surface bears a row of one celled thick, multicellular scales. They are 

63

arranged close to each other towards the apex of the branch. The older parts of 

the thallus bear two rows of scales. 

Fig.1.34. Riccia thallus (a) dorsal surface (b) ventral surface (c) smooth 

walled and tuberculate rhizoids 

The rhizoids absorb the nutrients and water from substratum, and in this way 

they perform the function of the roots. The rhizoids are of two types, i.e. smoothwalled 

and tuberculate. In each case the rhizoids are unicellular. They are absent 

in R. fluitans (aquatic species). 

Internal structure of the thallus 

The cross section of the thallus shows simple tissue arrangement. Two regions 

are distinctly seen. The dorsal region is capable of food preparation. It is provided with 

chlorophyllous tissues and the ventral region is storage in function as it contains 

parenchyma tissue which is colourless. However, the inter cellular spaces are absent 

and the cells are filled up with starch grains. From the single layered epidermis formed 

by the compactly arranged storage tissues several unicellular rhizoids (smooth walled 

or tuberculate) and muticellular scales are given out. 

Fig.1.35 Riccia thallus internal structure (a) transverse section of thallus 

(diagrammatic), (b) a part of T.S. of thallus showing cellular details 

64

Chlorophyllous tissue 

The dorsal region of the thallus consists of chlorophyllous cells. These cells with 

distinct chloroplasts in them are are arranged in vertical rows. There are regular air 

canals in between the two vertical rows of the chlorophyllous cells and this region is 

photosynthetic and synthesise carbohydrates. The epidermis of the dorsal surface of 

the thallus is discontinuous and open outside at several places by the opening of air 

canals. The epidermis is single layered. The exchange of gases takes place through 

the air canals. 

Reproduction 

The reproduction in Riccia takes place by means of (1) vegetative and (2) sexual 

methods. 

(1) Vegetative Reproduction 

(a) By the death and decay of the older parts of the thallus 

(b) By adventitious branches 

(c) By cell division or gemma formation 

(d) By tubers 

(e) By thick apices 

By adventitious Branches 

In several species of Riccia, the adventitious branches are produced on the ventral 

surface of the thallus. These branches get detached from the thalli and develop into 

new gametophytes. 

By Tubers 

In species such as R. discolor, the vegetative tubers develop at the apices of the 

branches of the thallus which face adverse conditions and develop into new plants on 

the approach of favourable conditions. 

By cell division or gemma formation 

Riccia may also reproduce vegetatively by cell division of the young rhizoids 

which develop into gemma-like structure. These structures give rise to new plants. 

Sexual reproduction 

Gemetophyte Generation 

Majority of the species of Riccia are homothallic (monoecious) i.e., and antheridia 

and archegonia are borne upon the same thallus. The species of this type are 

R. glauca and R. gangetica. The heterothallic (dioecious) species are also common. 

In such species the antherdia and archegonia develop separately on different thalli and 

the important species of this type are R. himalayensis and R. frostii. 

65

Sporophyte generation 

At the time of formation of spores, there is genotypic type of sex determination 

i.e., two spores of a tetrad develop into female and two into male gametophytes, after 

meiosis. 

Sexorgans 

The sex organs, antheridia and archegonia, are produced in the groove situated on 

the dorsal surface of the 

mature gametophyte in 

acropetal succession. 

In homothallic species, 

the alternate groups of 

antheridia and archegonia are 

found at a sufficient distance 

from the growing point, even 

though the sex organs start 

growing initially on the floor of 

the dorsal groove as they 

grow, the surrounding tissue 

also grows quickly. As a result 

of which a distinct chamber is 

seen surrounding the male and 

the female sex organs. They 

are the antheridial and 

archegonial chambers. 

Structure of mature 

antheridium 

A mature antheridium remains embedded in an 

antheridial chamber, which opens by an ostiole on the dorsal 

side of the thallus. The mature antheridium consists of a 

few celled stalk and the antheridium proper. The antheridium 

proper may be rounded or somewhat pointed at its apical 

end. A sterile single layered jacket-layer encircles the 

antheridium and protects it. The mature antheridium contains 

androcytes within the jacket layer. Each androcyte 

metamorphoses into an antherozoid. Each antherozoid is 

a curved structure with two flagella. 

Fig. 1.36. Riccia - Male sex organs. 

A, Vertical section of the thallus showing position 

of antheridium B, An antheridium 

66 

Fig. 1.37. Riccia - 

Antherozoid

Structure of mature 

archegonium 

Mature archegonium is a 

flask-shaped structure attached to 

the thallus by a short stalk. It 

consists of an elongated neck 

made up of 6 rows of cells and a 

bulbous venter. The six vertical 

rows of the cells enclose a neck 

canal. There are four cover cells 

at the top of neck canal. Prior to 

maturity, the four neck canal cells, 

found within the neck canal, 

disintegrate into mucilaginous 

mass. The venter has a single 

layered wall around it which is of 

12-20 cells in perimeter. The 

venter encloses a venter canal 

cell and the large egg. The 

venter canal cell disintegrates on 

maturity and only the large egg 

Fig. 1.38. Riccia - Female sex organs. 

A, Vertical section of the thallus showing position 

of archegonium B, An archegonium 

remains in the venter. The muilagenous mass absorbs water by imbibition and because 

of the pressure, the cover cells become separated from each other and an opening is 

formed. The mucilaginous mass extrudes through the opening and attracts the 

antherozoids, which enmass around the opening. 

The venter cells are stimutated by the process of fertilization, they divide periclinally 

and the wall of the venter benomes two celled in thickness, ultimately a two layered 

calyptra is formed inside which, the developing embryo is situated. 

Fertilization 

Water is indispensable for the process of fertilization. The antherozoids reach the 

mouth of the archegonuim through the medium of water. The mucilaginous mass of 

the neck region absorbs water and swells and as a result the cover cells get separated. 

This results in the formation of a passage which facilitates the entry of sperms or 

antherozoid. The antherozoids enter the mouth of the archegonuim, travel through the 

neck and reach the vicinity of the egg. One of the antherozoids penetrates egg cell and 

the fertilization is effected. Ultimately by the union of the nuclei of male and female 

gametes, a zygote is formed. The zygote contains 2n number of chromosomes i.e., the 

zygote is diploid. 

67

Sporophyte Generation 

The diploid zygote is the first cell of the sporophyte generation. This cell secretes 

a wall around it soon after the fertilization and enlarges in size and nearly fills the cavity 

of the venter. Afterwards it undergoes division and attains two celled stage. The upper 

cell is known as epibasal cell and lower cell is known as hypobasal cell. 

Sporophyte 

The two cells of the embryo 

(epibasal and hypobasal) divide further 

and give rise to a four celled stage of 

the embryo which is followed by eightcelled 

stage. 

At a later stage the embryo is 

differentiated into two regions. The outer 

layer is amphithecium and the inner 

mass of cells is endothecium. The 

amphithecium is protective in nature 

whereas endothecium gives rise to a 

mass of sporogenous cells. 

Sporemother cells are produced from 

sporogenous cells. Spore mother cells 

undergo meiotic division and tetrads are 

produced. Thus a tetrad of four spores 

is formed. The spores becomes separated from each other only on maturation. The 

spores are haploid in nature. On the death and decay of the thallus, the spores get free 

from the sporogonium. 

Structure of the spore 

The mature spore is three layered. The outer most, cutinized layer is exosporium, 

the middle layer is mesosporium which is thick walled and consists of three concentric 

zones. The inner most layer is endosporium. The spore is the beginning of the 

gametophytic generation. 

Life cycle 

Riccia shows alternation of generations a haploid gametophyte generation and a 

diploid sporophyte generation alternate each other. 

The haploid generation is called the gametophyte because it undergoes sexual 

reproduction to produce male and female gametes. Production of gametes involves 

mitosis, so the gametes are also haploid. The male and female gametes of Riccia fuse 

68 

Fig. 1.39. Riccia - Sporophyte 

A, Developing sporophyte. B, Mature 

sporophyte. C, Single spore

to form a diploid zygote which grows into the next generation the diploid sporophyte 

generation. Sporophyte produces spore tetrads. Production of spore involves meiosis. 

Hence, there is a return to the haploid condition. The haploid spores give rise to the 

gametophyte generation. In Riccia, the gametophyte generation is dominant. 

The life cycle of Riccia shows regular alternation of gametophytic and sporophyte 

generations. The two generations are morphologically different hence this type of 

alternation of generation is known as heteromorphic. 

Vegetative 

reproduction 

Riccia 

(gametophyte, n) 

Germ ination 

of spores 

Sexual 

reproduction 

antheridium (n) 

Sex organs (n) 

archegonium (n) 

spores (n) 

Gametophytic 

generation 

Meiosis 

spore mother cells (2n) 

antherozoid (n) 

Sporophytic 

generation 

fertilization 

zygote (2n) 

egg (n) 

sporophyte (2n) 

Fig.1.40 Riccia - Life Cycle Showing 

Alternation of generations 

69


One Mark 

1. Choose the correct answer : 

1. Riccia discolor is also known as 

(a) R. cruciata (b) R. himalayensis 

(c) R. kashyapii (d) R. gangetica 

2. Ricca is mainly confined to the following places 

(a) Dry shady place (b) Aquatic area 

(c) Damp shady place (d) Hot and dry place 

3. The scales that are seen in Riccia are 

(a) Multicellular (b) Two celled 

(c) Unicellur (d) Three celled 

2. Fill in the blanks 

1. R. discolor is also called as 

2. ......................... is an aquatic form. 

3. Riccia is commonly called as ............ of plant kingdom. 

Two Marks 

1. Where can we come across Riccia? 

2. Which plant is known as the amphibian of plant kingdom? Why it is called so? 

3. Give a brief account of the kinds of Rhizoids of Riccia thallus. 

4. Where the air canals are seen? Mention the role played by them. 

5. How vegetative reproduction is taking place in Riccia (give an out-line). 

Five Marks 

1. Describe the external structure of Riccia thallus. 

2. Describe vegetative reproduction in Riccia. 

3. Give an account of the internal structure of the thallus. 

4. Give a brief account of the structure of a mature archegonium and the process 

of fertilization. 

5. Write about the sporophyte of Riccia in detail. 

Ten Marks 

1. Write an essay on the internal and external features of Riccia. 

2. Describe the structure of sex organs of Riccia with suitable diagrams. 

3. Give an account of the types of reproduction that you can come across in 

Riccia. 

4. Trace the life cycle of Riccia with suitable diagrams and illustrations. 

5. What do you mean by heteromorphic alternation of generations? Explain this 

phenomenon with any one of the forms studied by you. 

70

2.6. Pteridophytes 

This division includes club mosses, horsetails and ferns. The oldest known 

pteridophytes are fossils from the end of the silurian period, 380 million years ago. 

Pteridophyta constitutes the earliest known vascular plants. Vascular plants are 

those plants that contain the vascular tissue that is the conducting tissues of xylem 

and phloem. Sometimes all vascular plants are included in one division the 

Tracheophyta . This is to emphasise the advance nature of vascular tissue over 

the simple conducting cells of some Bryophytes and Algae. Tracheophyta includes 

pteridophytes and the more advanced spermatophytes (seed bearing plants) 

as two subdivisions. 

Presence of vascular tissue is a feature of the sporophyte generation, which 

in the bryophytes is small and dependent on the gametophyte. The occurrence of 

vascular tissue in the the sporophyte is one reason why sporophyte generation has 

become the dominant one in all vascular plants. The vascular tissue of pteridophytes 

shows certain primitive features compared with flowering plants. The xylem of 

pteridophytes contains only tracheids rather than vessels and the phloem contains 

sieve cells rather than sieve tubes. 

Vascular tissue has two important roles to perform. Firstly it forms a transport 

system, conducting water and food around the multi- cellular body, thus leading to 

the development of large, complex bodies. Secondly, xylem, one of the vascular 

tissues, supports these large bodies since xylem contains lignified cells of great 

strength and rigidity. 

Salient features of Pteridophytes 

Pteridophytes are the vascular Cryptogams. They are seedless and they 

are the simplest plants among the Tracheophytes (Plants having vascular tissues). 

Pteridophytes were world wide in distribution and abundant in the geological past. 

Today, they are best represented by the ferns. The non-fern pteridophytes are 

comparatively less in number. These plants are mostly small and herbaceous. 

They grow well in moist, cool and shady places where water is available. 

Distinguishing characters of Pteridophytes 

1. The life cycle shows distinct heteromorphic alternation of generation. 

2. Plant body of Sporophyte is dominant phase. 

3. Sporophyte is differentiated into true root, stem and leaves. 

4. Vascular tissue i.e xylem and phloem are present. Xylem lacks vessels but 

tracheids are present. In phloem sieve tubes and companion cells are absent. 

5. Asexual reproduction takes place by spores. 

71

6. Most pteridophytes are homosporous i.e they produce one type of spores. 

A few show heterospory i.e they produce two types of spores 

microspores and megaspores. 

7. Spores are produced from spore mother cells after meiosis in multi-cellular 

sporangia. 

8. Sporangia bearing leaves are called sporophylls. 

9. Spores on germination develop into gametophyte which is haploid, multicellular, 

green and an 

independent structure. 

10. The gametophyte 

develops multi-cellular 

sex organs. The male 

sex organ is called 

antheridium and the 

female sex organ is called 

archegonium. 

11. Sex organs have a sterile 

jacket. 

12. Antherozoids are spirally 

coiled and multiflagellate. 

13. Fertilization takes place 

i n s i d e 

archegonium. 

14. Opening of 

sex organs 

and transfer 

of male 

gametes to 

archegonium 

for fertilization 

are dependent 

on water. 

15. Fertilized egg 

i.e zygote 

develops into 

embryo. 

Pteris 

Fig: 1.41. Microphyllous Pteridophyte - Selaginella 

Young leaf 

with circinnate 

vernation 

Rhizome 

Leaflet 

M other 

plant 

Some common 

Fig: 1.42. Ferns 

examples of microphyllous pteridophytes are Psilotum, Lycopodium, Selaginella, 

Isoetes, Equisetum etc. 

72 

Rhizophore 

Adiantum 

Daughter 

plant 

Pinna 

Leaf 

Rhizoides 

Rhizome 

A dventitious roots 

Dryopteris 

Strobilus 

Young leaf 

with circinnate 

vernation

Ferns represent a more specialized group of higher pteridophytes with larger 

leaves (megaphyllous). They are world wide in distribution and grow luxuriantly 

in forests, mountains, valleys etc. Some common examples of ferns are 

Nephrolepis, Ophioglossum, Osmunda, Pteris, Adiantum, Marsilea, Azolla, 

Salvinia etc. 

Characteristics of Pteridophytes 

Heterospory 

M icrosporophyll 

In some 

pteridophytes the 

gametophyte is 

protected by 

remaining in the spores 

of the previous 

sporophyte generation. 

In such cases there 

are two types of spore 

and the plants are 

therefore described as 

heterosporous. 

Plants producing only 

one type of spore, like 

Megasporophyll 

Megasporangium 

Megaspores 

M icrospores 

M icrosporangium 

Male 

Prothallus 

produces sperm 

Female 

prothallus 

produces ova 

Fig : 1.43. Diagramatic representation of heterospory 

the Bryophytes, are described as homosporous. 

In heterosporous plants two types of spores are produced. 1. large spores 

called megaspores and 2. small spores called mircrospores. Megaspores give 

rise to female gametophytes (prothalli). Female gametophyte bears the female 

sex organs namely archegonia. The microspores give rise to male gametophytes 

(Prothalli). This bears the male sex organs namely antheridia. Sperms (antherozoids) 

produced by the antheridia travel to the female sex organ namely archegonium 

found in female gametophyte. Both male and female gametophytes remain protected 

inside their respective spores. The microspore is small and they are produced in 

large numbers and they are dispersed by wind from the parent sporophyte, the 

male gametophyte that the microspore contains within is therefore dispersed with 

it. The evolution of heterospory is an important step towards the evolution of seed 

bearing plants. 

Economic importance of pteridophytes 

1. Ferns are grown as ornamental plants for their beautiful fronds. 

2. The rhizomes and petioles of the fern Dryopteris yield a vermifuge drug. 

3. The sporocarps of Marsilea ( a water fern) are used as food by certain 

tribal people. 

Leaf 

Stem 

73


One Mark 


1. The process of evolution of the seed habit is associated with the origin of 

_________ 

2. The dominant phase changed from________ to __________ as in all 

Pteridophytes,Gymnosperms and Angiosperms. 

Two Marks 

1. What is meant by Tracheophyta? 

2. Justify: the vascular tissue of pteridophyte is primitive when compared with 

flowering plants. 

3. What are the functions of vascular tissue? 

4. What are the advantages of seed development in Phaenerogams? 

5. Name any two economically important products of Pteridophytes. 

Five Marks 

1. What are the salient features of Pteridophytes? 

2. What is heterospory? What is it’s significance? 

Ten Marks 

1. List the strategies that the plants had to develop in order to survive on land. 

74

2.6.1 Nephrolepsis 

Division : Tracheophyta 

Sub division : Pteropsida 

Class : Leptosporangiatae 

Order : Filicales 

Family : Dennstaedtiaceae 

Genus : Nephrolepsis 

The genus Nephrolepsis is a tropical fern with 

about 30 species. Most of the species are found in 

terrestrial habitats. Some species are epiphytes e.g., 

N. volubilis and N. ramosa. Several species are 

grown as ornamental plants. In India, there are 5 

species. Of these, N. acuta and N. tuberosa are 

common species. 

Morphology of sporophyte 

leaves. 

The sporophyte consists of rhizome, roots and 

Rhizome : The rhizome is short and erect or suberect, 

producing elongated slender stolons. Some 

species have creeping rhizome with adpressed scales. 

The rhizome of N. tuberosa bears tubers which act 

as reservoirs for carbohydrates and water. The rhizome 

is covered with scales. 

Root : The roots arise from the thizome and stolon. 

The roots are adventitious and branched. 

Leaves : The leaves are long, narrow and 

herbaceous. They are pinnately compound and 

their length varies from 40 cm to 70 cm or more. 

The pinnae are sessile, subsessile or shortly 

petioled. They have usually rounded or cordate 

base. The veins are prominent and the veinlets 

are branched with open ends. The tips of veinlets 

are gland dotted and they extend upto the margins. 

75 

Fig.1.44. A-D Nephrolepsis 

tuberosa 

A, a leaf. B, a pinna, C. Soras, 

D. Sporangium (after beddome)

The petiloe, rachis and pinnae are covered with 

multicellular brown haris or scales 

called ramenta. 

Anatomy 

Rhizome: The Rhizome is 

differentiated into epidermis, 

hypodermis, ground tissue and stele. 

The stele is a meristele (Fig) A 

meristele is a part of dictyostele found 

between two neighbouring leaf gaps 

and appear as separate strand in a 

transverse section. A dictyostele is a 

solenostele with leaf gaps and distinct 

vascular strands. A solenostele is a Fig.1.45. TS of Rhizome 

condition when a mass of parenchyma 

cells found in the centre of the xylem. The epidermis is the protective layer with a 

thick layer of cuticle. The hypodermis is more or less continuous and heavily 

sclerotic. This region is followed by parenchymatous ground tissue with starch 

grains. 

The stele structure varies within the same rhizome. A mature rhizome with 

many leaves has dictyostele which 

gets separated into a number of 

strands called meristeles. Each 

meristele is surrounded by its own 

endodermis which is followed by 

pericycle. The pericycle is followed 

by phloem. The central region of 

stele is occupied by xylem. 

Root: The transverse section of 

root has three distinct parts - 

epiblema, cortex and vascular 

cylinder (fig). The epiblema is the 

outermost layer of thin walled cells. 

Some cells of this region produce 

unicellular root hairs. The cortex is Fig.1.46. TS of Roots 

divided into outer parenchymatous and inner sclerenchymatous regions. The latter 

provides machanical support to roots. The innermost region of cortex has 

endodermis. Next to this layer is pericycle. 

76

The vascular cylinder is diarch 

and exarch. A diarch condition 

consists of two protoxylem points. 

An exarch condition refers to 

presence of protoxylem away from 

the centre of the axis. 

Rachis or Petiole: The 

transverse section of rachis has 

epidermis, hypodermis, ground 

tissue and stele (fig). The epidermis 

consists of a single layer of cells with 

cuticle. This layer is followed by 2- 

Fig.1.47. TS of Rachis 

3 layered sclerenchymatous 

hypodermis. Next to this is a broad zone of parenchymatous ground tissue. At the 

centre, a ‘U’ shaped meristele is located. This stele is similar to that of rhizome. 

Pinna: The internal structure resembles a dicot leaf. The upper and lower 

epidermis are single layered. The outer walls of both epidermis have thick cuticle. 

The mesophyll region is differentiated into a columnar palisade parenchyma and 

loosely arranged spongy parenchyma cells. A concentric vascular bundle is found 

in the centre with a distinct bundle sheath. 

Reproduction 

Vegetative reproduction is by death and decay of the underground rhizome. 

The rhizome is dichotomously branched and grows indefinitely. When the death 

and decay of rhizome reaches up 

to the point of dichotomy, both 

the branches separate and each 

grows into a new plant. 

Reproduction by spores: 

Sori, formed on the lower side 

of the mature pinnae, are 

arranged in two rows; one on 

either side of the midvein. The 

sori are groups of sporengia. 

They are superficial in origin and 

arise at the tips of veinlets. They 

are distinct and maintain their Fig.1.48. Structure of mature sporangium 

individuality. Some species of 

Nephrolepis show fusion of adjacent sori. Each sorus has an indusium which covers 

the sorus. The indusium is reniform i.e., kidney-shaped, roundish or sub-orbicular. 

77

Each sporangium is mounted on a long stender stalk. The annulus consists of 

thick-walled cells extending from the base to almost three-fourth of the capsule. 

The distal end of annulus has a strip of thin-walled cells. This region is called 

stomium. Each sporangium produces 32-64 haploid spores after meiotic division 

of spore mother cells. 

At maturity, the annulus tears the sporangial wall from the stomium and turns 

backward. This kind of dehiscence leads to the release of spores. 

Gametophyte (Prothallus) 

Upon germination, each haploid spore develops into a multicellular 

chlorophyllous filament. The filament further develops into a flat, green coloured 

and more or less heart-shaped prothallus or gametophyte. A mature prothallus is 

3-8mm in diameter with rhizoids which anchor the prothallus in the soil. 

Fig.1.49. Structure of mature prothallus 

The male sexorgan (Antheridium) and female sexorgan (Archegonium) are 

produced in the prothallus. Antheridia are found in the basal central region and 

archegonia are found near the apical notch of the prothallus. 

Each mature antheridium produces 30-40 multiflagellate male gametes or 

antherozoids. 

A. B. 

Fig.1.50 Sex organs A. Mature Antheridium B.Mature Archegonium 

78

Each mature archegonium is differentiated into neck which is composed of 

four vertical rows of cells and a basal venter. The venter contains a single large 

ovum or egg. 

Fertilization: Water is essential for fertilization. After the release of 

antherozoids from antheridium, they swim in a thin film of water present on the 

surface of the prothallus. They are attracted towards the neck of archegonia by 

chemicals oozing out of the neck. Thus the antherozoids are directed towards the 

egg. Even though many antherozoids enter the neck only one fuses with the egg 

and forms zygote. 

The zygote, 

formed by the fusion 

of antherozoid and 

egg, is the starting 

point for the next 

sporophyte 

generation. It 

increases in size and 

almost completely 

occupies the venter. 

By repeated 

divisions, the zygote 

develops into an 

embryo consisting of 

shoot apex, 

cotyledon, foot and 

root. The foot 

absorbs nutrients for 

the developing 

Fig.1.51. Structure of embryo 

egg (n) 

Archegonium (n) 

Embryo (2n) 

Zygote (2n) 

Antherizoid (n) 

79 

Nephrolepis 

(sporophyte, 2n) 

Antheridium (n) 

Prothallus 

(gametophyte - n) 

Sporophyte 

generation 

Gametophyte 

Generation 

Sporophyll with sori (2n) 

Sporangium (2n) 

Spore mother cell (2n) 

Meiosis 

Spore tetrad (n) 

Spores (n) 

Spore germination 

Fig.1.52. Life Cycle - Nephrolepsis

embryo from the prothallus. The venter forms a protective cover called calyptra 

for the developing embryo. The root and cotyledon grow more rapidly than the 

shoot and finally form a new sporophyte plant. The prothallus gradually decays 

once the young sporophyte is well established. 


One Mark 


1. Gameophyte of Nephrolepis is otherwise known as 

a) Antheridium b) Male gamete c) Prothallus d) Antherozoid 

2. When a mass of parenchyma found in the centre xylem, the stele is called 

a) Dictyostele b) Solenostele c) meristele d) actinostele 

3. Multicellular prawn hairs or scales found on the rachis are called 

a) scale leaves b) ramenta c) vernation d) cirinat 


1. A cluster of sporangia is known as ............ 

2. The outermost cell layer that covers root is called ....................... 

3. The egg of Nephrolepis gametophyte is found in the.................... of 

archegonium. 

4. The gametophyte of Nephrolepis is attached to the soil with the help of 

Two Marks 

1. What is meant by diarch vascular cylinder? 

2. What is meant by exarch vascular cylinder? 

3. What is meant by meristele? 

Five Marks 

1. Draw the diagram of the sporangium of Nephrolepis. 

2. Describe the structure of antheridium. 

3. Describe the structure of archegonium. 

4. Explain what is meant by alternation of generation. 

Ten Marks 

1. Describe the structure of prothallus. 

2. Describe the lifecycle of Nephrolepis. 

3. Describe the transverse section of rhizome. 

4. Describe the transverse section of rhizome. 

5. Describe how a new sporophyte emerges from zygote. 

80

2.7 Spermatophytes (Gymnosperms) 

The most successful and advanced group of land plants are the 

spermatophytes (sperma – seed). One of the main problems that had to be 

faced by plants living on land was the vulnarability of their gametophyte generation. 

For example in ferns the gametophyte is a delicate prothallus and it produces the 

male gametes (sperms) which are dependent on water for swimming to reach the 

female gamete in archegonia. In seed plants, however, the gametophyte generation 

is protected and very much reduced. Three important developments have been 

made by seed plants. 1. The development of heterospory. 2. The 

development of seeds. 3. The development of non-swimming male gametes. 

Classification and Characteristic of Spermatophytes 

Division : Spermatophyta (seed bearing plants) 

General Characteristics 

Heterosporous - microscope 

= pollen grain, megaspore = 

embryo sac. The embryo sac 

remains completely enclosed in 

the ovule ; a fertilized ovule is a 

seed. Sporophyte is the dominant 

generation, gametophyte is very 

much reduced. Water is not 

needed for sexual reproduction 

because male gametes do not 

swim, complex vascular tissues 

in roots, stem and leaves are 

present. It includes two classes 

namely Gymnospermae and 

angiospermae. 

GYMNOSPERMS 

Salient features of Gymnosperms 

Gymnosperms represent a primitive group of seed bearing plant 

(Spermotophytes) in which the seeds are naked i.e. they are not covered by the 

fruit wall as in Angiosperms (the word Gymnos means naked and spermos means 

seed). This is because in Gymnosperms the ovules are exposed and they are not 

covered by ovary. Instead the ovules are borne directly on open carpellary leaves 

81 

Fig.1.53. Seeds of Gymnosperm and 

Angiosperm compared

called megasporophylls and hence they are naked and they develop into naked 

seeds after fertilization. 

Table : 1.5. Differences between class Gymnospermae 

and Angiospermae 

Class Gymnospermae 

(Cycads Conifers, and 

Ginkgos) 

1. No vessels in xylem, only 

tracheids(except Gnetales) no 

companion cells in phloem. 

2. Usually have cones on 

which sporangia and spores 

develop. 

3. Seeds are naked that is the 

seeds are exposed; they are 

not enclosed in ovary. 

Class Angiospermae(flowering 

plants) 

xylem has vessels, phloem 

contains companion cells 

Produce flowers in which 

sporangia and spores develop 

Seeds are enclosed in ovary. 

4. No fruit because no ovary After fertilization ovary 

develops into fruit. 

Gymnosperms were 

most abundant during the 

Mesozoic era (225 million 

years) ago. However, 

they form only a small 

part of the present day 

vegetation. There are 

about 70 genera and 900 

species of gymnosperms 

distributed in tropical and 

temperate regions. Most 

of them are Conifers 

mostly evergreen, with 

needle like leaves. They 

are found in the form of 

coniferous forests in the 

Himalayas in the Indian sub-continent. The common conifers are species of pine, 

fir, spruce,Cedar,Cupressus, Sequoia gigantia, (red wood tree which measures 

more than 100 meters in height). 

Distinguishing features of Gymnosperms 

1. Gymnosperms are woody perennial which are mainly trees and rarely shrubs. 

2. The life cycle of gymnosperms shows heteromorphic alternation of 

generations. 

3. They form an 

intermediate group 

between pteridophytes 

and Angiosperms i.e they 

are more advanced than 

pteridophytes but are 

primitive than 

angiosperms. 

4. The plant body is the 

sporophyte (diploid) 

mostly a tree with well 

developed roots, stem 

Fig.1.54. Gymnosperms 

and leaves. 

82

5. The sporophyte bears two types of fertile leaves, the microsporophyll that 

produces microspores and megasporophyll that produces megaspores. 

6. Mostly the spores are grouped into compact cones or strobili. 

7. Spores on germination develop into gametophytes which are very much reduced, 

microscopic and dependent on sporophyte. 

8. Ovules are naked. 

9. Pollination is mostly by wind (anemophilous). 

10. Fertilization involves only one fusion. Female gametophyte provides nutrition 

to the developing embryo. The endosperm (female gametophyte) is a prefertilization 

tissue and is haploid.sac) and the embryo (of the next sporophyte 

generation). All the nutrients for life are supplied 

11. Seeds are naked and not embedded in fruit. 

12. Vessels are absent in xylem (except Gnetales) 

Classification of Gymnosperms 

Chamberlain has classified gymnosperms into two classes 1. class 

Cycadophyta 2. Class Coniferophyta .The class Cycadophyta consists of plants 

with simple stem, thick cortex but thin wood and simple sporophylls. The class 

Coniferophyta consists of plants with profusely branched stem, thin cortex, thick 

wood and complex sporophylls. 

Economic importance of Gymnosperms 

1. Woods of many conifers are used in the manufacture of paper. eg. Pinus. 

Conifers are the source of soft wood for construction, packing and ply 

wood industry eg. Cedrus, Agathis 

2. Turpentine is obtained from the resin of Pinus. It is used as solvent in paint 

and polishes. It is also used medicinally for pain, bronchitis etc. 

3. Seeds of Pinus gerardiana are edible. 

4. Ephedrine is an alkaloid obtained from Ephedra. It is used in curing asthma 

and respiratory problems. 

5. Saw dust of conifers is used in making linoleum and plastics. 

6. Pinus species yield a resin called rosin which is used in water proofing and 

sealing joints. 

7. Araucaria is an ornamental plant. 

83

Ovum 

(n) 

Spores (n) 

Meiosis 

Capsule 

(2n) 

Pteridophyte (Homosporous) 

Zygote 

(2n) 

Fertilization 

Sporophyte 

2n 

Sperm 

(n) 

Archegonia Antheridia 

(n) (n) 

Gametophyte 

(Prothallus) 

(n) 

(2.a) 

Bryophyte 

G ametophyte 

(n) 

AntheridiaArchegonia 

(n) (n) 

Sperm 

(n ) 

Sorus (2n) 

Meiosis 

Sporangia 

(2n ) 

Spores 

(n ) 

Ovum 

(n ) 

Zygote 

(2n) 

Fertilization 

Ovum 

(n) 

Sporophyte 

2n 

Sperm 

(n) 

Archegonia Antheridia 

(n) (n) 

Embryo 

(2n) 

Zygote 

(2n) 

Fertilization 

Pteridophyte (Heterosporous) 

O 

(2.b) 

Sporophyte 

2n 

Meiosis 

M icro (2n) 

Sporangia 

M icro 

Spores 

(n) 

Gametophyte(n) 

O Gametophyte 

+ 

(n) 

Gymnosperm 

Sporophytic 

Generation 

Gametophytic 

Generation 

Meiosis 

Mega (2n) 

Sporangia 

Mega 

Spores (n) 

Male cone 

(2n) 

Female cone 

Microsporophyll (2n) 

(2n) 


(2n) 

Microsporangium 

(2n) 


(2n) 

Microspore 

mother cell Megaspore 

(2n) 

mother cell 

(2n) 

Meiosis 

Meiosis 

Zygote 

(2n ) 

Fertilization 

84 

Egg (n) Antherozoid (n) 

Microspore (n) Megaspore (n) 

Male 

gametophyte (n) 

Female 

gametophyte (n) 

Antheridium (n) 

Achegonium Archegonium (n) (n) 

(1) (3) 

Fig.1.55. Graphical representation of life cycles of various plant groups


One Mark 


1. The most successful and advanced group of land plants are____________ 

2. All seed plants are ________ 

3. The most extreme reduction of gametophyte has taken place in__________. 

4. The equivalent structure to a megasporangium, in a seed plant is called an 

__________. 

5. The equivalent structure to a microsporangium, in a seed plant is called 

_________ 

Two Marks 

1. Name the three important developments that have been made by the seed 

plants. 

2. Define heterospory. 

3. Justify the statement: a seed is a complex structure containing cells from three 

generations. 

4. Why do we call the seeds of gymnosperms as naked? 

5. Name the two classes of Gymnospermae. 

Five Marks 

1. Discuss the advantages associated with seed habit. 

2. List the differences between Gymnospermae and Angiospermae. 

3. Write the salient features of Gymnosperms. 

4. Write about the economic importance of gymnosperms. 

85

2.7.1 Cycas 

Division : Cycadophyta 

Class : Cycadopsida 

Order : Cycadales 

Family : Cycadaceae 

Genus : Cycas 

Gymnosperms are plants which produce naked seeds i.e., plants which lack ovary 

and hence do not produce fruits. Cycas belongs to this group of plants. 

The genus cycas is the most widely distributed genus of the order cycadales. 

There are about 20 species which grow in the wilderness in China, Japan, Australia, 

Africa, Nepal, Bangladesh, Burma and India. C. circinalis, C. pectinata, C. rumphii 

and C. beddomei, are found in the wilderness in India. C. revoluta is grown in 

gardens in India. 

Species of Cycas are of considerable economic importance. Starch is extracted 

from several species of cycas. Young succulent leaves are used as vegetable in some 

parts of India. Several species of 

cycas are of medicinal value. The 

juice of young leaves of C. 

circinalis is used as a remedy for 

stomach disorders, flatulence, blood 

vomiting and skin diseases. The 

decoction of young seeds of this 

species is purgative and emetic. A 

tincture prepared from the seeds 

of C. revoluta is used to relieve 

headache, giddiness and sore 

throat. 

Morphology of sporophyte: 

Cycas is an evergreen slowgrowing 

palm-like small tree with 

an average height of 1.5 to 3 

meters (fig). It is commonly found 

in dry habitats. It also grows well 

in gardens of tropical countries Fig. 1.56. Cycas - Habit 

including India. The sporophyte is 

differentiated into roots, stem, and leaves. 

Roots: There are two types of roots in cycas 1) Normal roots, 2) Negatively 

geotropic roots called coralloid roots. 

86

Normal roots: The long-lived primary root is usually thick and short but the 

lateral roots are thin and long. These roots are positively geotropic. Their main functions 

are anchorage and absorption of water and mineral 

nutrients. 

Coralloid 

roots: These 

Fig. 1.57. Coralloid roots 

roots are 

negatively 

geotropic and 

grow on the 

surface of the 

soil. They are dichotomously branched and 

appear as coralline masses (fig). A specific 

algal zone with colonies of Anabaena or 

other blue green algae is present in the cortex 

of these roots. The algal cells may help in 

N 2 

fixation. These roots respire through 

special openings called lenticels. 

Stem: The young stem is tuberous Fig. 1.58. A part of mature stem 

a n d 

subterranean and its apical part is covered with 

brown scale leaves. The old stem is thick, columnar 

and woody. It is covered with persistent and woody 

leaf bases. The stem is usually unbranched, but 

sometimes due to shoot tip injury, the stem branches 

dichotomously. 

Leaves: Cycas has dimorphic leaves namely 

1) Foliage or assimilatory leaves and 2) Scale leaves. 

i) Foliage or assimilatory leaves 

Large, pinnately compound (fig) foliage leaves 

form a crown at the top of the stem. Each leaf has 

80-100 pairs of leaflets. They are arranged on both 

sides of the rachis in opposite or alternate manner. 

The leaflets are sessile, elongated and ovate or 

lanceolate with flat or revolute margins. The tip of 

each leaflet is acute or spiny. Each leaflet has a 

single midvein. Lateral veins are absent. 

Fig. 1.59. Cycas Leaf-let 

A, Compound leaf. B.Upper portion of a leaf-let 

C. Young leaf showing circinate vernation of leaf-lets 

87

The rachis of a very young leaf is circinate with circinately coiled leaflets like 

those of ferns. 

ii) Scale leaves 

These are small, rough, dry and triangular in shape. They protect the shoot apex 

and other aerial parts. They do not produce starch by photosynthesis. The foliage and 

scale leaves are arranged in close alternate whorls at the apex of the stem. 

Anatomy 

Normal root: A cross section of normal root (fig) consists of epiblema, cortex 

and central vascular tissue. 

Epiblema: It is composed of a single layer of thin-walled cells. 

(a) 

(b) 

Fig. 1.60. Cycas - Normal young root 

(A) Ground plan TS (B) A portion enlarged 

Cortex: It is a multilayered zone of thin-walled parenchymatous cells. These 

cells are filled with starch. Tannin cells and mucilage cells are also present in the 

cortex. The innermost layer of the cortex is endodermis. Pericycle is a multi-layered 

zone found next to endodermis. 

Vascular tissue: This tissue forms a central diarch stele. The diarch steel refers 

to the presence of two patches of protoxylem points. The xylem consists of xylem 

tracheids. A tracheid is one celled, non-living, elongated xylem element with thick 

88

lignified and pitted cell walls. The xylem is exarch i.e., the protoxylem is pointing towards 

the periphery while the metaxylem is located near the centre of roots. The pith is either 

reduced or completely absent. 

The normal roots exhibit 

secondary growth, which starts by the 

formation of cambium strips that are 

formed inner to the primary phloem 

strands (fig). These cambium strips cut 

off secondary phloem towards the 

outer side and secondary xylem 

towards the inner side. Due to the 

development of secondary structures 

the primary phloem is crushed while 

the primary xylem is found in the 

centre. A distinct layer of cork 

cambium (phellogen) arisis in the outer 

region of cortex which gives rise to 

cork (phellem) on its outer side and 

secondary cortex (phelloderm) on its 

inner side. Cork, cork cambium and 

cork cortex or secondary cortex are 

collectively known as periderm. 

Coralloid roots: The internal 

structure of coralloid roots is similar to 

that of normal roots except in certain 

respects. The cortex of coralloid root 

is differentiated into i) outer cortex 

composed of polygonal cells, ii) inner 

cortex consisted of thin-walled 

parenchymatous cells and iii) middle 

cortex made up of a single layer of 

loosely connected thin-walled and 

radially elongated cells with blue green 

algal forms such as Anabaena or 

Nostoc. Coralloid roots show little or 

no secondary growth. 

Fig.1.61 Coralloid root 

A. Ground plan TS B. A portion enlarged 

Stem : The stem is irregular in outline due to the presence of numerous persistent 

leaf bases. Its internal structure is similar to that of dicots of Angiosperms. Young stem 

of cycas is differentiated into epidermis, cortex and vascular cylinder (fig). The epidermis 

89

is the outermost layer of stem covered with a thick cuticle. Cortex forms the major 

part of the stem. It is composed of parenchymatous cells with rich starch grains. The 

cortex is traversed by several 

mucilagenous canals and many leaf 

traces. The inner most layer of cortex 

is endodermis which is followed by 

pericycle. However, these two regions 

are not distinctly seen. 

In the young stem, vascular 

region is very small when compared 

to the cortical zone. There are several 

vascular bundles arranged in a ring. 

The vascular bundles are conjoint, 

collateral, endarch and open. The 

individual bundles are separated by 

parenchymatous medullary rays. The 

xylem consists of tracheids and 

paraenchyma. Xylem vessels are Fig.1.62 Young Stem TS 

absent. The phloem consists of sieve 

tubes and phloem parenchyma. There are no companion cells. 

There is a parenchymatous pith present in the centre of the stem. The pith cells 

are rich in starch and some cells contain tannin and mucilagenous substances. 

Secondary growth i.e., the formation of secondary xylem and secondary phloem 

from cambium as found in dicot stems, is observed in old stems of cycas. In addition to 

secondary xylem and secondary phloem, the cambium also forms parenchymatous 

medullary rays. A well developed stem of cycas is called manoxylic because the wood 

is not compact due to well developed pith, cortex and broad medullary rays with limited 

vasculature. 

Rachis : Transverse section of rachis is more or less circular in outline. It has two 

rows of leaflets inserted on one side. The internal structure is differentiated into epidermis, 

hypodermis, ground tissue and vascular tissue (fig). 

Epidermis is covered with a thick cuticle. The epidermis is interrupted by sunken 

stomata. The epidermis is followed by hypodermis. The hypodermis consists of outer 

thin-walled chlorenchymatous cells (2-3 layers) and the inner thick walled 

sclerenchymatous cells (4-5 layers). The ground tissue consists of parenchymatous 

cells with mucilage canals. 

90

The vascular bundles are arranged in an inverted omega-shaped manner. The 

bundles are conjoint, collateral, open and diploxylic. Diploxylic condition refers to the 

presence of centrifugal and centripetal xylem. 

Leaflet : Transverse section of cycas leaflet shows the following tissues i) upper 

and lower epidermis, ii) hypodermis, iii) mesophyll, iv) transfusion tissue and v) vascular 

bundles. 

i) The upper and lower epidermis are the outermost cellular layers (one celled 

thick) of the upper and lower sides respectively of the leaflets. Both of them are 

covered by thick cuticle. The upper epidermis is continuous, whereas the lower 

epidermis is interrupted by sunken stomata. 

ii) Hypodermis : This layer is made up of sclerenchymatous cells. The hypodermal 

layer protects the plant from over-heating and excessive transpiration. 

iii) 

iv) 

Fig.1.63. Rachis A. Ground plan B. A portion enlarged 

Mesophyll : This tissue consists of palisade and spongy parenchyma cells. The 

palisade layer is a single continuous layer of column-like cells. The spongy 

parenchyma consists of several layers of loosely arranged oval or irregular cells. 

Both pallisade and spongy parenchyma cells are rich in chloroplasts. 

Transfusion tissue : This tissue consists of two small groups of short and wide 

tracheid-like cells with thickenings / pits on their walls. A few layers of transversely 

91

elongated cells are present in both the wings between palisade and spongy 

parenchyma cells. These layers are called accessory transfusion tissue or 

secondary transfusion tissue. 

v) Vascular bundle : There is only one vascular bundle present in the midrib region 

of the leaflet. It is conjoint, collateral, open and diploxylic. The triangular centrifugal 

xylem is well-developed with endarch protoxylem. Phloem is arc-shaped and 

remains separated by cambium. Phloem consists of sieve tubes and phloem 

parenchyma. Companion cells are absent. 

A 

B 

Fig. 1.64. Leaf-let A. TS Groun plan. B. TS of a portion through midrib 

92

Reproduction : Cycas reproduces by vegetative and sexual means. 

Vegetative reproduction 

Vegetative reproduction is by the formation of adventitious buds or bulbils . 

The bulbils develop from the basal part of stem especially from cortical cells. They are 

found between the persistent leaf bases. They are more or less oval shaped. Several 

scale leaves are arranged spirally and compactly over a dormant stem in a bulbil. Upon 

detachment from the stem, a bulbil germinates to produce a new plant. A bulbil from 

male plant produces a new male plant while a bulbil from female plant produces a new 

female plant. 

Sexual Reproduction : Cycas is 

strictly dioecious ie., male and female 

plants are distinctly different from each 

other. . 

The male plant of Cycas produces 

male strobilus (cone) at the apex of 

the stem in between the crown of foliage 

leaves. Each male cone is a shortly 

stalked compact, oval or conical woody 

structure. It is 40-80 cm in length, perhaps 

the largest among plants. Each male cone 

consists of several microsporophylls 

which are arranged spirally around a 

central axis. Each microsporophyll is a 

woody, brown coloured and more or less 

horizontally flattened structure with a 

narrow base and an expanded upper 

portion. The upper part is expanded and 

becomes pointed at its tip. The narrow 

basal part is attached to the cone axis. 

Each microsporophyll contains an 

Fig.1.65. Microsporophyll 

upper (adaxial) and a lower (abaxial) A. Entire, cone B. 

surface. Thousands of microporangia are longitudinal section 

present in the middle region of the lower 

surface in the form of groups of microsporangia. Each such group is called sorus with 

3-5 microsporangia. 

In the transverse section of a microsporophyll there are several shortly-stalked 

oval or sac-like microsporangia. Each microsporangium is covered by 3 distinct layers 

93

of cells. Pollen grains or microspores are produced at the end of meiotic division of 

microspore mother cells found in the microsporangium. 

Fig.1.65. Microsporophyll A. Adaxial surface, B. Abaxial surface C. Sori, 

D. Microsprophyll TS 

Male gametophyte 

Each microspore on pollen develops into male gametophyte partly even before 

the release of pollens from microsporangium. The transfer of pollens from male plant 

to the female plant is called pollination. At this stage, the male gametophyte has a 

prothallial cell, a generative cell and a tube cell. Dispersal of pollens is effected by wind 

94

(anemophyllous). Further 

development of male 

gametophyte starts only 

after the pollen reaches 

nucellar surface of the ovule 

where the pollen 

germinates to produce 

pollen tube. The pollen tube 

carries two top-shaped 

sperms. Each sperm 

contains thousands of cilia 

Fig.1.67 Pollen grain 

Fig.1.68 Pollen tube 

with male gametes 

. By means cilia, the sperms move freely in the pollen tube. 

The pollen tube. The pollen tube penetrates the nucellar region of the ovule and 

subsequently delivers the male gametes into the archegonial chamber. 

The female plant produces megasporophylls that are not organised into cones and 

instead they occur in close spirals in acropetal succession around the stem apex (fig). 

New megasporophylls are produced in large numbers every year. The megasporophylls 

of a year occupy the region between the successive whorls of leaves. The growth of 

the female plant is monopodial; the axis contines to grow as it produces foliage leaves 

and megasporophylls. 

A 

B 

Fig.1.69 Cycas 

A. Cluster of megasporophylls at the apex of the stem B. Single megasporophyll 

95

The megasporophylls 

are considered to be 

modified leaves. They are 

flat, dorsiventral and 

measuring 15-30 cm in length 

A megasporophyll is 

differentiated into a basal 

stalk and an upper pinnate 

lamina. Ovules are formed 

on the lateral sids of the 

stalk. The number of ovules 

per megasporophyll varies 

from 2-10 depending upon 

the species. 

Ovule : The ovule of 

cycas is orthotropous and 

unitegmic. It is sessile or 

shortly stalked and perhaps Fig.1.70. Structure of ovule LS 

the largest ovule (about 6 

cm length and 4 cm width) in the plant kingdom. 

Each ovule consists of a large nucellus surrounded by a single integument. 

The integument remains fused with the body of the ovule except at the apex of the 

nucellus where it forms a nucellar beak and an opening called micropyle. The 

opposite end of the microphyle is called chalaza. The integument is very thick and 

is differentiated into three layers - the outer and inner fleshy layers and a hard and 

stony middle layer. Some cells in the nucellar beak dissolve to form a pollen chamber. 

The young ovule is green and hairy whereas the mature one is red or orange 

without hairs. 

One of the deeply situated cells in the nucellus differentiates into megaspore 

mother cell and divides meiotically to produce 4 linearly arranged haploid 

megaspores. Of the four megaspores, the upper three cells degenerate while the 

lowermost acts as functional megaspore. 

Female gametophyte : The functional megaspore develops into a large, 

haploid multicellular tissue called female prothallus or endosperm. The nucellus is 

used up as the female gametophyte develops. At this stage, some superficial cells 

of the female gametophyte at the micropylar end enlarge and develop into 2-8 

archegonia. Each archegonium has a large egg nucleus and venter canal nucleus. 

The arehegonial chamber is found above the archegonia. 

96

Fertilization : The fusion of male and female gametes is called fertilization. 

The pollen tube of the pollen releases sperms or male gametes into the archegonial 

chamber. Normally, only one male gamete enters each archegonium and fuses 

with the egg thus resulting in the formation of zygote. Only one egg, in any one of 

the archegonia, is fertilized. The diploid zygote develops into embryo. The embryo 

takes about one year for its complete development. The ovule ultimately gets 

transformed into seed. 

C ycas 

Sporophyte (2n) 

Male plant (2n) 

Female plant (2n) 

Male cone (2n) 


(2n) 

Microsporophyll 

(2n) 

Seed germination 


(2n) 

Microsporangium 

(2n) 

Seed (2n) 

Embryo (2n) 

Megaspore 

mother cell 

(2n) 

Meiosis 

Functional 

megaspore 

(n) 

Female gametophyte 

Archegonium (n) 

Microspore 

mother cell 

(2n) 

Meiosis 

Microspore 

(n) 

Zygote (2n) 

Fertilization 

Fig.1.71. Life cycle of a Cycas 

97 

egg (n) 

Sperm (n) 

Male gametophyte 

(n)

SELF-EVALUATION 

One Mark 

Choose the correct answer : 

1. A special type of root of cycas that exhibits negative geotropism is called 

a) prop root b) normal root c) coralloid root d) aerial root 

2. The following species is grown in gardens in India 

a) cycas circinalis b) c. revoluta c) c. pectinata d) c. romphii 

3. The following alga is found in collalloid roots of cycas 

a) anabaena b) ulothrix c) volvox d) chlamydomonas 


1. The innermost layer of cortex is called 

2. The outermost layer of root is called 

3. The female sex organ of cycas is called 

4. The pollen grains of cycas are dispersed by 

Two marks 

1. What is transfusion tissue? 

2. What is meant by manoxylic wood? 

3. What is meant by bulbil? 

4. What is dioecious condition? 

Five marks 

1. Describe the external structure of a microsporophyll. 

2. Describe the L.S. of male cone. 

3. Describe the structure of a pollen grain. 

4. Brief the economic importance of cycas. 

5. Describe the L.S. of seed. 

Ten marks 

1. Describe the T.S. of corralloid root. 

2. Describe the T.S. of leaflet. 

3. Describe the internal structure of stem. 

4. Describe the internal structure of microsporophyll. 

5. Describe the structure of ovule? 

98

II. CELL BIOLOGY 

1. The Cell - Basic Unit of Life 

A cell is a structural and functional unit of all living organisms. It is microscopic 

and capable of independent existence. All living things are made up of cells. The 

outward differences among the various biological forms may bewilder us. But 

underlying these differences is a powerful uniformity. That is all biological systems 

are composed of same types of molecules and they all employ similar principles of 

organization at the cellular level. We shall see for example, that all living organisms 

employ the same genetic code and a similar machinery for protein synthesis. 

Organisms contain organs, organs composed of tissues, tissues are made up 

of cells; and cells are formed of organelles and organelles are made up of molecules. 

However, in all living organisms, the cell is the functional unit.All of biology revolves 

around the activity of the cell. Loewy and Siekevitz defined cell as a unit of an 

organism delimited by a plasma membrane in animal cells and cell wall and plasma 

membrane in plant cells. Thus cell forms the basic unit of life. 

A brief history about the discovery of cells 

The study of cell is impossible without microscope. Anton van Leewenhoek 

(1632-1723) studied the structure of bacteria, protozoa spermatozoa, red blood 

cells under the simple microscope which he examined under a simple microscope 

that was designed by him. The word cell was first coined by Robert Hooke in 

1665 to designate the empty honey-comb like structures viewed in a thin section of 

bottle cork which he examined. 

In 1838, the German botanist Schleiden proposed that all plants are made up 

of plant cells. Then in 1839 his colleague, the anatomist Theodore Schwann 

studied and concluded that all animals are also composed of cells. Even at that 

time the real nature of a cell was a big question. Cell theory was again rewritten 

by Rudolf Virchow in 1858. 

Robert Brown in 1831 discovered the presence of nucleus in the cells of 

orchid roots. This was an important discovery. Purkinje coined the term protoplasm 

for the slimy substance that is found inside the cells. In the 20 th century, Various 

modern micro techniques have been employed in cytological investigation. With 

the invention of electron microscope in the year 1932 more and more information 

99

about the cell and various 

organellses of the cells 

become available to us. On 

the basis of the structure, 

the cells are classified into 

prokaryotic and 

eukaryotic cells. 

Eukaryotic cells vary 

very much in shape and size. 

The smallest cells are found 

among bacteria (0.2 to 50 

microns). The largest plant 

cell is the ovule of Cycas. 

The shape of the cells also 

varies considerably. It may 

be spherical, polygonal, oval, 

rectangular, cylindrical, 

ellipsoidal etc., 

Dynamic nature of cell 

A cell in an adult 

organism can be viewed as 

a steady - state system. The 

DNA is constantly read out 

into a particular set of 

mRNA (transcription) 

which specify a particular 

set of proteins 

(translation). As these 

proteins function they are 

being degraded and replaced 

by new ones and the system 

is so balanced that the cell 

neither grows, shrinks, nor changes its function. Considering this static view of the 

cell, however, one should not miss the all-important dynamic aspect of cellular life. 

The dynamics of a cell can be best understood by examining the course of a 

cell’s life. A new cell is formed when one cell divides or when two cells, (a sperm 

and an egg) fuse. Both these events start a cell-replication programme. This usually 

involves a period of cell growth, during which proteins are made and DNA replicated, 

followed by cell division when a cell divides into two daughter cells. Whether a 

100

Table 2.1 Differences between plant and animal cell 

Plant cell 

1. Plant cell has an outer rigid cell 

wall, m ade up of cellulose 

2. Plant cell has a distinct, definite 

shape because of the rigid cell 

wall. So, the shape of cell in 

permanent. 

3. Plant cell contains plastids. Most 

im portant of this is the green 

chloroplast. 

4. Vacuoles are fewer and larger. 

5. Centrosom e is present only in the 

cells of som e low er plants. 

Animal cell 

Cell wall is absent. Plasm a 

m embrane is the outerm ost 

covering. 

The shape of the animal cell is not so 

definite. It can change its shape. 

Plastids are absent. 

Vacuoles are either absent or very 

small in number and size. 

All the animal cells have centrosomes 

6. Dictyosome (Golgi complex) is 

dispersed throughout the 

cytoplasm. It comprises stacks of 

single membranous lamellar 

discs. 

Golgi complex is organized in the 

cytoplasm. It appears as shallow 

saucer shaped body or narrow neck 

bowl-like form. It consists of 

interconnecting tubules in distal 

region. 

7. Lysosomes are found only in the 

eukaryotic plant cells. 

8. Plant cell is larger than the anim al 

cell. 

9. M ostly, starch is the storage 

material. 

Found in all cells. 

Anim al cell is sm all in size. 

Glycogen is the storage material 

10.During cytoplasmic division a 

cell plate is formed in the centre 

of the cell. 

given cell will grow and divide is a highly regulated decision of the body, ensuring 

that adult organism replaces worn out cells or makes new cells in response to a 

new need. The best example for the latter is the growth of muscle in response to 

exercise or damage. However, in one major and devastating disease namely cancer, 

the cells multiply even though there is no need in the body. the understand how 

101 

During cytoplasmic division a furrow 

appears from the periphery to the 

centre of the cell.

cells become cancerous, biologists have intensely studied the mechanism that 

controls the growth and division of cells. 

Cell Cycle 

The cell cycle follows a regular timing mechanism. Most eukaryotic cells live 

according to an internal clock, that is they proceed through a sequence of phases, 

called cell cycle. In the cell cycle DNA is Duplicated during synthesis (S) 

Phase and the copies are distributed to daughter cells during mitotic (M) phase. 

Most growing plant and animal cells take 10-20 hours to double in number 

and some duplicate at a much slower rate. 

The most complicated example of cellular dynamics occurs during 

differentiation i.e when a cell changes to carry out a specialized function. This 

process often involves changes in the morphology of a cell based on the function 

it is to perform. This highlights the biological principle that “form follows function” 

Unchecked cell growth and multiplication produce a mass of cells, a tumor. 

Programmed Cell Death (PCD) plays a very important role by balancing cell 

growth and multiplication. In addition, cell death also eliminates unecessary cells. 

Plant cells differ from animal cells in many ways. These differences are 

tabulated in page 53. 


One Mark 


1. The process in which DNA is constantly read out into a particular set of 

mRNA is called 

a. translation b.protein synthesis c.DNA duplication d.transcription 

2. The process of changing the form in order to carry out a specialized function 

is called 

a. differentiation b.growth c.cell division d.cell elongation 

Two Marks 

1. Define:Cell cycle 

2. What is meant by cell differentiation. 

3. Explain the statement: “form follows function” 

4. What is PCD? 

Ten Marks 

1. Tabulate the differences between a plant cell and an animal cell. 

102

2. Cell Theory 

In the year (1839) Schleiden and Schwann have jointly proposed the “Cell 

Theory” It states that all living organisms are made up of cells and cells are the 

structural and functional units of all organisms. 

Development of Cell Theory 

If we study the step by step development of cell theory we will understand 

how scientific methodology operates. It includes the following steps 1.observation 

2.Hypothesis 3.Formulation of theory 4.modification of theory (if it warrants). 

Observations were made by Schleiden (1804 - 1881) a German botanist. He 

examined a large variety of plants and found that all of them were composed of 

cells. In 1838 he concluded that cells are the ultimate structural units of all plant 

tissues. 

Schwann, a German Zoologist studied many types of animals and found that 

animal cells lack a cell wall and they are covered by a membrane. He also stated 

that animal cells and plant cells were basically identical but for the cell wall. He 

observed that both contain nucleus and a clear substance around it. He defined 

the cell as a membrane bound nucleus containing structure. He proposed a 

hypothesis that the bodies of animals and plants are composed of cells and their 

products. 

Schleiden and Schwann both together discussed Schwann’s hypothesis and 

they formulated cell theory. The important aspects of cell theory are: 

1. All living organisms are made up of minute units, the cells which are the 

smallest entities that can be called living. 

2. Each cell is made up of protoplasm with a nucleus and bounded by plasma 

membrane with or without a cell wall. 

3. All cells are basically alike in their structure and metabolic activities. 

4. Function of an organism is the sum total of activities and interaction of its 

constituent cells. 

Exception to cell Theory 

1. Viruses are biologists’ puzzle. They are an exception to cell theory. They 

lack protoplasm, the essential part of the cell. 

2. Bacteria and cyanobacteria (Blue Green algae) lack well organized 

nucleus. 

3. Some of the protozons are acellular. 

103

4. The coenocytic hyphae of some fungi eg. Rhizopus have undivided mass 

of protoplasm, in which many nuclei remain scattered. 

5. Red Blood Corpuscles (RBC) and mature sieve tubes are without nuclei. 

A cell may grow, secrete, divide or die while its adjacent cells may lie in a 

different physiological state. Many of the subsequent findings about the cell like 

this had necessitated modification in cell theory. The modified form of cell theory 

has been given the higher status as cell principle or cell Doctrine. 

Cell Principle or Cell Doctrine 

The important features of cell doctrine are: 

1. All organisms are made up of cells. 

2. New cells are produced from the pre-existing cells. 

3. Cell is a structural and functional unit of all living organisms. 

4. A cell contains hereditary information which is passed on from cell to cell 

during cell division. 

5. All the cells are basically the same in chemical composition and metabolic 

activities. 

6. The structure and functionof the cell are controlled by DNA. 

7. Sometimes the dead cells may remain functional as tracheids and vessels 

in plants and horny cells in animals. 

Self evaluation 

One Mark 


1. An exception to cell theory is 

a.viruses b. bryophyte c. seed plant d. pteridophyte 


1. ............ and ............ proposed cell theory. 

2. Cells are the ............ and ............ units of life. 

3. The modified cell theory is called ............ 

Two Marks 

1. Name the steps involved in scientific methodology. 

2. State the cell theory as proposed by Schleiden and Schwann 

3. Name any two exceptions to cell theory. 

Five Marks 

1. State the important features of cell doctrine. 

Ten Marks 

1. Describe the development of cell theory. 

104

3. Prokaryotic and Eukaryotic Cell 

(Plant Cells) 

All living things found on the planet earth are divided into two major groups 

namely, prokaryotes and Eukaryotes based on the types of cells these organisms 

possess. Prokaryotic cells lack a well defined nucleus and have a simplified internal 

organization. Eukaryotic cells have a more complicated internal structure including 

a well defined, membrane - limited nucleus. Bacteria and Cyanobacteria are 

prokaryotes. Fungi, plants and animals are eukaryotes. 

Prokaryotes 

In general, Prokaryotes consist of a single closed compartment containing the 

cytosol and bounded by the plasma membrane. Although bacterial cells do not 

have a well defined nucleus, the genetic material, DNA, is condensed into the 

central region of the cell. In all prokaryotic cells, most of or all the genetic 

information resides in a single circlular DNA molecule, in the central region of the 

cell. This region is often referred ot as incipient nucleus or nucleoid. In addition, 

most ribosomes, the cell’s protein synthesizing centres are found in the DNA-free 

region of the cell. Some bacteria also have an invagination of the cell membrane 

called a mesosome, which is associated with synthesis of DNA and secretion of 

proteins. Thus we can not say that bacterial cells are completely devoid of internal 

organization. 

Bacterial cells possess a cell wall which lies adjacent to the external side of 

the plasma membrane. The cell wall is composed of layers of peptidoglycan, a 

complex of proteins and oligosaccharides. It protects the cell and maintains its 

shape. 

Some bacteria (eg E.coli) have a thin cell wall and an unusual outermembrane 

separated from the cell wall by the periplasmic space. Such bacteria are not stained 

by Gram staining technique and thus are classified as Gram-negative bacteria. 

Other bacteria (eg.Bacillus polymyxa) that have a thicker cell wall without an 

outer membrane take the Gram stain and thus are classified as Gram positive 

bacteria. 

Ultra structure of a prokaryotic cell 

The bacterium is surrounded by two definite membranes separated by the 

periplasmic space. The outer layer is rigid, serves for mechanical protection and is 

designated as the cell wall. The chemical composition of the cell wall is rather 

105

complex; it contains peptidoglycan, polysaccharides, lipid and protein molecules. 

One of the most abundant polypetides, porin, forms channels that allow for the 

diffusion of solutes. The plasma membrane is a lipoprotein structure serving as a 

molecular barrier with the surrounding medium. the plasma membrane controls 

the entry and exit of small molecules and ions. The enzymes involved in the oxidation 

of matabolites (i.e. the respiratory chain) as well as the photosystems used in 

photosynthesis, are present in the plasma membrane of prokaryotes. 

The bacterial chromosome is a single circular molecule of naked DNA tightly 

coiled within the nucleoid which appears in the electron microscope as a lighter 

region of the protoplasm. It is amazing to note that the DNA of E.coli which 

measures about 1 mm long when uncoiled,contains all the genetic information of 

the organism. In this case, there is sufficient information to code for 2000 to 3000 

different protiens. 

The single chromosome or the DNA molecule is circular and at one point it is 

attached to the plasma membrane and it is believed that this attachment may help 

in the separation of two chromosomes after DNA replication. 

In addition to a chromosome, certain bacteria contain a small, 

extrachromosomal circular DNA called plasmid. The plasmid is responsible for 

the antibiotic resistance in some bacteria. These plasmids are very much used in 

genetic engineering where the plasmids are separated and reincorporated, genes 

(specific pieces of DNA) can be inserted into plasmids, which are then transplanted 

into bacteria using the techniques of genetic engineering. 

106

Surrounding the DNA in the darker region of the protoplasm are 20,000 to 

30,000 particles called ribosomes. These are composed of RNA and proteins and 

are the sites of protein synthesis. Ribosomes exist in groups called polyribosomes 

or polysomes. Each ribosome consists of a large and a small sub unit. the remainder 

of the cell is filled with H 2 

O, various RNAs, protein molecules (including enzymes) 

and various smaller molecules. 

Certain motile bacteria have numerous, thin hair like processes of variable 

length called flagella. Flagella are used for locomotion. In contrast with the flagella 

of eukaryotic cells which contain 9+2 micortubles each flagellum in bacteria is 

made of a single fibril. 

It was Fox et al who divided the living organisms into two kingdoms Prokaryota 

and Eukaryota. Prokaryotes are in turn classified into two major sub groups 1) the 

Archae bacteria and 2) Eubacteria. Cyanobacteria are included in the group 

Eubacteria. the Cyanobacterial prokaryotes, commonly called bluegreen algae, 

are photosynthetic. In cyanobacterial cells, the photosynthetic, respiratory and 

genetic apparatuses are present but not delimited from each other by any bounding 

membrane of their own. No sharp boundaries divide the cell into special regions. 

But, there are several cell components with characteristic fine structure. These 

are distributed throughout the cell in patterns varying from species to species and 

also in different developmental stages in the same species. 

These cyanobacterial cells have an elaborate photosynthetic membrane system, 

composed of simple thylakoids and a central nucleoplasmic area which is usually 

fibrillar or granular or both. The cell also includes various kinds of granular inclusions, 

a rigid, several layered cell wall and a fibrous sheath over the cell wall.The 

characteristic collective properties of Cyanobacteria include oxygenic 

photosynthesis, chromatic adaptation, nitrogen fixation and a capacity for cellular 

differentiation by the formation of heterocysts, akinetes and hormogonia. 

Eukaryotes 

Eukaryotes comprise all members of Plant Kingdom, Fungi and Animal 

Kingdoms, including the unicellular fungus Yeast, and protozoans. Eukaryotic cells, 

like prorkaryotic cells are surrounded by a plasma membrane. However, unlike 

prokaryotic cells, most eukaryotic cells contain internal membrane bound organelles. 

Each type of organelle plays a unique role in the growth and metabolism of 

the cell, and each contains a set of enzymes that catalyze requisite chemical 

reactions. 

The largest organelle in a eukaryotic cell is generally the nucleus, which 

houses most of the cellular DNA. The DNA of eukaryotic cells is distributed 

among 1 to about 50 long linear structures called chromosomes. The number 

107

Table 2.2 The differences between Prokaryotes and Eukaryotes 

Property Prokaryotes Eukaryotes 

Size 

Most of them are very small. Some are 

larger than 50 µm. 

Most are large cells (10-100µm). Some 

are larger than 1 mm. 

General 

Characteristics 

All are m icrobes. Unicellular or 

colonial. The nucleoid is not membrane 

bound. 

Some are microbes; most are large 

organism s. All possess a membranebound 

nucleus. 

Cell Division 

Sexual system 

Development 

No mitosis or meiosis. Mainly b y binary 

fission or budding. 

Absent in most forms, when present 

unidirectional transfer of genetic 

material from donor to recipient. 

No m ulti-cellular development from 

diploid zygotes. No extensive tissue 

differentiation. 

M itosis and m eiosis types of cell 

division occur. 

Present in most forms, equal male and 

female participation in fertilization. 

Haploid forms are produced by meiosis 

and diploid from zygotes. M ulticellular 

organism s show extensive 

tissue differentiation. 

Flagella Type 

Some have simple bacterial flagella 

composed of only one fibril. 

Flagella are of 9 + 2 type 

Cell Wall 

Organelles 

Ribosomes 

DNA 

Made up of peptidoglycan 

(mucopeptide). Cellulose is absent. 

Membrane bound organelles such as 

endoplasm ic reticulum , golgi complex, 

m itochondria, chloroplasts and 

vacuoles are absent. 

Ribosomes are smaller made of 70s 

units (s refers to Svedberg unit, the 

sedimentation coefficient of a particle 

in the ultra centrifuge). 

Genetic material (DNA) is not found in 

well-organized chromosomes. 

Cell w all is m ade up of cellulose in 

plants and chitin in fungi. 

Membrane bound organelles such as 

endoplasm ic reticulum , golgi complex, 

m itochondria, chloroplasts and 

vacuoles are present. 

Ribosomes are larger and made of 80s 

units. 

Genetic m aterial is found in well 

organized chromosomes. 

108

and size of the chromosomes are the same in all cells of an organism but vary 

among different species of organisms. The total DNA ( the genetic information) 

in the chromosomes of an organism is referred to as its genome. In addition to the 

nucleus, several other organelles are present in nearly all eukaryotic cells, the 

mitochondria in which the cell's energy metabolism is carried out, the rough and 

smooth endoplasmic reticula, a network of membranes in which proteins and 

lipids are synthesized and peroxysomes, in which fatty acids and amino acids 

are degraded. Chloroplasts, the site of photosynthesis are found only in plants 

and some single celled organisms. Both plant cells and some single celled eukaryotes 

contain one or more vacuoles, large, fluid – filled organelles in which nutrients and 

waste compounds are stored and some degradative reactions occur. The cytosol 

of eukaryotic cells contains an array of fibrous proteins collectively called the 

cytoskeleton. Cytosol is the soluble part of the cytoplasm. It is located between 

the cell organelles. The plant cell has a rigid cell wall composed of cellulose and 

other polymers. The cell wall contributes to the strength and rigidity of plant cell. 

Some familiar prokaryotes are: Bacteria, filamentous bacteria 

(Actinomycetes) and Cyanobacteria. 

Some familiar eukaryotes are: Fungi, plants and animals. 


One Mark 


1. The extra-chromosomal DNA found in the bacterium E.coli is called 

a. measosome b. nucleoid c. incipient nucleus d. plasmid 


1. Bacteria having a thin wall and an outer membrane separated from the cell 

wall are usually Gram———————. 

2. The plasmid is responsible for —————— of the bacterium. 

3. Plasmids are very much used in ——————— 

4. Ribosomes that exist in groups are called———————— 

Two Marks 

1. What is meant by incipient nucleus. 

2. What are the uses of plasmid? 

3. Distinguish a prokaryotic cell form a eukaryotic cell. 

Five Marks 

1. Describe the ultra structure of a prokaryotic cell. 

Ten Marks 

1. Tabulate the differences between prokaryotes and eukaryotes. 

109

4. Light Microscope and Electron Microscope 

(TEM & SEM) 

The modern, complete understanding of cell architecture is based on several 

types of microscopy. Schleiden and Schwann using a primitive light microscope, 

first described individual cells as the fundamental unit of life and light microscopy 

continued to play a major role in biological research. The development of electron 

microscopes has greatly extended the ability to resolve sub-cellular particles and it 

has provided new information on the organization of plant and animal tissues. The 

nature of the image depends on the type of light or electron microscope used and 

on the way in which the cell or tissue has been prepared for observation. 

Light microscopy 

The compound microscope which is most commonly used today contains 

several lenses that magnify the image of a specimen under study. The total 

magnification of the object is a product of the magnification of the individual lenses; 

if the objective lens magnifies 100-fold (a 100 × lens, usually employed) and the 

eye piece magnifies 10-fold, the final magnification recorded by the human eye or 

on film will be 1000-fold (100 × 10). 

The limit of resolution of a light microscope using visible light is about 0.2µm 

(200nm). No matter how many times the images is magnified, the microscope 

can never resolve objects that are less than ≈0.2µm apart or reveal details smaller 

than ≈0.2µm in size 

Samples for light microscopy are usually fixed, sectioned and stained. 

Specimens for light microscopy are usually fixed with a solution combining alcohol 

or formaldehyde, compounds that denature most protein and nucleic acids. Usually 

the sample is then embedded in paraffin or plastic and cut into sections of one or 

a few micrometers thick using a microtome. Then these sections are stained using 

appropriate stains. 

Transmission Electron Microscopy 

The fundamental principles of electron microscopy are similar to those of 

light microscopy, the major difference is that in electron microscope electro 

magnetic lenses and not optical lenses are used. Also it focuses a high velocity 

electron beam instead of visible light. The electrons are absorbed by atoms in the 

110

air and that is the reason 

why the entire tube between 

the electron source and the 

viewing screen is 

maintained under an ultra 

high vacuum. 

The TEM directs a 

beam of electrons through 

a specimen. Electrons are 

emitted by a tungsten 

cathode when it is 

electrically heated. A 

condenser lens focuses the 

electron beam on to the sample 

objective and projects them on to a 

viewing screen or on a piece of 

photographic film. 

The minimum distance D at which 

two objects can be distinguished is 

proportional to the wavelength λ of the 

light that illuminates the objects. Thus 

the limit of resolution for the electron 

microscope is theoretically 0.005nm 

or 40,000 times better than that of 

unaided human eye. But in reality a 

resolution of 0.10nm can be obtained 

with TEM, about 2000 times better 

than the resolution of light 

microscopes. 

111

Scanning Electron Microscope 

SEM generally has a lower resolving power than the TEM. It is very useful 

for providing three-dimensional images of the surface of microscopic objects. In 

this electrons are focused by means of lenses in to a very fine point. The interaction 

of electrons with the specimen results in the release of different forms of radiation 

(eg secondary electrons) from the surface of the specimen. These radiations are 

then captured by an appropriate detector, amplified and then imaged on a television 

screen. 

Other important techniques in EM include the use of ultra thin sections of 

embedded material; a method of freeze-drying the specimen, which prevents the 

distortion caused by conventional drying procedure; and the use of negative staining 

with an electro dense material such as phosphotungstic acid or Uranyl salts. These 

heavy metal salts provide enough contrast to detect the details of the specimen. 


One Mark 


1. The ............. value of D, the better will be the resolution. 

2. The resolution of a microscope lens is numercally equivalent to ............. 

3. The purpose of using heavy metals in scanning electron microscopy is to 

provide enough ............. to detect the details of the specimens. 

4. The compound microscope uses ............. lenses to magnify the objects. 

Two Marks 

1. Define:resolving power of a microscope 

Ten Marks 

1. Explain the structure and principle used in light microscope. 

2. Explain the structure and principle used in Transmission electron 

microscope. 

112

5.Cell Wall 

The cells of all plants, bacteria and fungi have a rigid, protective covering 

outside the plasma membrane called cell wall. The presence of cell wall in plant 

cells distinguishes them from animal cells. Among the vascular plants only certain 

cells connected with the reproductive processes, are naked, all other cells have 

walls. The cell wall was first observed by Hooke in the year 1865 in cork cells. 

Originally it was thought that the cell wall was a non-living secretion of the 

protoplasm, but now it is known to be metabolically active and is capable of growth 

and at least during its growth, contains protoplasmic material. 

Formation of the cell wall 

During the telophasic stage of mitosis, the phragmoplast widens and becomes 

barrel shaped. At the same time, on the equatorial plane the cell plate i.e the first 

evident partition between the daughter protoplasts, begins to form inside the 

phragmoplast. In the area where the cell plate forms, the fibres of the phragmoplast 

become indistinct and are restricted to the circumference of the cell plate. When 

the cell plate is completely formed the phargmoplast disappears completely. At 

this stage thin lamellae are laid down by the daughter protoplasts on both the sides 

of the cell plate. The cell plate gradually undergoes changes to form the intercellular 

substances referred to as the middle lamella. 

Structure of the cell wall 

A typical plant cell has the following three parts. 1.Middle lamella 2.Primary 

wall 3.Secondary Wall 

Chemical Composition 

The chemical composition of cell wall varies in different kingdoms. In bacteria 

the cell wall is composed of peptidoglycan, in Fungi it is made up of chitin. The 

plant cell wall is made up of cellulose. Besides cellulose certain other chemicals 

such as hemicellulose, pectin, lignin, cutin, suberin, silica may also be seen deposited 

on the wall. 

Middle lamella 

It is a thin amorphous cement like layer between two adjacent cells. Middle 

lamella is the first layer, which is deposited at the time of cytokinesis. It is optically 

inactive (isotropic). It is made up of calcium and magnesium pectates. In addition 

to these substances proteins are also present. 

113

Primary wall 

It is the first formed wall of the cell which is produced inner to the middle 

lamella. It is thin, elastic and extensible in growing cells. It is optically active 

(anisotropic). It grows by addition of more wall material within the existing one. 

Such a growth is termed as intussusception. Some cells like the parenchymatous 

cells and meristematic cells have only the primary wall. The primary wall consists 

of a loose network of cellulose microfibrils embedded in a gel like matrix or 

ground substances. In most of the plants the micro fibrils are made up of cellulose. 

The micro fibrils are oriented variously according to shape and thickness of the 

wall. The matrix of the primary wall in which the micro fibrils are embedded is 

mainly composed of water, hemicellulose, pectin and glycoprotein. Pectin is the 

filling material of the matrix. Hemicellulose binds the microfibrils with the matrix 

and the glycoproteins control the orientation of the microfibrils. 

Secondary Wall 

A thick secondary wall is laid inner to the primary wall after the cell has 

reached maturity. It is laid down is succession of at least three layers often named 

S 1 

, S 2 

and S 3 

. It grows in thickness by accretion (apposition) i.e deposition of 

materials over the existing structures. The central layer (S 2 

)is usually the thickest 

layer. In some cells however, the number of layers may be more than three. The 

formation of secondary 

wall is not uniform in all the 

cells. This results in the 

differentiation of various 

types of cells, such as 

parenchyma, collenchyma, 

fibres and tracheids. 

The micro fibrils of 

secondary wall are 

compactly arranged with 

different orientation in 

different layers embedded 

in a matrix of pectin and 

hemicellulose. substances 

like lignin, suberin, minerals, 

waxes, tannins, resins, 

gums, inorganic salts such 

as calcium carbonate, 

calcium oxalate, silica etc may be deposited in the secondary wall. The secondary 

114

wall is very strongly anisotropic 

and layering can be observed in 

it. 

Fine structure of the cell wall 

particularly that of the secondary 

wall, has been intensively studied. 

This study was stimulated 

because of its importance to the 

fibre, paper and other industries. 

Cell wall is built of a system of 

microscopic threads the micro 

fibrils, which are grouped together 

in larger bundles. the layering 

seen in the secondary wall is 

often the result of the different 

density of the micro fibrils. The 

secondary wall consists of two 

continuous interpenetrating 

systems one of which is the 

cellulose micro fibrils and the 

other, the continuous 

system of 

microcapillary 

spaces. These 

spaces may the filled 

with lignin, cutin, 

suberin, hemicellulose 

and other organic 

substances and 

sometimes even some 

mineral crystals. 

The cellulose 

molecules consist of 

long chains of linked 

glucose residues. The 

chain molecules are 

arranged in bundles 

which are generally 

termed micellae. The 

115

hypothesis of the presence of micellae was proposed by Nageli. According to 

Frey-wyssling and Muhlethaler the thread like cellulose molecules are arranged in 

bundles. Each such bundle which forms an elementary fibril consists of about 

36 cellulose molecules. The elementary fibril is mostly crytalline. 

Plasmodesmata 

The cell wall is not totally complete around the cell. It is interrupted by narrow 

pores carrying fine strands of cytoplasm, which interlink the contents of the cells. 

They are called plasmodesmata. They form a protoplasmic continuum called 

symplast. It consists of a canal, lined by plasma membrane. It has a simple or 

branched tubule known as desmotubule. Desmotubule is an extension of 

endoplasmic reticulum. Plasmodesmata serves as a passage for many substances 

to pass through. It is also believed that they have a role in the relay of stimuli. 

Pits 

Pits are the areas on the cell wall on which the secondary wall is not laid 

down. The pits of adjacent cells are opposite to each other. Each pit has a pit 

chamber and a pit membrane. The pit membrane consists of middle lamella and 

primary wall. Pit membrane has many minute pores and thus they are permeable. 

Pits are of two types 1.Simple pits 2.Bordered pits. In simple pits the 

width of the pit chamber is uniform. There is no secondary wall in the simple pit. 

In bordered pit the secondary wall partly overhangs the pit. Pits help in the 

translocation of substances between two adjacent cells. Generally each pit has a 

complementary pit lying exactly opposite 

to it in the wall of the neighbouring cell. 

Such pits form a morphological and 

functional unit called the pit pair. 

Functions of cell wall 

1. It gives definite shape to the cell. 

2. It protects the internal protoplasm 

against injury. 

3. It gives rigidity to the cell 

4. It prevents the bursting of plant cells 

due to endosmosis. 

5. The walls of xylem vessels, tracheids 

and sieve tubes are specialized for long 

distance transport. 

6. In many cases, the cell wall takes part in offense and defense. 

116


One Mark 


1. The addition of wall materials within the existing one is called 

a.accretion b.intussusception c.apposition d.deposition 


1. The cell wall of bacterium is made up of ——————— 

2. The cell wall of a typical plant cell is made up of —————— 

3. The cell wall of a fungus is made up of —————— 

4. The addition of wall materials over the existing one is called————— 

Two Marks 

1. Name the three important components of a typical plant cell wall. 

2. What is middle lamella? 

3. What is meant by growth by intussusception? 

4. What are micellae? 

5. Name the two continuous interpenetrating systems found in secondary wall. 

6. What is a pit membrane? 

7. What are bordered pits? 

8. Define:symplast. 

9. What is desmotubule? 

Five Marks 

1. What is plasmodesmata? Explain 

2. What are pits? Explain their types. 

3. Discuss the functions of cell wall. 

Ten Marks 

1. Describe the fine structure of cell wall. 

117

6. Cell Membrane 

All the prokaryotic and eukaryotic cells are enclosed by an elastic thin 

covering called plasma membrane. It is selectively permeable since it allows 

only certain substances to enter or leave the cell through it. In addition to this 

eukaryotic cells possess intracellular membranes collectively called cytoplasmic 

membrane system, that surround the vacuole and cell organelles. Plasma membrane 

and the sub-cellular membranes are together known as biological membranes. 

Ultra structure of the cell membrane 

Cell membranes are 

about 75A° thick. Under the 

electron microscope they 

appear to consist of 3 layers. 

1. an outer electron 

dense layer of about 

20A° thick 

2. an inner electron dense 

layer of about 20A° 

thick. 

3. a middle pale coloured 

layer about 35A° thick. 

The outer and inner 

layers are formed of protein 

molecules whereas the 

middle one is composed of 

two layers of phospholipid molecules. Such a trilaminar structure is called “Unit 

membrane” which is a basic concept of all membranes. 

Fluid mosaic Model 

Many models have been proposed to explain the molecular structure of plasma 

membrane. Fluid mosaic model was proposed by Singer and Nicholson (1972) 

and it is widely accepted by all. According to this model the cell membrane has 

quasifluid structure. All cellular membranes line closed compartments and have 

a cytosolic and an exoplasmic face. Membranes are formed of lipids and priteins. 

According to this model the membrane is viewed as a two dimensional mosaic of 

phospholipids and protein molecules. 

118

Lipids 

The lipid molecules form a continuous bilayer. The protein molecules are 

arranged as extrinsic proteins on the surface of lipid bilayer and as intrinsic 

proteins that penetrate the lipid bilayer either wholloy or partially. The lipid bilayer 

is formed of a double layer of phospholipid molecules. They are amphipathic 

molecules i.e. they have a hydrophilic and hydrophobic part. The arrangement of 

phospholipids forms a water resistant barrier. So that only lipid soluble substances 

can pass through readily but not water soluble substances. 

The phospholipid bilayer forms the basic structure of all biomembranes which 

also contain proteins, glycoproteins, cholesterol and other steroids and glycolipids. 

The presence of specific sets of membrane proteins permits each type of membrane 

to carryout distinctive functions. 

Proteins 

Proteins are arranged in two forms. 

1. Extrinsic or peripheral proteins:These are superficially attached to 

either face of lipid bimolecular membrane and are easily removable by 

physical methods. 

2. Intrinsic or Integral proteins:These proteins penetrate the lipid either 

wholly or partially and are tightly held by strong bonds. In order to remove 

them, the whole membrane has to be disrupted. The integral proteins 

occur in various forms and perform many functions. 

Functions of plasma membrane 

In all cells the plasma membrane has several essential functions to perform. 

These include transporting nutrients into and metabolic wastes out of the cell 

preventing unwanted materials from entering the cell. In short, the intercellular 

and intra cellular transport is regulated by plasma membrane. The plasma membrane 

maintains the proper ionic composition pH(~7.2) and osmotic pressure of the cytosol. 

To carry out all these functions, the plasma membrane contains specific trasport 

proteins that permit the passage of certain small molecules but not others. Several 

of these proteins use the energy relaeased by ATP hydrolysis to pump ions and 

other molecules into or out of the cell against concentration gradients. Small charged 

molecules such as ATP and amino acids can diffuse freely within the cytosol but 

are restricted in their ability to leave or enter it across the plasma membrane. 

119

In addition to these universal functions, the plasma membrane has other 

important functions to perform. Enzymes bound to the plasma membrane catalyze 

reactions that would occur with difficulty in an aqueous environment. The plasma 

membranes of many types of eukaryotic cells also contain receptor proteins 

that bind specific signalling molecules like hormones, growth factors, 

neurotransmitters etc. leading to various cellular responses. 

Like the entire cell, each organelle in eukaryotic cells is bounded by a unit 

membrane containing a unique set of proteins essential for its proper functioning. 

Membrane Transport 

Based on the permeability a membrane is said to be: 

1. Permeable: If a substance passes readily through the membrane 

2. Impermeable: If a substance does not pass through the membrane 

3. Selectively permeable: If the membrane allows some of the substances 

to pass through but does not allow all the substances to pass through it. 

The permeability of a membrane depends on 1)the size of pores in the Plasma 

membrane. 2)The size of the substance molecules 3)The charge on the substance 

molecules. 

All the biological membranes are selectively permeable. Its permeability 

properties ensure that essential molecules such as glucose, amino acids and lipids 

readily enter the cell, metabolic intermediates remain in the cell and waste 

compounds leave the cell. In short it allows the cell to maintain a constant internal 

environment. 

Substances are transported across the membrane either by: 

1. Passive Transport or 2. Active Transport 

Passive Transport 

Physical processes 

Passive Transport of materials across the membrane requires no energy by 

the cell and it is unaided by the transport proteins. The physical processes through 

which substances get into the cell are 1.Diffusion 2.Osmosis 

Diffusion 

Diffusion is the movement of molecules of any substance from a region of it’s 

higher to a region of it’s lower concentration (down its own concentration gradient) 

to spread uniformly in the dispersion medium on account of their random kinetic 

motion. 

120

The rate of diffusion is directly proportional to 

1. the concentration of the substance 

2. temperature of the medium 

3. area of the diffusion pathway 

The diffusion is inversely proportional to 

1. the size of the substance molecules 

2. the molecular weight of the substance molecule 

3. the distance over which the molecules have to diffuse 

Diffusion through Biomembranes 

Gases and small hydrophobic molecules diffuse directly across the phospholipid 

bilayer at a rate proportional to their ability to dissolve in a liquid hydro carbon. 

Transport of molecules takes place along the concentration gradient and no 

metabolic energy is expended in this process. This can be described as ‘down hill 

transport’. Diffusion through the bio membrane takes place in two ways. 

1. Diffusion of fat-soluble substances through plasma membrane simply by 

dissolving in the lipid bilayer. 

2. Diffusion of water soluble substances and ions: This takes place through 

pores in the membranes. 

Diffusion of charged particles water soluble substances and ions such as 

K + Cl − − 

and HCO 3 

diffuse through the pores in the membranes. An ion diffuses 

from the side richer in like charges to the side with an excess of opposite charges. 

The difference of electrical charges between the two sides of a membrance is 

called electro chemical gradient. 

The integral proteins of the membrane act as protein channels extending 

through the membrane. The movement of gas molecules occurs down its pressure 

gradient. 

Osmosis 

It is the special type of diffusion where the water or solvent diffuses through 

a selectively permeable membrane from a region of high solvent concentration to 

a region of low solvent concentration. 

Role of Osmosis 

1. It helps in absorption of water from the soil by root hairs. 

121

2. Osmosis helps in cell to cell movement of water. 

3. Osmosis helps to develop the turgor pressure which helps in opening and 

closing of stomata. (For more about Osmosis see unit 5.4) 

Uniporter Catalyzed Transport 

The plasma membrane of most cells (animal or plant) contains several 

uniporters that enable amino acids, nucleosides, sugars and other small molecules 

to enter and leave cells down their concentration gradients. Similar to enzymes, 

uniporters accelerate a reaction that is thermodynamically favoured. This type of 

movement sometimes is referred to as facilitated transport or facilitated 

diffusion. 

Three main features distinguish uniport transport from passive diffusion. 

1. the rate of transport is far higher than predicted 2.transport is specific 3.transport 

occurs via a limited number of transporter proteins rather than through out the 

phospholipids bilayer. 

Active transport 

It is vital process. It is the 

movement of molecules or ions against 

the concentration gradient. i.e the 

molecules or ions move from the 

region of lower concentration towards 

the region of higher concentration. The 

movement of molecules can be 

compared with the uphill movement 

of water. 

Energy is required to counteract the force of diffusion and the energy comes 

from ATP produced by oxidative phosphorylation or by concentration gradient of 

ions. Thus active transport is defined as athe energy dependent transport of 

molecules or ions across a semi permeable membrane against the concentration 

gradient. 

Active transport takes place with the help of carrier proteins that are present 

in the plasma membrane. In the plasma membrane there are a number of carrier 

molecules called permeases or translocases present. For each type of solute 

molecule there is a specific carrier molecule. It has got two binding sites; one for 

the transportant and other for ATP molecule. The carrier proteins bind the 

transportant molecule on the outer side of the plasma membrane. This results in 

the formation of carrier-transportant-complex. As the ATP molecule binds 

122

itself to the other binding site of the carrier protein it is hydrolysed to form ADP 

and energy is released. This energy brings conformational change in the carrier 

transportant-complex and the transportant is carried through the channel on the 

other side of the membrane. The carrier molecule regains its original form and 

repeats the precess. 

There are two forces which govern the movement of ions across selectively 

permeable membranes, the membrane electric potential and the ion 

concentration gradient. ATP driven ion pumps generate and maintain ionic 

gradients across the plasma membrane. 

Endocytosis and exocytosis 

Endocytosis and exocytosis are active processes involving bulk transport of 

materials through membranes, either into cells(endocytosis) or out of cells 

(exocytosis). 

Endocytosis occurs by an in folding or extension of the plasma membrane to 

form a vesicle or vacuole or vauole. It is of two types. 

1. Phagocytosis:(cell eating)-Substances are taken up in solid form. Cells 

involving in this process are called phagocytes and said to be phagocytic. 

(eg.) some white blood cells. A phagocytic vacuole is formed during the 

uptake. 

2. Pinocytosis (cell drinking)-Substances are taken up in liquid form. Vesicles 

which are very small are formed during intake. Pinocytosis is often 

associated with amoebiod protozozns, and in certain kidney cells involved 

in fluid exchange. It can also occur in plant cells. 

123

Exocytosis is the reverse of endocytosis by which materials are removed 

from cells such as undigested remains from food vacuoles. 


One Mark 


1. Active transport of molecules take place 

a. along the concentration gradient 

b. along the electric gradient 

c. along the pressure gradient 

d. against the concentration gradient 

2. Phagocytosis is also known as 

a. cell eating b. cell death c. cell drinking d. cell lysis 


1. All the biological membranes are —————— 

2. In passive transport method, transport of molecules takes place—— the 

concentration gradient. 

Two Marks 

1. Define: Biological membrane. 

2. What are amphipathic molecules? 

3. What are extrinsic proteins? 

4. What are intrinsic proteins? 

5. Define: semi-permeable membrane. 

6. Define: Passive transport/Active transport 

7. Define: Diffusion/Osmosis 

8. Name any two factors on which permeability of a membrane depends on. 

9. What is the role of osmosis in plants? 

10. What is meant by facilitated transport? 

11. Distinguish uniport transport method from passive diffusion. 

12. Define: Phagocytosis/Pinocytosis/exocytosis 

124

Five Marks 

1. List the functions of plasma membrane. 

2. Define diffusion. Discuss the various factors that affect the rate of 

diffusion. 

3. Describe Uniporter Catalyzed transport. 

4. Describe active transport of substances across the membranes. 

Ten Marks 

1. Describe the fluid mosaic model of cell membrane. 

125

7. Cell Organelles 

The internal architecture of cells and central metabolic pathways are similar 

in all plants, animals and unicellular eukaryotic organisma (eg. Yeast) All eukaryotic 

cells contain a membrane bound nucleus and numerous other organelles in their 

cytosol. Unique proteins in the interior and membranes of each type of organelle 

largely determine it’s specific functional characteristics. 

A Typical plant cell contains the following organelles and parts: 

1. Mitochondria 

They are bounded by two membranes with the inner one extensively folded. 

Enzymes in the inner mitochondrial membrane and central matrix carry out terminal 

stages of sugar and lipid oxidation coupled with ATP synthesis. 

2. Chloroplasts 

They are the sites of Photosynthesis. They are found only in plant cells. They 

are surrounded by an inner and outer membrane, a complex system of thylakoid 

membranes in their interior contains the pigments and enzymes that absorb light 

and produce ATP. 

3. Nucleus 

It is surrounded by an inner and outer membrane. These contain numerous 

pores through which materials pass between the nucleus and cytosol. The outer 

nuclear membrane is continuous with the rough endoplasmic reticulum. The nuclear 

membrane resembles the plasma membrane in its function. The nucleus mainly 

contains DNA organized into linear structures called chromosomes. 

4. Endoplasmic reticulum 

These are a network of inter connected membranes. Two types of Endoplasmic 

Reticulum are recognised. 1.Rough E.R 2. Smooth E.R 

Rough ER 

In this kind of ER, ribosomes are present on the surface. The endoplasmic 

reticulum is responsible for protein synthesis in a cell. Ribosomes are sub organelles 

in which the amino acids are actually bound together to form proteins. There are 

spaces within the folds of ER membrane and they are known as Cisternae. 

126

Smooth ER 

This type of ER does not have ribosomes. 

5. Golgi Body or Golgi Apparatus(G.A.) (Dictyosomes) 

Golgi body is a series of flattened sacs usually curled at the edges. Proteins 

which were formed on ribosomes of rough endoplasmic reticulum are processed 

in G.A. After processing, the final product is discharged from the G.A. At this 

time the G.A. bulges and breaks away to form vesicle known as secretory vesicle. 

The vesicles move outward to the cell membrane and either insert their protein 

contents in the membrane or release these contents outside the cell. 

6. Vacuoles 

The Vacuoles form about 75% of the plant cell. In the vacuole the plant 

stores nutrients as well as toxic wastes. If pressure increases within the vacuole 

it can increase the size of the cell. In this case the cell will become swollen. If the 

pressure increases further the cell will get destroyed. 

7. Ribosomes 

Ribosomes are found in cells, both prokaryotic and eukaryotic except in mature 

sperm cells and RBCs. In eukaryotic cells they occur freely in the cytoplasm and 

also found attached to the outer surface of rough ER. Ribosomes are the sites of 

protein synthesis 

8. Plasma Membrane 

In all the cells the plasma membrane has several functions to perform. These 

include transporting nutrients into and metabolic wastes out of the cell. It is formed 

of lipids and proteins. 

9. Microbodies 

These are spherical organelles bound by a single membrane. They are the 

sites of glyoxylate cycle in plants. 

10. Cell wall 

The cells of all plants have cell wall. It has three parts. 1. Middle lamella 2. 

Primary wall 3. Secondary wall.It gives definite shape to the plant cell. 

Nucleus 

Nuclus is the largest organelle in eukaryotic cells. It is surrounded by two 

membranes. Each one is a phospholipid bilayer containing many different types of 

proteins. The inner nuclear membrane defines the nucleus itself. In many cells the 

127

Table 2.3. Structure and functions of various cell organelles and parts 

Diagram Structure Functions 

It has an envelope made up of two 

membranes, the inner is folded to form 

cristae. Matrix with ribosomes is 

present. A circular DNA is also there. 

ICristae are the sties of oxidative 

phosphorylation and electron transport. 

Matrix is the site of Krebs’ cycle reactions. 

128 


membranes. Contains gel like stroma 

and a system of membranes called 

grana. Ribosomes and a circular DNA 

are present in the stroma 

Photosynthesis takes place here. It is a 

process in which light energy is converted 

into chemical energy. 


membranes. They have nuclear pores. It 

contains nucleolus and chromatin. 

Nuclear division is the basis of cell 

replication and thus reproduction. 

Chromosomes contain DNA, the molecule 

responsible for inheritance. 

Structure : Consists of membrane - 

bounded sacs called cisterae. 

Smooth ER, (no ribosomes) is the site of 

lipid synthesis. Rough ER (with ribosomes) 

transports proteins made by the ribosomes 

through the cisterae.

It is formed by a stack of flattened 

membrane bound sacs, called cisternae. 

Often involved in secretion. 

Vacuoles 

It is bound by a single membrane called 

the tonoplast. It contains cell sap. 

Stores various substances including waste 

products. It helps in the osmotic properties 

of the cell. 

129 

It consists of a large and a small sub unit. 

They are made of protein and RNA. 

Ribosome are found in mitochondria 

and chloroplasts also. They may form 

polysomes i.e. collection of ribosomes 

strung along messenger RNA. 

They are the sites of protein synthesis. 

Two layers of lipid (bilayer) sandwiched 

between two protein layers. 

Spherical organelle bound by a single 

membrane. 

Being a differentially permeable membrane 

it controls the exchange of substances 

between the cell and its environment. 

They are the sites of glyoxylate cycle in 

plants. 

It consists of cellulolose microfibrils in 

a matrix of hemicellulose and pectic 

substances. Secondary thickening may 

be seen 

It provides mechanical support and 

protection.

outer nuclear membrane is 

continuous with the rough ER and 

the space between the inner and 

outer nuclear membrane is 

continuous with the lumen of the 

rough ER. 

The two nuclear membranes 

appear to fuse at the nuclear 

pores. These ring like pores are 

constructed of a specific set of 

membrane proteins and these act 

like channels that regulate the 

movement of substances between 

thenucleus and the cytosol. 

In a growing of differentiating cell, the nucleus is metabolically active, 

producing DNA and RNA. The RNA is exported through nuclear pores to the 

cytoplasm for use in protein synthesis. In 'resting' cells, the nucleus is inactive or 

dormant and minimal synthesis of DNA and RNA takes place. 

In a nucleus that is not dividing, the chromosomes are dispersed and not thick 

enough to be observed in the light microscope. Only during cell divisons the 

chromosomes become visible b y light microscopy. Chromosomes form the 

physical basis of heredity. Genes, the chemical basis of heredity, are arranged in 

linear fashion on the chromosomes. A sub organelle of the nucleus, the nucleolus 

is easily recognized under light microscope. Most of the ribosomal RNA of a cell 

is synthesized in the nucleolus. The finished or partly finished ribosomal sub units 

pass through a nuclear pore into the cytosol. 

The non nucleolar regions of the nucleus is called the nucleoplasm. It has 

very high DNA concentration. Fibrous proteins called lamins form a two dimensional 

network along the inner surface of the inner membrane giving it shape and apparently 

binding DNA to it. During the early stages of cell division breakdown of this 

network occurs. 

Functions of Nucleus 

1. It controls all the metabolic activities of the cell by controlling the 

synthesis of enzymes required. 

2. Nucleus controls the inheritance of characters from parents to offspring. 

3. Nucleus controls cell division. 

130

Mitochondria 

A Mitochondrion is also called as the “Power house of the cell” because it 

stores and releases the energy of the cell. The energy released is used to form 

ATP (Adenosine 

Triphosphate) 

Mitochondria are the 

principal sites of ATP 

production in aerobic cells. 

Most eukaryotic cells 

contain many 

mitochondria, which 

occupy up to 25 percent 

of the volume of the 

cytoplasm. These 

complex organelles are 

among the largest 

organelles generally 

exceeded in size only by 

the nucleus, vacuoles and 

chloroplasts. Typically the 

mitochondria are sausage-shaped but these may be granular, filamentous, rodshaped, 

spherical or thread like. 

Mitcohondria contain two very different membranes an outer one and an 

inner one, separated by the inter membrane space. 

The outer membrane is composed of about half lipid and half protein. The 

inner membrane is less permeable. It is composed of about 20 percent lipid and 

80 percent protein. The surface area of the inner membrane is greatly increased 

by a large number of infoldings, or cristae that protrude into the matrix. 

Structure of the cristae membrane 

The inner of the cristae membrane (i.e the surface towards the matrix) is 

covered with numerous (infinite) stalked particles. These are called F1 Particles, 

elementary particles or sub units. These particles project into the matrix. Each 

F1 particle has 3 parts, viz, the head piece, the stalk and the base piece. The 

respiratory chain consists of enzymes and co-enzymes which constitute the 

Electron Transport System, (ETS) in the mitochondrion. These enzymes and 

co-enzymes of the ETS act as the electron acceptors in the aerobic respiration 

reaction. (Oxidative Phosphorylation). 

131

In non photosynthetic cells the principal fuels for ATP synthesis are fatty 

acids and glucose. The complete aerobic degradation of glucose to CO 2 

and H 2 

O 

is coupled to synthesis of as many as 38 molecules of ATP. In eukaryotic cells, the 

initial stages of glucose degradation occur in the cytosol, where 2 ATP molecules 

per glucose molecule are generated. The terminal stages including those involving 

phosphorylation coupled to final oxidation by oxygen are carried out by exzymes 

in the mitochondrial matirix and cristae. As many as 36 ATP molecules per glucose 

molecule are generated in mitochondria although this value can vary because much 

of the energy released in mitochondrial oxidation can be used for other purposes 

(e.g heat generation and the transport of molecules into or out of the mitochondrion) 

making less enery available for ATP synthesis. Similarly, virtually all the ATP 

formed during the oxidation of fatty acids to CO 2 

is generated in the mitochondrion. 

Thus the mitochondrion can be regarded as the “Power plant” of the cell. 

Mitochondria as semi-autonomous organelles 

Mitochondria are self perpetuating semi autonomous bodies. These arise new 

by the division of existing mitochondria. These are also regarded as intra cellular 

parasitic prokaryotes that have established symbiotic relationship with the cell. 

The mitochondrial matrix contains DNA molecules which are circular and 70s 

ribosomes, tRNA and enzymes for functioning of mitochondrial genes. 

Plastids 

Plastids are the largest cytoplasmic organelles bounded by double membrane. 

These are found in most of the plant cells and in some photosynthetic protists. 

These are absent in prokaryotes and in animal cells. Plastids are of three types 

namely chloroplasts, chromoplasts and leucoplasts. 

Chromoplasts are coloured plastids other than green. They are found in 

coloured parts of plants such as petals of the flower, pericarp of the fruits etc. 

Leucoplasts are the colourless plastids. These colourless plastids are involved 

in the storage of carbothydrates, fats and oils and proteins. The plastids which 

store carbohydrates are called amyloplasts. The plastids storing fats and oils are 

called elaioplasts. The plastids storing protein are called proteinoplasts. 

Chloroplast 

Chloroplasts can be as long as 10ìm and are typically 0.5 - 2.0 ìm thick, but 

they vary in size and shape in different cells, especially among the algae. Like 

mitochondrion, the chloroplast is surrounded by an outer and inner membrane. In 

addition to this, chloroplasts contain an internal system of extensive inter connected 

membrane-limited sacs called thylakoids which are flattened to form disks. These 

are often grouped in stakes of 20-50 thylakoids to from what are called grana and 

embedded in a matrix called stroma. 

132

Stroma, a semi fluid, colourless, colloidal complex contains DNA, RNA, 

ribiosomes and several enzymes. The DNA of chloroplast is circular. The 

ribosomes are of 70s type. The matrix of higher plant’s chloroplasts may contain 

starch as storage product. Thylakoids may occur attached to the inner membrane 

of the chloroplast envelop. 

About 40-100 grana may occur in a chloroplast. Many membranous tubules 

called stroma lamellae (intergranal thylakoids) interconnect thylakoids of different 

grana. Thylakoid membrane contains photosynthetic pigments. 

The thylakoid membrane contains green pigments (Chlorophylls) and other 

pigments and enzymes that absorb light and generate ATP during photosynthesis. 

Part of this ATP is used by enzymes located in stroma to the convert CO 2 

into 

three carbon (3C) intermediates which are then exported to the cytosol and 

converted to sugars. 

The molecular mechanism by which ATP is formed is very similar in 

mitochondria and chloroplasts. Chloroplasts and mitochondria have other features 

also in common. Both migrate often from place to place within cells and both 

contain their own DNA which code for some of the key organellar proteins. These 

proteins are synthesized in the ribosomes within the organelle. However, most of 

the proteins in each of these organelles are encoded in the nuclear DNA and are 

synthesized in the cytosol. These proteins are then incorporated into the organelles. 

133

Ribosomes 

Ribosomes are small 

subspherical granular 

organelles, not enclosed by 

any membrane. They are 

composed of ribonucleo 

proteins and they are the site 

of protein synthesis. 

They occur in large 

number. Each ribosome is 

150-250A in diameter and 

consists of two unequal sub 

units, a larger dome shaped 

and a smaller ovoid one. The 

smaller sub unit fits over the 

larger one like a cap.These 

twosu units occur separately 

in the cytoplasm and join to form ribosomes only at the time of protein synthesis. 

At the time of protein synthesis many ribosomes line up and join an mRNA chain 

to synthesise many copies of a particular polypeptide. Such a string of ribosomes 

is called polysome. 

Ribosomes occur in cytoplasmic matrix and in some cell organelles. 

Accordingly, they are called cytoplasmic ribosomes or organelle ribosomes. The 

organelle ribosomes are found in plastids and mitochondria. The cytoplasmic 

ribosomes may remain free in the cytoplasmic matrix or attached to the surface of 

the endoplasmic reticulum. The attached ribosomes generally transfer their proteins 

to cisternae of endoplasmic reticulum for transport to other parts both inside and 

outside the cell. 

Depending upon size or sedimentation coefficient(s), ribosomes are of two 

types. 70s and 80s. 70s type of ribosomes are found in all prokaryotic cells and 

80s type are found in eukaryotic cells. S is Svedberg unit which is a measure of 

particle size with which the particle sediments in a centrifuge. In eukaryotic cells, 

synthesis of ribosomes occurs inside the nucleolus. Ribosomal RNA are synthesized 

in the nucleolus. The ribosomal proteins are synthesized in the cytoplasm and shift 

to the nucleolus for the formation of ribosomal sub units by complexing with rRNA. 

The sub units pass out into the cytoplasm through the nuclear pores. In prokaryotic 

cells, both ribosomal RNAs and proteins are synthesized in the cytoplasm. Thus 

the ribosomes act as the protein factories of the cell. 

134


One Mark 


1. The spaces inside the folds of ER membrane are known as 

a. thylakoids b.cisternae c.mesosomes d.periplasmic space 

2. These are colourless plastids 

a.chromoplasts b.chloroplasts c.elaioplasts d.leucoplasts 

3. The internal system of inter-connected membrane-limited sacs of 

chloroplasts are called 

a. grana b.stroma c.thylakoids d. cisternae 


1. DNA is organized into linear structures called————— 

2. The endoplasmic reticulum is responsible for————— in a cell. 

3. ———— are the sites of protein synthesis. 

4. ————form the physical basis of heredity. 

Match 

Power house of a cell 

-Chromosomes 

Site of protein synthesis 

-Genes 

Controls all metabolic activities of cell -Mitochondria 

Physical basis of heredity 

-Ribosomes 

Chemical basis of heredity 

-Nucleus 

Two Marks 

1. What are the main functions of a nucleus? 

2. Give reasons: Mitochondria are semi autonomous organelles. 

3. Name the three kinds of plastids. 

4. Name any two common properties shared by chloroplasts and 

mitochondria. 

5. What is a polysome? 

6. Distinguish the ribosomes of prokaryotic cells from that of eukaryotic 

cells. 

Five Marks 

1. Draw a plant cell and label it’s parts. 

2. Explain the ultrastructure of chloroplast. 

135

Cell Cycle 

8. Cell Division 

As we have discussed in the earlier chapter, the cell cycle amazingly follows 

a regular timing mechanism. Most eukaryotic cells live according to an internal 

clock, that is, they proceed through a sequence of phases, called the cell cycle. 

During the cell cycle DNA is duplicated during the synthesis(S) phase and the 

copies are distributed to the daughter cells during mitotic(M) phase. Most growing 

plant and animal cells take 10-20 hours to double in number and some duplicate at 

a much slower rate. 

A multi cellular organism usually starts it’s life as a single cell (zygote). The 

multiplication of this single cell and it’s descendants determine the growth and 

development of the organism and this is achieved by cell division. Cell division is a 

complex process by which cellular material is equally divided between daughter 

cells. Cell division in living things are of three kinds. They are 1.Amitosis 2.Mitosis 

3. Meiosis. 

Amitosis 

It is a simple type of division where the cell contents including nucleus divide 

into two equal halves by an 

inwardly growing constriction 

in the middle of the cell. This 

type of cell division is common 

in prokaryotes. 

Mitotic cell cycle 

It is represented by DNA 

duplication followed by nuclear 

division (Karyokinesis) which 

in turn is followed by 

cytokinesis. Mitotic cell 

division was first described by 

W. Flemming in 1882. In the 

same year, mitosis in plants 

was described by 

Strasburger. 

136

In plants, active mitotic cell division takes place in apices. In higher animals 

mitotic cell division is said to be diffused, distributed all over the body. 

Mitotic cell cycle consists of long interphase(which is sub divided into G 1 

, S 

and G 2 

phases), a short M stage (or mitotic stage, subdivided into prophase 

metaphase, anaphase and telophase) and cytokinesis. The duration of interphase 

and M-phase varies in different cells. 

Interphase 

It is the stage in between two successive cell divisions during which the cell 

prepares itself for the process by synthesizing new nucleic acids and proteins. 

Chromosomes appear as chromatin network. Interphase consists of the following 

three sub stages. 

i) G 1 

or Gap-1 phase 

This phase starts immediately after cell division. The cell grows in size and 

there is synthesis of new proteins and RNA needed for various metabolic activities 

of the cell. A non-dividing cell does not proceed beyond G 1 

phase. The 

differentiating cells are said to be in G 0 

stage. 

ii) S-or Synthetic Phase 

During this phase there is duplication of DNA. Thus each chromosome now 

is composedof two sister chromatids. 

iii) G 2 

or Gap-2Phase 

The proteins responsible for the formation of spindle fibres are synthesised 

during this stage. 

Mitosis 

Mitosis is divided into the following 4 sub stages. 

1.Prophase 2. Metaphase 3.Anaphase 4. Telophase 

1. Prophase 

The chromatin network begins to coil and each chromosome becomes distinct 

as long thread like structure. Each chromosome at this stage has two chromatids 

that lie side by side and held together by centromere. The nucleus gradually 

disappears. The nuclear membrane also starts disappearing. 

2. Metaphase 

The disappearance of nuclear membrane and nucleolus marks the beginning 

of metaphase.The chromosomes become shorter by further coiling. Finally, the 

chromosomes become distinct and visible under compound microscope. The 

137

chromosomes orient themselves in the equator of the cell in such a way that all the 

centromeres are arranged in the equator forming metaphase plate or equatorial 

plate. Out of the two chromatids of each chromosome, one faces one pole and the 

other one faces the opposite pole. At the same time spindle fibres arising from the 

opposite poles are seen attached to the centromeres. The fibres are made up of 

proteins rich in sulphur containing amino acids. 

At late metaphase, the centromeres divide and now the chromatids of 

each chromosome are ready to be separated. 

3. Anaphase 

Division of centromere marks the beginning of anaphase. The spindle fibres 

start contracting and this contraction pulls the two groups of chromosomes towards 

the opposite poles. As the chromosomes move toward opposite poles they assume 

V or J or I shaped configuration with the centromere proceeding towards the 

poles with chromosome arms trailing behind. Such variable shapes of the 

chromosomes are due to the variable position of centromere. 

Telophase 

At the end of anaphase, chromosomes reach the opposite poles and they 

uncoil, elongate and become thin and invisible. The nuclear membrane and the 

nucleouls reappear. thus, two daughter nuclei are formed, one at each pole. 

138

Cytokinesis 

The division of the cytoplam is called cytokinesis and it follows the nuclear 

division by the formation of cell wall between the two daughter nuclei. The formation 

of cell wall begins as a cell plate also known as phragmoplast formed by the 

aggregation of vesicles produced by Golgi bodies. These vesicles which contain 

cell wall materials fuse with one another to form cellmembranes and cell walls. 

Thus, at the end of mitosis, two identical daughter cells are formed. 

Significance of Mitosis 

1. As a result of mitosis two daughter cells which are identical to each other 

and identical to the mother cell are formed. 

2. Mitotic cell division ensures that the daughter cells possess a genetical 

identity, both quantitatively and qualitatively. 

3. Mitosis forms the basis of continuation of organisms. 

4. Asexual reproduction of lower plants is possible only by mitosis. 

5. Vegetative reproduction in higher plants by grafting, tissue culture method 

are also a consequence of mitosis. 

6. Mitosis is the common method of multiplication of cells that helps in the 

growth and development of multi-cellular organism. 

7. Mitosis helps in the regeneration of lost of damaged tissue and in wound 

healing. 

8. The chromosomal number is maintained constant by mitosis for each 

species. 

Meiosis 

Meiosis is a process of cell division of the reproductive cells of both plants 

and animals in which the diploid number of chromosomes is reduced to haploid. 

Meiosis is also known as reduction division(RD) since the number of 

chromosomes is reduced to half. It takes place only in the reproductive cells during 

the formation of gametes. Meiosis consists of two complete divisions. As a result 

of this a diploid cell produces four haploid cells. The two divisions of meiosis are 

meiosis I or heterotypic division and meiosis II or homotypic division. The first 

division is meiotic or reductional in which the number of chromosomes is reduced 

to half and the second division is mitotic or equational. 

In all the sexually reproducing organism the chromosome number remains 

constant generation after generation. During sexual reproduction the two gametes 

139

male and female, each having single set oof chromosomes (n) fuse to form a 

zygote.The zygote thus contains twice as many chromosome as a gamete 

(n+n=2n). In these two sets of chromosomes one set is derived from the male 

parent and the other set from the female parent. This is how diploids come to 

possess tow identical sets of chromosomes called homologous chromosomes 

Meiosis may take place in the life cycle of a plant during any one of the following 

events. 

1. At the time of spore formation ie. During the formation of pollen grains in 

anther and megaspores in ovules. 

2. At the time of gamete formation 

3. At the time of zygote germination. 

Each meiotic division cycle is divided into same four stages as in mitosis. 

Prophase, Metaphase, Anaphase and Telophase. The name of each stage is 

followed by I or II depending on which division of cycle is involved. 

Meiosis I 

It consists of four stages namely. 

1. Prophase I 

2. Metaphase I 

140

3. Anaphase I 

4. Telophase I 

Prophase I 

It is the first stage of first meiosis. This is the longest phase of the meiotic 

division. It includes 5 sub stages namely 

141

1.Leptotene 2.Zygotene 3.Pachytene 4.Diplotene 5.Diakinesis 

1. Leptotene 

The word leptotene means ‘thin thread’. The chromosomes uncoil and 

become large and thinner. Each chromosome consists of two chromatids. 

2. Zygotene 

Homologous chromosomes come to-gether and lie side by side throughout 

their length. This is called pairing or synapsis. The paired chromosomes are now 

called bivalents. The adjacent non-sister chromatids are joined together at certain 

posints called chiasmata. 

3. Pachytene 

The chromosomes condense further and become very shorter and thicker. 

They are very distinct now. The two sister chromatids of each homologous 

chromosome become clearly visible. The bivalent thus becomes a tetrad with 

four chromatids. In the region of chiasmata, segments of non-sister chromatids of 

the homologous chromosomes are exchanged and this process is called crossing 

over 

4. Diplotene 

The homologous chromosomes condense further. They begin to separate from 

each other except at the chiasmata. Due to this separation the dual nature of a 

bivalent becomes apparent and hence the name diplotene. 

5. Diakinesis 

The Chromosomes continue to contract. The separation of chromosome 

becomes complete due to terminalisation. The separation starts from the 

centromeres and goes towards the end and hence the name terminalisation: 

The nucleolus and nuclear membrane disappear and spindle formation starts. 

Metaphase I 

The spindle fibres become prominent. The bivalents align on the equatorial 

plane. Spindle fibres from opposite poles get attached to the centromeres of 

homologous chromosomes. 

Anaphase I 

The two chromosomes of each bivalent (with chromatids still attached to the 

centromere) separate from each other and move to the opposite poles of the cell. 

Thus, only one chromosome of each homologous pair reachers each pole. 

142

Consequently at each pole only half the number of chromosomes (haploid) is 

received. These chromosomes are, however not the same as existed at the 

beginning of prophase. Each chromosome consists of one of its original chromatids 

and the other has a mixture of segments of its own and a segment of chromatid 

from its homologue (due to crossing over). 

Telephase I 

This is the last stage of meiosis I. Reorganization of the chromosomes at 

poles occurs to form two haploid nuclei. Nuclear membrane and nucleolus 

re-appear. The spindle disappears. There is no cytokinesis after meiosis I. The 

second meiotic division may follow immediately or after a short inter phase. The 

DNA of the two haploid nuclei does not replicate. 

Meiosis II 

The second meiotic division is very much similar to mitosis. 

Prophase II 

The events of prophase II are similar to mitotic prophase. Nucleolus and 

nuclear membrane disappear. Spindle fibres are formed at each pole. 

Metaphase II 

Chromosomes move to the centre of the equatorial plane. They get attached 

to spindle fibres centromere. 

Anaphase II 

The sister chromatids separate from one another and are pulled to opposite 

poles of the spindle due to contraction of the spindle fibres. 

Telophase II 

The chromosomes begin to uncoil and become thin. They reorganize into 

nucleus with the reappearance of nucleolus and nuclear membrane in each pole. 

Cytokinesis follows and four haploid daughter cells are formed and thus the 

meiotic division is completed. 

Significance of Meiosis 

1. Meiosis helps to maintain the chromosome number constant in each 

plant and animal species. In meiosis four haploid daughter cells are 

formed from a single diploid cell. This is very important in sexual 

reproduction during the formation of gametes. 

2. The occurrence of crossing over results in the recombination of genes. 

143

3. The recombination of genes results in genetic variation. 

4. The genetic variations form raw materials for evolution 


One Mark 


1. During this phase there is a duplication of DNA 

a. G 1 

Phase b.S phase c. G 2 

Phase d. interphase 

2. Cytokinesis is the division of 

a.cytoplasm b.nucleus c.chloroplast d.centriole 

3. Terminalisation takes place during 

a. pachytene b.zygotene c.leptotene d. diakinesis 

Two Marks 

1. Define crossing over. 

2. What is a tetrad? 

3. What is a bivalent? 

Five Marks 

1. Explain cell cycle 

2. Write notes on: significance of mitosis/significance of meiosis 

Ten Marks 

1. Describe mitosis. Add a note on it’s significane. 

2. Explain the various stages of I meiosis/II meiosis 

144

III. PLANT MORPHOLOGY 

1. Root, Stem and Leaf 

Morphology is the branch of biology that deals with form, size and structure 

of various organs of the living organisms. Each and every living organism has a 

definite form. Study of the external structure or morphology helps us to identify 

and distinguish living organisms. Knowledge of morphology of plants is also 

helpful in the study of various other fields such as genetics, plant breeding, genetic 

engineering, horticulture, crop protection and others. 

Morphology of flowering plants or Angiosperms. 

The plants which we commonly see in the gardens and road-side belong to 

the largest group of plants called flowering plants or Angiosperms. (angio=box 

Entire Plant 

L.S. of Flower 

Shoot 

system 

Leaf 

Flower 

Fruit 

Stamen 

Stamen 

Anther 

Filament 

Petal 

Ovary 

Sepal 

Gynoecium 

Pollen 

grains 

Stigma 

Style 

Ovary 

Fruit 

Stem 

Root 

system 

Primary 

Root 

Secondary 

Root 

Seedling 

Seed 

Fig : 3.1. Parts of a typical angiospermic plant (mustard) 

145

sperm=seed). The word derives its origin from the fact that the ovules are enclosed 

in a box like organ called ovary. Hence the seeds are enclosed in the fruit. 

Angiosperms include more than 2,20,000 species exhibiting a wide spectrum of 

forms and occupying a wide range of habitats. Such a wide range of flowering 

plants are identified, described and classified based on their morphology and 

anatomy. 

Parts of a Flowering Plant 

Any common flowering plant consists of a long cylindrical axis which is 

differentiated into an underground root system and an aerial shoot system. The 

root system consists of root and its lateral branches. The shoot system has a 

stem, a system of branches and leaves. Root, stem and leaves together constitute 

the vegetative organs of the plant body and they do not take part in the process of 

reproduction. The flowering plants on attaining maturity produce flowers, fruits 

and seeds. These are called the reproductive organs of the plant. 

Root System 

The root system is typically a non-green underground descending portion of 

the plant axis. It gives rise to many lateral roots. The roots do not have nodes and 

internodes. 

General Characteristic features of the root 

1. Root is positively geotropic and negatively phototropic. 

2. Roots are generally non-green in colour since they do not have chlorophyll 

pigments and hence they cannot perform photosynthesis. 

3. Roots do not have nodes and internodes; these do not bear leaves and buds. 

4. The lateral branches of the roots are endogenous in origin i.e they arise 

from the inner tissue called pericycle of the primary root. 

Regions of a typical root 

The following four regions are distinguished in a root from apex upwards. 

1. Root Cap: It is a cap like structure that covers the apex of the root. The 

main function of the root cap is to protect the root apex. 

2. Meristematic Zone or Zone of cell division: This is the growing tip of 

the root. It lies a little beyond the root cap. The cells of this region are actively 

dividing and continuously increase in number. 

3. Zone of elongation: It is a region that lies just above the meristematic 

zone. The cells of this zone increase in size. This zone helps in the growth in 

length of the plant root. 

146

4. Zone of cell differentiation : 

(Cell maturation) This is a zone that lies 

above the zone of elongation. In this 

zone the cells differentiate into different 

types. They form the tissues like the 

epidermis, cortex and vascular bundles. 

In this region a number of root hairs 

are also present. The root hairs are 

responsible for absorbing water and 

minerals from the soil. 

Types of Root System 

There are two types of root system 

1. Tap root system 

2. Adventitious root system 

Tap root system 

It develops from the radicle of the embryo. The radicle grows in to the primary 

or tap root. It produces branches called secondary roots. These branch to 

produce what are called tertiary roots. This may further branch to produce fine 

rootlets. The tap root with all 

its branches constitutes the 

Tap root system 

tap root system. Tap root 

Fibrous root system 

system is the characteristic 

feature of most of the dicot 

plants. 

Adventitious root 

system 

Root developing from 

any part of the plant other 

than the radicle is called 

adventitious root. It may 

develop from the base of the 

stem or nodes or internodes. 

The adventitious roots of a 

plant along with their 

branches constitute the 

adventitious root system. 

Region of 

elongation 

Region of 

cell division 

Root cap 

Fig : 3.2. Regions of a typical root 

Fig : 3.3. Types of root system 

Region of 

maturation 

147

In most of the monocots the primary root of the seedling is short lived and 

lateral roots arise from various regions of the plant body. They are thread like and 

are of equal size and length. These are collectively called fibrous root system. 

It is commonly found in monocot plants like maize, sugarcane and wheat. 

Functions of roots 

Roots perform two kinds of function namely primary and secondary function. 

The primary functions are performed by all the roots in general. In some plants 

the roots perform certain additional functions in order to meet some special needs. 

These are called secondary functions of the roots. In order to perform these special 

functions the roots show modification in their structure accordingly. 

Primary functions 

1. Absorption: The main function of any root system is absorption of water 

and minerals from the soil with the help of root hairs. 

2. Anchorage: The roots help to fix the plant firmly in the soil. 

Secondary functions: 

The following are some of the secondary functions performed by the roots in 

addition to the primary functions mentioned above. 

1. Storage of food 2. Additional support 3. Haustorial function 

4. Assimilation 5. Respiration 6. Symbiosis 

Root Modifications 

Besides primary functions like absorption and anchorage some roots also 

perform certain additional functions in order to meet some specific needs. These 

roots are modified in their structure to perform these special functions. 

Modification of Taproot 

1. Storage Roots: 

In some plants the tap root or the primary root becomes thick and fleshy due 

to the storage of food materials. These are called root tubers or tuberous roots. 

They are classified in to three types based on their shape. 

a. Conical : In this type the root tuber is conical in its shape i.e. it is broad at 

the base and tapers gradually towards the apex. eg. Carrot 

b. Fusiform : The root is swollen in the middle and tapers towards the base 

and the apex. eg. Radish 

c. Napiform: The root tuber has a top-like appearance. It is very broad at the 

base and suddenly tapers like a tail at the apex eg. Beet root. 

148

2. Respiratory or 

breathing roots 

In plants which grow in 

marshy places like in 

Avicennia, the soil becomes 

saturated with water and 

aeration is very poor. In these 

cases erect roots arise from 

the ordinary roots that lie 

buried in the saline water. 

These erect roots are called 

pneumatophores. They have 

a large number of breathing 

pores or (pheumatothodes) 

for the exchange of gases. 

Modifications of 

adventitious roots 

1. Storage Roots: 

In some plants the 

adventitious roots store food 

and become fleshy and 

swollen. It may assume the 

following shapes. 

a. Tuberous Roots: 

These are without any 

definite shape. Eg. 

Sweet Potato 

b. Fasciculated Root: In 

this type the tuberous 

Fusiform - Radish Napiform - Beetroot 

Conical - Carrot 

Fig: 3.4. Storage roots 

Pnem atophores 

Pores 

Fig : 3.5. Respiratory Roots (Avicennia) 

roots occur in clusters at the base of the stem eg. Asparagus, Dahlia. 

c. Nodulose Roots: In this type the roots become swollen near the tips. Eg. 

Mango ginger and turmeric 

2. Roots modified for additional support 

A. Stilt roots: These adventitious roots arise from the first few nodes of the 

stem. These penetrate obliquely down in to the soil and give support to the plant. 

eg. Maize, sugarcane and pandanus. 

149

B. Prop roots: These 

roots give mechanical 

support to the aerial 

branches as in banyan tree. 

These lateral branches grow 

vertically downwards into 

the soil. Gradually, the 

roots become thick and stout 

and act as pillars. 

3. Roots modified for 

other vital functions 

Tuberous - Sweet Potato 

Fasciculated - Dahlia 

a) Epiphytic roots: 

These are adventitious roots 

found in some orchids that 

grow as epiphytes upon the 

branches of other trees. 

These epiphytes develop 

Fig : 3.6. Storage - Adventitious roots 

special kinds of aerial 

roots which hang freely in 

Banyan 

Sugarcane 

the air. These aerial roots 

possess a special sponge 

like tissue called velamen. 

Velamen helps in 

absorbing the atmospheric 

moisture and stores them 

since these plants do not 

have direct contact with the 

soil. 

b) Photosynthetic or 

assimilatory roots: 

In some plants the 

adventitious roots 

become green and Fig : 3.7. Stilt & Prop roots 

carry on 

photosynthesis. These roots are called photosynthetic or assimilatory roots: 

In Tinospora roots arise as green hanging threads from the nodes of the 

stem during rainy season. They assimilate co 2 

in the presence of sunlight. 

150

Vanda 

Tinospora 

Leaf 

Photosynthetic 

root 

Support 

Fig : 3.9. Photosynthetic root 

Tree trunk 

Cuscuta 

Aerial roots 

Fig : 3.8. Epiphytic roots 

c) Parasitic roots or haustoria : These 

roots are found in non-green parasitic plants. 

Parasitic plants are those plants which cannot 

make their own food and they have to obtain their 

food from the host. Adventitious roots are given 

out from the nodes of these plants and these 

penetrate into the host tissue and enter in to its 

conducting tissue. From the conducting tissues 

of the host they acquire the required food 

materials.eg. Cuscuta 

Shoot System 

Fig : 3.10. Parasitic roots 

The plumule of the embryo grows into the stem which forms the main axis 

of the plant. The stem along with the leafy branches constitutes the shoot system 

of the plant. 

Characteristic features of the stem 

1. The stem is the ascending portion of the main axis of the plant. 

2. It is positively phototropic and negatively geotropic. 

3. It has well developed nodes and internodes. 

4. It has a terminal bud at the apex. 

5. The stem bears flowers and fruits. 

151

6. Lateral branches of the stem are exogenous in origin i.e they arise from 

the tissues which are in the periphery of the main axis (cortex) 

Buds: Buds are the young shoot, yet to develop. They have compressed axis 

in which the internodes are not elongated and the young leaves are closed and 

crowded. When these buds develop the internodes elongate and the leaves spread 

out. 

When a bud is found at the apex of the main 

Bryophyllum 

stem or branch it is called terminal bud or apical 

bud. When a bud arises in the axil of a leaf, it is 

known as axillary bud. Certain buds develop in 

positions other than the normal. Such buds are 

known as adventitious buds. e.g. Bryophyllum. 

In this buds arise on the leaves. These are called 

epiphyllous buds. 

Buds 

Functions of Stem: The primary functions 

of stem is 1. to support the branches and leaves. 

2. It conducts water and minerals from the roots 

to the leaves and the food materials from the 

leaves to the roots. The secondary functions of 

the stem are 1. Storage eg. Potato 2. 

Perennation e.g. Ginger 3. Vegetative Fig : 3.11. Epiphyllous buds 

Propagation e.g Potato 4. Photosynthesis e.g 

Opuntia 

Modifications of stem 

In many plants in addition to the normal functions mentioned above the stem 

performs certain additional functions. In these plants they show structural 

modifications. The additional functions may be 1. Storage of food 2. Perennation 

3. Vegetative propagation 4. Photosynthesis. 

Modified stems are grouped into the following three categories. 

1. Aerial modifications 2. Sub aerial modifications 3. Under ground 

modifications 

1.Aerial modifications 

In some plants, stem undergoes modification to a great degree to perform 

certain special functions. These are 

1. Tendrils 2. Thorns 3. Phylloclade 4. Cladode 5. Bulbil. We will discuss 

about phylloclade and cladode in detail. 

152

Phylloclade: These are 

green, flattened or cylindrical 

stems with nodes and internodes. 

The leaves are reduced to spines 

to reduce the loss of water by 

transpiration since these plants 

grow in xerophytic conditions. 

The stem becomes flat like a leaf 

and performs the functions of 

photosynthesis. eg. Opuntia. In 

this the phylloclade i.e. the stem 

performing the function of leaf 

becomes succulent due to 

storage of water and food. 

Opuntia 

Asparagus 

Phylloclade 

Spine 

Fig : 3.12. Phylloclade & Cladode 

Cladode 

Scaly 

leaf 

Cladode: These are green, cylindrical or flattened stem branches of limited 

growth. These are usually of one internode as in Asparagus. Their stem nature is 

evident by the fact that they bear buds, scales and flowers. 

2.Sub-aerial modifications: 

This type of modification is found in many herbaceous plants with a thin, 

delicate and weak stem. In such plants a part of the stem is aerial and the remaining 

part lives underground. These plants bear adventitious roots and aerial branches 

at their nodes. They propagate quickly by vegetative methods. Sub-aerial modified 

stems are of the following types: 

1. Runner 2. Sucker 3. Stolon 4. Offset 

We will discuss about Runner and Sucker. 

1. Runner : It has long and thin internodes and the branches creep over the 

surface of the soil. They develop adventitious roots from the lower sides of 

the nodes. From the axil of the scale leaves at the nodes arise aerial branches. 

Runners grow in all directions from the mother plant. On detachment from 

the mother plant the daughter plant propagate in a similar manner. Thus very 

soon a whole area is covered by many plants from a single plant. Eg. Doob 

grass, oxalis 

2. Sucker: It is a modified runner. In this the runner originates as a lateral 

branch from the underground axillary bud of an aerial shoot. It grows down 

in to the soil obliquely for some distance and then grows upwards. The sucker 

has nodes and internodes and in the nodal region it bears scale leaves and 

axillary buds above and adventitious roots below. Eg. Chrysanthemum 

153

3. Underground modifications: 

Oxalis 

Some plants develop non-green 

underground stem which are perennial i.e 

they live for many years. These store 

reserve food, and are adapted for 

perennation. During favorable conditions 

underground stems give rise to aerial 

Runner 

shoots. With the onset of unfavourable 

Chrysanthemum 

conditions the aerial shoots die. During 

this period the underground stems remain 

dormant. 

These underground stems can be 

distinguished from the roots by the 

Sucker 

following. 

Fig : 3.13. Runner & Sucker 

a. Presence of nodes and internodes 

b. Presence of scale leaves and adventitious roots arising from the nodes. 

c. Presence of axillary and terminal bud. 

The four different types of underground stem are 1. Rhizome 2. Tuber 

3. Bulb 4. Corm 

a. Rhizome: Rhizomes are horizontal, thick, stout underground stems. They are 

swollen with the storage of food materials. They have nodes and internodes. 

The nodes have brown scaly leaves which protect the axillary buds. The 

nodes bear adventitious roots on the lower side. At the onset of favourable 

condition the axillary and terminal buds grow into aerial shoots. These aerial 

shoots die on the approach of unfavourable condition. eg.Ginger, Turmeric 

Advantages of Rhizomes: Rhizomes are very good means of perennation. 

They help to tide over the unfavourable conditions like drought etc. They serve as 

store houses of food which is safely protected from the grazing of animals. Since 

aerial shoots arise from the buds of the rhizome they are useful in vegetative 

propagation also. 

b. Tuber: Tubers are the swollen tips of special underground branches. They 

are different from rhizomes in that they are stouter, with slender internodes 

and the adventitious roots are generally absent. The tuber bears many scale 

leaves with axillary buds in the nodes. Potato is a common example for a 

tuber. It has got small depressions on it called the eye of the potato. It bears 

the bud. When the tuber is planted in the soil the buds develop into branches 

154

Scale 

leaf 

Ginger 

Bud 

Potato 

Eye spot 

Node 

Bud 

Fig : 3.14. Rizome and Tuber 

at the expense of the food material stored in the tuber. Some of these branches 

become aerial and green and erect while others grow horizontally underground 

and their tips become swollen with food materials. 

Leaf 

Leaves are green, thin flattened lateral outgrowths of the stem. They are 

borne at the nodes of the stem. Leaves are the chief organs of photosynthesis. 

The green leaves of the plant are collectively called as foliage of the plant. 

Parts of a Leaf 

Morphology of 

Flowering plants 

The three main 

Banana Leaf 

Apex 

Clitoria 

parts of a typical leaf 

are 1. Leaf base 2. Blade 

Petiole 3. Lamina 

Vein 

Midrib 

Leaf base : The 

part of the leaf which is 

attached to the stem or 

a branch is called leaf 

base. In some plants 

the leaf has a swollen Petiole 

Sheathing 

leaf base. It is known 

leaf base 

Stipule 

Leaf Base 

as pulvinus eg. The 

compound leaves of the 

family Fabaceae. In 

Fig : 3.15. Parts of a typical leaf & Pulvinus &Sheathing leaf base 

monocots the leaf base 

is very broad and flat and it clasps a part of the node of the stem as in maize and 

in banana. It is called sheathing leaf base. 

Stipules: In most of the dicotyledonous plants, the leaf-base bears two lateral 

appendages called the stipules. Leaves which have the stipules are called stipulate. 

155

The leaves without stipules are called exstipulate. The main function of the 

stipule is to protect the leaf in the bud. 

Petiole : Petiole connects the lamina with the stem or the branch. A leaf is 

said to be petiolate when it has a petiole. It is said to be sessile when the leaf 

does not have a petiole. 

Leaf blade: It is also known as lamina. This is the most important, green 

part of the leaf which is mainly concerned with the manufacture of food. The 

lamina is traversed by the midrib from which arise numerous lateral veins and 

thin veinlets. 

Venation 

The arrangement of veins in the leaf blade or lamina is called venation. It is 

mainly of two types namely Reticulate venation and Parallel venation. 

1. Reticulate 

Venation: This type of 

venation is common in 

all dicot leaves. In this 

type of venation there is 

a prominent vein called 

the midrib from which 

arise many small veins 

which finally form a net 

like structure in the 

lamina. It is of two 

types 

Pinnately reticulate 

venation : In this type of 

venation there is only one 

midrib in the center which 

forms many lateral branches to 

form a net work. eg. Mango 

2. Parallel Venation: In 

this type of venation all the 

veins run parallel to each other. 

Most of the monocot leaves 

have parallel venation. It is of 

two types. 

Pinnately 

Reticulate 

Pinnately 

Parallel 

Palmately 

Reticulate 

(Divergent) 

Fig : 3.16. Types of Reticulate Venation 

Palmately Parallel 

(C onvergent) 

Fig : 3.17. Types of Parallel Venation 

Palmately 

Reticulate 

(Convergent) 

Palmately Parallel 

(D ivergent) 

156

VENATION 

Reticulate 

Parallel 

Pinnately Palmately Pinnately Palmately 

Reticulate Reticulate Parallel Parallel 

a. Pinnateley Parallel venation : In this type, there is a prominent midrib in 

the centre. From this arise many veins perpendicularly and run parallel to each 

other eg. Banana. 

b. Palmately parallel 

Alternate - Polyalthia 

venation : In this type several 

veins arise from the tip of the 

petiole and they all run parallel 

to each other and unite at the 

apex. In grass they converge 

at the apex and hence it is 

called convergent. In 

Borassus (Palmyra) all the 

main veins spread out towards Superposed - Guava 

Decussate - Calotropis 

the periphery. Hence it is 

called divergent. 

Phyllotaxy: The 

arrangement of leaves on the 

stem or the branches is known 

as phyllotaxy. The purpose 

of phyllotaxy is to avoid 

overcrowding of leaves so as 

to expose the leaves maximum 

Ternate - Nerium 

W horled - Allamanda 

to the sunlight for 

photosynthesis. The four main 

types of phyllotaxy are 

1. Alternate 2. Opposite 

3. Ternate 4. Whorled. 

Fig : 3.18. Types of Phyllotaxy 

157

1.Alternate phyllotaxy: In this type the leaves are arranged alternatively in 

the nodes. There is only one leaf at each node. eg. Polyalthia. 

2.Opposite Phyllotaxy: In this type of arrangement two leaves are present 

at each node, lying opposite to each other. It is of two types: 

a) Opposite superposed: The pairs of leaves arranged in successive nodes 

are in the same direction i.e two opposite leaves at a node lie exactly above 

those at the lower node eg. Guava 

b) Opposite decussate: In this type of phyllotaxy one pair of leaves are 

placed at right angles to the next upper or lower pair of leaves. Eg. Calotropis 

3.Ternate Phyllotaxy : In this type there are three leaves attached at each 

node eg. Nerium 

4. Whorled : In this type, more than three leaves are present in a whorl at 

each node eg. Alamanda. 

Simple and compound leaves 

Simple Leaf: A leaf is said to be simple in which the leaf blade or lamina is 

entire. It may be with incision or without incision. e.g. Mango 

Compound leaf: Here the lamina is divided in to a number of leaf like lobes 

called the leaflets. The leaflets are borne on a common axis and they do not bear 

any axillary buds in their axils. The two types of compound leaf are: 

1. Pinnately compound leaves 2. Palmately compound leaves 

Pinnately compound leaves 

In a pinnately compound leaf, the leaflets are borne on a common axis called 

the rachis. The leaflets are known as the pinnae. The pinnately compound leaf 

may be of the type 1. Unipinnate 2. Bipinnate 3. Tripinnate 4. Decompound 

Unipinnate 

(Paripinnate) 

Unipinnate 

(Im paripinnate) 

Bipinnate 

Tripinnate 

Fig : 3.19. Types of Pinnately compound leaves 

158

1.Unipinnate: In 

Digitate - Bombax 

this type the pinnae are 

borne directly on the 

rachis. When the 

number of leaflets is 

odd, it is said to be 

imparipinnate eg. 

Digitate - Gynadropsis 

U nifoliate - Pum m elo 

Neem .When the 

number of leaflets is 

even it is said to be 

paripinnate eg. 

Tamarind. 

Fig : 3.20. Types of Palmately compound leaves 

2.Bipinnate: In this type of compound leaves, the primary rachis is branched 

to produce secondary rachis which bear the leaflets. eg. Acacia. 

3.Tripinnate: In this type the secondary rachis produces the tertiary rachis 

which bear the leaflets eg. Moringa 

4.Decompound : When the compound leaf is more than thrice pinnate it is 

said to be decompound. eg. Coriander 

Palmately compound leaf 

When all the leaflets are attached at a common point at the tip of the petiole, 

it is known as palmately compound leaf. According to the number of leaflets 

present the compound leaf may be 1. unifoliate (eg. Lemon) 2. Bifoliate 

(eg.Zornia diphylla) 3. Trifoliate (eg. Oxalis) 4. quadrifoliate (eg. Marsilia) 

5. Multifoliate (eg. Bombax) 

Leaf Modification 

The primary functions of leaf are photosynthesis and transpiration. But in 

many plants the leaves are modified to perform some additional functions. These 

are called as leaf modifications Some of the leaf modifications are: 

Table : 3.1. Differences between a simple leaf and a compound leaf: 

Simple Leaf 

Compound Leaf 

1. Axillary bud is present in the Axillary bud is present in the axil of a compound 

axil of a simple leaf 

leaf. But the leaflets of a compound leaf do not 

have them. 

2. Stipules are present at the baseof 

Stipules are not present at the base of the 

simple leaves. 

Leaflets. 

3. The simple leaf may have incisions The compound leaves are divided into distinct 

but these incisions are not deep enough to parts called leaflets. 

divide the blade into leaflets. 

159

1. Leaf tendrils (eg. Wild pea) 2. Leaf 

hooks (eg. Bignonia) 3. Leaf spines (eg. 

Zizyphus) 4. Phyllode (eg. Acacia) 5. 

Pitcher (Nepenthes) 6. Bladder eg. 

(Utricularia) 

1. Leaf tendrils : Here the stem is 

very weak and hence they have some 

special organs for attachment to the 

support. Tendril is a slender wiry coiled 

structure which helps in climbing the 

support. In Lathyrus the entire leaf is 

modified into tendril. In Smilax the 

stipules become modified into tendril. 

2 Leaf hooks: In this the leaves are 

modified into hooks and help the plant to 

climb the support. In Bignonia unguiscati 

, the three terminal leaflets of the 

compound leaves become stiff, corved and 

claw like hooks. 

Stipules 

Tendril 

(Leaf) 

3. Leaf-spines : In this type the leaves become wholly 

or partially modified into sharp pointed structures known 

as spines. This modification helps the plant to cut down 

transpiration and also protects the plants against the attacks 

of grazing animals. Any part of the leaf may get modified 

in to spine. e.g. Zizyphus 

4. Phyllode: In Acacia the petiole or any part of the 

rachis becomes flattened or winged taking the shape of the 

leaf and turning green in colour. This flattened 

or winged petiole or rachis is known as the 

phyllode. The normal leaf which is pinnately 

compound develops in the young stage, but 

soon falls off. The phyllode then performs all 

the functions of the leaf. The wing of the 

phyllode normally develops in the vertical 

direction so that sunlight cannot fall on its 

surface; this reduces evaporation of water. 

There are about 300 species of Australian 

Acacia, all showing the phyllode. 

Lathyrus 

Phyllode 

(Petiole) 

Smilax 

Zizyphus 

Tendril 

(Stipules) 

Stipule 

Fig : 3.22. Leaf Spines 

Acacia 

Stem 

Fig : 3.23. Phyllode 

Bignonia 

Hooks 

Fig : 3.21. Leaf Tendrils & Leaf Hooks 

Leaflet 

160

5. Pitcher 

In the pitcher plant 

(Nepenthes) the leaf becomes 

Lid 

modified into a pitcher. There is 

a slender stalk which coils like a 

tendril holding the pitcher vertical 

Mouth of 

and the basal portion is flattened 

Pitcher 

like a leaf. The pitcher is 

provided with a lid which covers 

the mouth. The function of the 

pitcher is to capture and digest 

insets. The lamina is modified 

into pitcher. The rim of the 

pitcher is beautifully coloured 

and it is provided with a row of 

nectar glands for attracting 

Fig : 3.24. Insectivorous Plant 

insects. The inner wall of the 

pitcher is provided with glands secreting a watery fluid. There are also hairs 

pointed downwards below the rim. This 

arrangement prevents the insects entering the 

pitcher from escaping out. The insects get 

drowned in the fluid and it is digested by 

the enzymes secreted by the glands. Thus 

the plant is able to get nitrogenous food. 

6. Bladder 

In utricularia some of the much 

dissected leaves are modified into bladders. 

These bladders serve as floats for the aquatic 

plants and for trapping the insects. 

Nepenthes 

Utricularia 

Whole plant 

Bladder 

Tw ig 

Single 

Bladder 

Bristles 

Bladder 

Fig : 3.25. Bladder Plant 

161


One Mark 


1. The type of phyllotaxy found in Calotropis is 

a. alternate b.opposite decussate c. opposite superposed d. ternate 


1. In Bignonia unguiscati__________ become stiff, claw like hooks. 

2. In ____________, tuberous roots which have no definite shape are seen. 

Match 

1. Moringa - Pneumatophores 

2.Lemon - Tripinnate 

3.Acacia - Bladder 

4.Utricularia - Phyllode 

5.Lathyrus - Unifoliate 

6.Avicennia - Tendril 

Two Marks 

1. What is meant by exogenous / endogenous origin? 

2. Name any two vegetative organs/ reproductive organs of a flowering plant. 

3. Write any two characteristic features of root/ shoot. 

4. Define: adventitious roots/ root cap/ meristematic zone/ pulvinus/ bud 

5. What is an epiphyllous bud? 

6. What are the advantages of rhizome? 

7. What are pneumatophores? 

Five Marks 

1. Describe the parts of a typical root. 

2. Describe the two types of root system with suitable examples. 

3. Write about the functions of roots? 

4. Describe phyllode/ phylloclade 

5. Describe the pitcher plant. 

6. Distinguish a simple leaf from a compound leaf. 

Ten Marks 

1. Describe the modifications of Tap root system/ adventitious root system/ stem/ 

leaf. 

2. Describe the various types of venation/ phyllotaxy 

162

2. Inflorescence 

The reproductive organs of flowering plants are flowers. The flowers are 

produced after a period of vegetative growth. The flowers may be borne singly or 

in clusters. When borne singly they are said to be solitary (eg) Hibiscus rosa 

sinensis (shoe flower), if in clusters they form an inflorescence. 

Inflorescence 

When several flowers arise in a cluster on a common axis, the structure is 

referred to as an inflorescence. The common axis is the inflorescence axis which 

is also called rachis or peduncle. Several single flowers are attached to the 

inflorescence axis. In case of plants possessing underground rhizomes, the rachis 

or peduncle arises directly from the rhizome. Such a rachis is referred to as scape. 

In the case of lotus, the scape gives rise to a solitary flower. In plants like onion, 

the scape gives rise to an inflorescence. 

Based on the location, the inflorescence may be classified into 3 types. 

(i) Terminal Inflorescence (ii) Intercalary Inflorescence and (iii) Axillary 

Inflorescence. 

In plants like Callistemon the inflorescence is found in between the stem. 

This is called intercalary inflorescence. 

Generally, based on the arrangement, structure and organisation of flowers on 

the axis, inflorescence are classified into various types. There are four major 

types. 

i) Racemose 

ii) Cymose 

iii) Mixed and 

iv) Special types 

Racemose Inflorescence 

In this type, the inflorescence axis shows unlimited growth. Several flowers 

arise in acropetal succession on the axis. The younger flowers are found at the tip 

and older flowers are found towards the base of the inflorescence axis. The order 

of opening of flowers is centripetal i.e. from the periphery towards the centre. 

Racemose inflorescence may be sub-divided into various types based on branching 

of inflorescence axis, length of the axis and presence or absence of pedicels in 

flowers. 

163

i. Main axis elolngated 

Here the inflorescence axis is very much elongated and bears pedicellate or 

sessile flowers. This may include several types. 

164

Simple Raceme 

This is a very simple type of inflorescence. The axis shows unlimited growth. 

Numerous pedicellate flowers are arranged from base to apex in acropetal 

succession. Each flower arises in the axil of a bract eg. Crotolaria retusa, Cleome 

viscosa. 

Compound Raceme or Panicle 

In this type, the inflorescence axis is branched. Each branch shows flowers 

arranged as in a simple raceme i.e. in acropetal sucession. eg. Mangifera. 

Spike 

This inflorescence shows an axis of unlimited growth as in raceme but the 

flowers are sessile and are arranged in acropetal order eg. Achyranthes and Piper 

longum. 

Sim ple Raceme - Crotolaria 

Compound Raceme - Mangifera 

Spike - Achyranthes 

Spadix - Arum Compound Spadix - Cocos 

M ain Axis Elongated 

Fig : 3.26. Types of Racemose Inflorescence 

165

Compound Spike 

The inflorescence axis is branched and each branch is referred to as spikelet. 

Each spikelet bears a few flowers only. The base of the inflorescence shows a 

pair of bracts called glumes. Each flower has a bract called lemma and a bracteole 

called palea eg. Oryza (Paddy). 

Spadix 

The inflorescence axis is swollen and fleshy. Numerous sessile flowers 

arranged in acropetal order are embedded in the axis. The entire inflorescence is 

protected and covered by a large bract called spathe. The base of the axis bears 

female flowers, and the sterile flowers and male flowers are borne towards the 

top. The tip of the inflorescence axis does not bear flowers. eg. Arum, Colocasia. 

Compound Spadix 

The swollen and fleshy inflorescence axis is branched and bears sessile flowers. 

There is a thick and large boat-shaped bract called spathe covering the inflorescence 

eg. Cocos. 

ii. Main Axis Shortened 

Here the main axis shows reduced growth and is shortened. Corymb belongs 

to this type. 

Corymb 

The inflorescence 

axis in this type is not 

elongated as in raceme. 

The pedicels of the 

flowers are of unequal 

length. The older 

flowers have long 

pedicels and the 

Corymb Caesalpinia 

younger flowers show 

short pedicels. So all 

Fig : 3.27. Main Axis Shortened 

flowers appear at the same level. eg. Caesalpinia. 

iii. Main Axis ending in flowers 

There are two types under this-umbel and compound umbel. 

Umbel 

The main axis may be simple or branched. But the vertical growth of the axis 

is suddenly stopped and a whorl of bracts arise at the tip of the inflorescence. This 

166

is called involucre of bracts from the axils of which arise flowers having pedicels 

of equal length. The flowers are in acropetal order and present at the same level. 

eg. Allium cepa (Onion). 

Compound Umbel 

The main axis of the umbel inflorescence produces an involucre of bracts 

which give rise to branches called rays from their axils. Each ray produces an 

involucre of bracts at its tip from the axils of which arise flowers having pedicels 

of equal length in acropetal order. Each such umbel is called an umbellet. eg. 

Daucas carota (carrot). 

Umbel - Allium cepa 

Compound Umbel - Daucas carota 

Main axis ending in flowers 

Floret 

Receptacle 

Involucre 

Head or capitulum - Helianthus 

M ain axis flattened 

Fig : 3.28. Types of Racemose Inflorescence 

167

Main axis flattened 

The main axis is flattened and assumes various shapes. On the flattened axis 

flowers are arranged. 

There are two types under this - head or capitulum and compound head. 

Head or Capitulum 

The main axis of the inflorescence is flattened and functions as the thalamus. 

This bears numerous florets in acropetal order. The inflorescence is surrounded 

by an involucre of bracts which are green in colour and protect the young flowers 

and fruits. 

The florets of the inflorescence are sessile and are of two types. 1. The 

tubular or disc florets and 2. The ligulate or ray florets. Based on the type of 

florets present, the head inflorescence may be of two types - Homogamous head 

and Heterogamous head. 

Homogamous Head 

This type shows florets of a single kind only which may be ray or disc florets 

eg. Vernonia shows only disc florets and Launaea shows ray florets only. 

Heterogamous Head 

The florets present here belong to both ray and disc type. The disc florets are 

present in the centre of the thalamus while the ray florets radiate outwards from 

the margins of the thalamus. eg. Helianthus, Tridax. 

Compound Head 

In Lagasca mollis the inflorescence axis is branched and each branch bears 

a head inflorescence. 

Cymose Inflorescence 

Inflorescence axis shows limited growth. The tip of the inflorescence stops 

growing after producing a flower. The lateral pair of bracts at the base of the 

flower give rise to lateral branches each of which ends in a flower. Similarly the 

lateral pair of bracts of each of these branches may also form branches. In this 

way flowers are formed in basipetal order i.e. from apex to base. The older 

flowers is at the tip and younger flower is at the base and the order of opening of 

flowers is centrifugal i.e. from cnetre to periphery. The flowers are few in number. 

Cymose inflorescence is of various types. 

168

Simple Cyme 

The stem or the axil of leaf may show a single flower which shows a joint on 

the pedicel. Such flowers are referred to as terminal solitary cyme and axillary 

solitary cyme respectively. eg. Papaver - Terminal solitary cyme, Hibiscus - 

Axillary solitary cyme. 

Simple Dichasium 

It is a group of three flowers. The inflorescence axis ends in a flower. the two 

lateral bracts at the base of the flower give rise to branches ending in a flower. 

Thus, there are three flowers in the inflorescence and the central flower is the 

oldest eg. Jasminum. 

Simple Dichasium - Jasminum 

M onochasial H elicoid 

Cyme - Hamelia 

Monochasial Scorpioid 

Cyme - Heliotropium 

Compound Dichasium - Clerodendron 

Polychasial Cym e - Nerium 

Fig : 3.29. Types of Cymose inflorescence 

169

Compound Dichasium 

The tip of the inflorescence ends in a flower. From the lateral bracts of this 

flower a pair of branches arise, each ending in a flower. Each of the branches 

bears a pair of bracts and these also give rise to a pair of lateral branches each. 

Thus symmetrical bunches of three flowers each are formed where the central 

flower is the oldest. eg. Clerodendron. 

Monochasial Cyme 

The inflorescence axis terminates in a flower. Of the two lateral bracts only 

one bears flowers. Such a cyme is called a monochasial cyme. This is of two 

types - Helicoid cyme and Scorpioid cyme. 

Helicoid Cyme 

The main axis terminates in a flower. The lateral branches arising from the 

axile of bracts are on one side only giving rise to a helical appearance. eg. Hamelia 

patens. 

Scorpioid Cyme 

The main axis stops growing after producing a flower.The lateral branches 

arising from the axil of bracts are produced alternately to the left and to the right in 

a zig-zag manner eg. Heliotropium. 

Polychasial Cyme 

The main axis terminates in a flower. The lateral branches formed from the 

bract continue to branch repeatedly eg. Nerium. 

III. Mixed Inflorescence 

In this type of inflorescence, the axis starts as a racemose inflorescence and 

shows branching in a cymose fashion. There are different types under this. 

Thyrsus 

The main axis of the inflorescence shows a number of simple dichasial cymes 

arranged in a racemose manner eg. Ocimum. 

Verticillaster 

A pair of dichasial cymes arise from the axils of opposite flowers. Later 

these grow as monochasial scorpioid cymes around the stem eg. Leucas. 

Mixed Spadix 

In Musa, several cymose clusters are arranged on the swollen inflorescence 

axis from base to apex. Each cymose cluster is surrounded by a large bract called 

spathe. 

170

IV. Special Type of 

Inflorescence 

The type of 

inflorescence which cannot 

be included in racemose type 

or cymose type is called 

special type. There are 

several kinds of special type 

inflorescence. 

Cyathium 

This is found in the 

genus Euphorbia. The 

inflorescence is reduced to 

look like a single flower. The 

bracts are united to form a 

cup - like structure enclosing 

a convex receptacle. There are 

a number of reduced unisexual 

flowers on the receptacle. There 

is a single female flower in the 

centre of the receptacle. It is 

naked, represented by the 

gynoecuim only and borne on a 

long stalk. Around the female 

flower five groups of naked male 

flowers are arranged in a 

monochasial scorpioid cymes. 

The male flower is represented 

by a single stamen arising in the 

axil of a bract. The top of the 

inflorescence shows the presence 

of beautiful nectaries. eg. 

Euphorbia cyathophora. 

Hypanthodium 

Here the receptacle is 

concave and cup shaped. The 

Thyrus - Ocimum 

Verticillaster - Leucas 

Flowers 

Spathe 

Entire V.S 

Mixed spadix - Musa 

Fig : 3.30. Types of Mixed Inflorescence 

V.S. of 

inflorescence. 

Entire 

Hypanthodium - Ficus 

Female 

Flower 

Gall 

Flower 

Cyathium - Euphorbia 

Spathe 

Axis 

Male 

Flower 

Vertical 

Section 

Fig : 3.31. Types of Special Inflorescence 

171

upper end has an opening called ostiole, which is protected by scales. Inside the 

receptacle, three types of flowers are present. Male flowers are present in the 

upper part, female flowers towards the base and the neutral flowers are found in 

the middle between the male and female flowers eg. Ficus. 

Coenanthium 

Here the receptacle is fleshy and appears like a circular disc like structure. 

The centre of the disc contains female flowers and around these are present the 

male flowers eg. Dorstenia. 

SELF EVALAUTION 

One Mark 

Choose the correct anser 

1. Spike is a type of 

a. Racemose inflorescence b. Cymose inflorescence 

c. Mixed inflorescence d. Special inflorescence 

2. Dorstenia an example for 

a. raceme b. panicle c. spadix d. coenanthium 

3. This is a homogamous head with ray florets 

a. Vernonia b. Tridax c. Launaea d. Helianthus 

4. Musa in an example for 

a. spadix b. mixed spadix c. compound spadix d. none of the above 

5. Flowers are unisexual in 

a. cyathium b. thyrsus c. verticillaster d. cyme 

Two marks 

1. Define : Ligulate floret / Hypanthodium / Corymb / Involucre / Umbellet 

Five Marks 

1. Describe the different types of mixed inflorescence with examples 

2. Give an account of head inflorescence 

3. Classify cymose inflorescence and explain any two of them. 

4. Give an account of special types of inflorescence. 

Ten Marks 

1. Give an outline classifications of racemose types of inflorescence. 

2. Write an essay on the various types of racemose inflorescence? 

3. Describe the various types of cymose inflorescence. 

172

3. Flowers, Fruits and Seeds 

Structure and types of flower 

A flower is a modified condensed shoot specialized to carry out sexual 

reproduction in higher plants. Like a branch, it arises in the axil of a small leaf-like 

structure called bract. The terminal part of the axis of a flower, which supports all 

the floral appendages (i.e., sepals, petals, stamens and carpels) is called 

receptacle (thalamus or torus). The receptacle consists of several crowded 

nodes which are separated by condensed internodes. The internode of the branch 

that lies below the receptacle is called pedicel. A bract is usually situated at the 

base of pedicel. Sometimes small leaf-like structures are present in the middle of 

pedicel. They are called bracteoles. 

FLOWER - A Metamorphosed Shoot 

The concept that the flower is a modified or a metamorphosed shoot for the 

purpose of reproduction is an old one and the concept is gradually developed 

through the past and is accepted at the present by a majority of morphologists. 

Linnaeus expressed this idea in his Philosophia Botanica (1751) by the phrase 

“vegetative metamorphosis”. This concept that floral leaves were a modification 

of vegetative leaves was further elaborated by Caspar Wolff and Decandolle. 

The ‘foliar theory’ of the flower of the earlier botanists is held today by many 

though modified in one form or other by other botanists. 

That the flower is a modified shoot, is only a figurative expression, and implies 

that the floral leaves are vegetative leaves and transformed to do a different function 

of reproduction, in the place of the ordinary function of photosynthesis. 

Evidences to support that flower is a modified shoot 

1. The position of flower buds and shoot buds is same, i.e., they are terminal 

or axillary in position. 

2. In some plants, the flower buds are modified into vegetative buds or bulbils, 

eg. Agave, Onion, etc. 

3. In some plants, the thalamus elongates to form a vegetative branch or 

another flower above the first flower, e.g. Rose. 

4. In Nymphaea (Water Lily), the flowers show all transitional stages 

between a sepal and petal and between a petal and stamen. 

173

5. In Gynandropsis gynandra, the thalamus elongates and shows long 

internodes between the floral organs. 

6. In rose, the sepals are similar in morphology to leaves. 

7. In Degeneria, the stamens are expanded like leaves and the carpels appear 

like folded leaves without differentiating into stigma and style. 

8. Anatomy of the thalamus, pedicel and stem show close similarities. The 

vascular supply of different floral appendages resemble the vascular supply 

of ordinary vegetative leaves. 

Position of flower 

A flower is usually seen either at the axil of a leaf or at the apices of the stem 

and its branches. Accordingly, the flower is described as axillary and terminal 

respectively. 

Flower, whether solitary or in inflorescence, usually has a short stalk called 

pedicel. A flower with stalk is described as pedicellete and a flower without 

stalk is called sessile. 

174

Parts of a flower 

A typical flower consists of following parts: 

1. Bracts and Bracteoles 

2. Thalamus 

3. Whorls of flower 

a. Calyx 

b. Corolla 

c. Androecium 

d. Gynoecium 

Essential and Non-Essential Parts 

Of the four parts of a flower, androecium and gynoecium are known as 

essential organs because they have a direct role in reproduction i.e. pollination and 

fertilization which lead to development of fruit and seeds from flower. The calyx 

and corolla do not have a direct role in these processes. Hence they are described 

as non-essential organs or accessory organs. 

Bracts and Bracteoles 

Bracts are special leaves at whose axil flowers develop. For example, in an 

axillary flower, the leaf from whose axil the flower develops becomes the bract. 

But bracts are not always present. If a bract is found, the flower is called bracteate; 

if it is absent then the flower is described as ebracteate. When bracts are present, 

they protect flower buds in the young stage. Sometimes small and thin bract-like 

structures are present on the pedicel between the flower and the bract. These are 

called bracteoles. the bracteoles may be one or two in number. Flowers having 

bracteoles are described as bracteolate and flowers where bracteoles are absent 

are called ebracteolate. 

The receptacle (thalamus) 

The thalamus is the short floral axis, with compressed nodes and internodes 

on which various floral leaves are inserted. 

Variation of the Receptacle 

In a few cases, internodes become distinct and elongated. The elongated 

internode between the calyx and corolla is the anthophore as in Caryophyllaceae. 

The internode elongated between the corolla and the androecium is called the 

androphore eg. Passiflora (family - Passifloraceae). 

175

The elongated internode between the androecium and the gynoecium is called 

the gynophore as in Capparis [Capparidaceae] When both androphore and 

gynophore are present, they are called gynandrophore or androgynophore e.g. 

Gynandropsis. When the thalamus is prolongated beyond the ovary, it is called 

the carpophore as in the Coriander, Foeniculum etc. 

Insertion of floral leaves on the thalamus 

Hypogyny 

When the thalamus 

is convex or elongated, 

the carpel occupies the 

top most position on it. 

The other floral members 

(sepals, petals, and 

stamens) are placed 

below them. This mode of 

arrangement is called 

hypogyny. The flower is 

described as hypogynous. 

The ovary is known as 

superior. eg. Malvaceae, 

Annonaceae etc. 

Epigyny 

Hypogyny 

Stamen 

Petal 

Ovary 

Sepal 

Thalamus 

Perigyny 

Petal 

Stamen 

Epigyny 

Ovary 

Sepal 

Thalamus 

Fig : 3.33. Insertion of floral leaves 

Petal 

Stamen 

Sepal 

Ovary 

Thalamus 

When the thalamus is cup shaped, the lower part of the ovary is situated at 

the bottom of the cup and also fused with the inner wall of thalamus. The other 

floral members appear to be inserted upon the ovary. This mode on arrangement 

is called epigyny. Then the flower is said to be epigynous. the ovary is said to be 

inferior. eg. Asteraceae, Cucurbitaceae, Rubiaceae etc. 

Perigyny 

In this condition, the receptacle is flat or slightly cup-shaped. The carpels are 

situated at its centre and other floral members are inserted on its margin. This 

mode of arrangement is called perigyny. The flower is known as perigynous. In 

this case, the ovary is still described as half inferior. eg. Fabaceae, Rosaceae 

etc. 

The Perianth 

Most flowers of monocot plants have perianth, where there is no difference 

between calyx and corolla. In families of monocotyledons, the perianth is brightly 

176

coloured and highly developed, which is known as Petaloid perianth as in Gloriosa 

superba. Some families of dicotyledons have also petaloid perianth e.g. 

Polygonaceae. 

The function of the perianth leaves is to protect the inner part of the flower. 

When brightly coloured, they attract insects for pollination. 

Calyx 

The calyx is the outermost whorl of a flower composed of sepals. The sepals 

are usually green in colour, but sometimes, become brightly coloured then, said to 

be petaloid as in Caesalpinia pulcherrima. in Musseanda frondosa the sepals 

are transformed into large, yellow or white and leafy structure. 

The primary function of the calyx is protective. It protects the inner parts of 

the flower from mechanical injury, rain and excessive sun shine, and from drying 

out in the bud condition. Green in colour, it can also do the phosynthetic function. 

When petaloid, it performs the function of attracting insects for pollination. When 

spiny, its function is defensive and as pappus, it helps in the dispersal of fruit. 

The calyx may be regular or irregular. The sepals are free from one another 

and is said to be ploysepalous, when united united, gamosepalous. 

Variations of calyx 

The calyx may sometimes be absent or modified into scaly structure as in 

Sunflower. 

In some cases, it is modified into a bunch of hair - like structures called 

pappus eg. Vernonia. 

Duration of Calyx 

After the opening of the flower, the calyx usually falls off but it may persist in 

some cases. 

According to its duration, it may be described as follows: 

1. Caducous or Fugacious:Sometimes the calyx falls off, even before 

flowers are opened and such a calyx is said to be caducous.eg.Papaver, 

magnolia etc. 

2. Deciduous:When it falls off after the opening of the flower, it is said to 

be deciduous. (eg) Nelumbo 

3. Persistent: In somw other cases, when the calyx persists (unwithered) 

even after fruit formation, it is said to be persistent. eg. Brinjal, 

177

Corolla 

4. Accresent: Calyx not only persistent but also grows along with 

development of the fruit. eg. Physalis. 

The corolla is the second accessory floral whorl consisting of petals. 

The petals of the corolla are usually variously coloured and of delicate texture. 

They may be free (polypetalous) or united (gamopetalous). The primary function 

of the corolla is to attract insects for polinatioin and also serves to protect the 

essential organs. 

Shape of the petals in the corolla 

I. Clawed: The petal is narroe and slender at the base as a claw eg. Petals of 

Cruciferae. 

Cruciform 

Caryophyllaceous 

Rosaceous 

Papilionaceous 

Tubular Campanulate Infundibuliform Rotate 

Hypocrateriform 

Urceolate Bilabiate Personate Ligulate 

Fig : 3.34. Forms of corolla 

178

II. 

Filmbriate: Petals fringed with hairy, teeth-like structure_eg. Dianthus 

III. Laciniate: Petal divided into several long more or less equal segments. 

IV. Spurred: Corola with a long hollow projection called spur eg. Delphinium majus. 

V. Saccate: THe lower part of the corrolla tube gets dilated to form a sac-like 

structure eg. Antirhinum. 

Forms of Corolla 

A Polytalous and Regular 

i. Cruciform: When the corolla consists of four clawed petals arranged at 

right angles to one another. eg. Brassica, Radish, etc., 

ii. 

Caryphyllaceaus: when the corolla consists of five clawed petals with 

spreading limbs; claws and limbs are at right angles to one another. eg. 

Carroypyllaceae 

iii. Rasaceous: when the corolla consits of five spreading petals, without any 

claw eg. Wild Rose. 

B Polyetalous and Irregular 

i. Papillonaceous: whent he corolla consists of 5 petals, one large - the 

vexillum or standard petal which is posteior and outemost, two lateralsalae 

or wings at the sides and two partially fused structures - the keel or carina. 

eg. Pea, etc. (Fam. Fabaceae). 

ii. Orchidaceous: flowers with a peculiarity of combining calyx and corolla: 

One member, the petal in from of the stamen and stigma, differs from the 

rest in shape and in being nectarigerous: It is called a labellum. eg. 

Habenaria. 

C Gamopetalous and Regular 

I. Tubular: Corolla tube is more or less cylindrical. Eg. Disc florests of 

Helianthis 

II. Companulate: when the corolla tube is inflated below and winded out at 

the top. It looks bell-shaped eg. Cucurbita maxima 

III. Infundibuliform: corolla is funnel-shaped structure. eg. Datura 

IV. 

Rotate: When the corolla tube is short with spreading limbs at right angle 

to it. It looks like a wheel in shape eg. Solanum 

179

V. Salver-Shaped or Hypocrateriform - Corolla tube is long and narrow 

with spreading limbs. eg. Vinca. 

VI. Urceolate: un-shaped Corolla tube is inflated in the middle but narrow 

above and below, as inBryphyllum calycinum 

D. Gamopetalous and Irregular 

i. Bilabiate: Limb of the corolla is divided into two projecting lips eg. Ocimum 

ii. Personate:Corolla shows bilabiate condition with mouth closed by the 

projecting lip.et.Antirrhium 

iii. Ligulate:Strap-shaped. When the corolla tube is short and tubular at the 

base but flat above like a strap. eg. Ray florets of Asteraceae 

Aestivation 

The mode of arrangement of either sepals or petals of a flower in bud condition 

is said to be an Aestivation. 

The Aestivation is of the following types 

1. Valvate Aestivation 

Sepals or petals in a whorl just meet by their edges without overlapping. eg. 

Sepals of Hibiscus. 

2. Twisted Aestivation 

In this mode of aestivation one margin of each sepal or petal overlaps the 

next one, and the other margin is overlapped by a preceding one. Here the over 

lapping is regular in one direction-clockwise or anticlockwise. eg. Petals of Hibiscus 

3. Imbricate 

In this type, one sepal or petal is internal or being overlapped on both the 

margins and one sepal or petal is external with both of its margins overlapping. Of 

the remaining sepals or petals, one margin is overlapping and the other margin 

overlapped. 

There are two types of imbricate aestivation descendingly imbricate and 

ascendingly imbricate. 

a. Descendingly Imbricate or Vexillary Aestivation: In this type of aestivation 

the posterior petal of overlaps one margin of the two lateral petals. 

The other margin of these two lateral petals overlaps the two anterior petals, 

which are united. Thus the overlapping is in descending order and hence the 

name eg. Corolla of Fabaceae. 

180

Valvate Twisted Descendingly 

Imbricate 

Ascendingly 

Imbricate 

Quincuncial 

Fig : 3.35. Different types of Aestivation 

Ascendingly imbricate aestivation : In this type the posterior odd petal is 

innermost being overlapped by one margin of the two lateral petals. The 

other margin of the two lateral petals is overlapped by the two anterior petals. 

Here the overlapping of petals begins from the anterior side proceeding towards 

the posterior side. This is just opposite of descendingly imbricate aestivation. 

eg. Petals of Caesalpiniaceae. 

4. Quincuncial 

It is modification of imbricate aestivation in which two petals are internal, two are 

external and the fifth one has one margin external and the other margin internal. eg. 

Guava 

Androecium 

It is the third whorl of the flower. It is considered as the male part of the flower. 

The androecium is made up of stamens or microsporophylls. Each stamen has a slender 

stalk called filament, bearing the anther (microsporangial sorus). Usually the anther 

consists of two lobes. The two lobes of an anther are connected by a tissue called 

connective. Each anther lobe has two pollen sacs (microsporangia). Each pollen 

sac consists of innumerable Pollen grains (microspores). 

181

Sterile stamen or staminode 

In some plants, a stamen may not develop any fertile anther. Such sterile stamens 

are called staminodes eg. Cassia. 

1. Cohesion of Stamens 

i. Monadelphous: All the stamens of a flower are united in one bundle by 

fusion of their filaments only. The anthers are free, eg. Hibiscus, Abutilon, 

etc. 

ii. Diadelphous: All the stamens of a flower are united in two bundles by 

fusion of their filaments only. The anthers are free, eg. Clitoria 

iii. Polyadelphous: Filaments of all the stamens unite to form more than two 

bundles. The anthers are free, eg. Citrus. 

iv. Syngenesious: Anthers of all the stamens of the flower unite to forma 

cylinder around the style. The filaments are free, eg. Asteraceae. 

Monadelphous 

Diadelphous 

Syngenesious 

Synandrous 

Polyadelphous 

v. Synandrous: Anthers as well as the filaments are fused throughout their 

whole length, eg. Cucurbitaceae 

vi. Polyandrous: Stamens are indefinite and free, eg. Ranunculus. 

2. Adhesion of stamens 

Fig : 3.36. Cohesion of stamens 

i. Epipetalous: Stamens adhre to the petals by their filaments and hence 

appearing to arise from them, eg. Solanum, Ocimum, etc. 

ii. Epitepalous (Epiphyllous): When stamens united with the perianth leaves, 

the stamens are said to be Epitepalous. eg. Asphodelus. (Spider lilly) 

182

iii. 

Gynandrous: Stamens adhere to the carpels either throughout their length 

of by their anthers only. eg. Calotropis. 

3. Length of stamens 

i. Didynamous: Out of four 

stamens in a flower, two are 

long and two are short, eg. 

Ocimum 

ii. Tetradynamous: Out of six 

stamens in a flower, two outer 

are short and four inner are long, 

eg. Mustard. 

4. Position of stamens 

i. Inserted: Stamens shorter 

than corolla tube. 

ii. Exerted: Stamens longer than 

the corolla tube, protruding outwards. 

5. Number of antherlobes 

i. Dithecous: Anthers have two lobes with four microsporangia or pollen sacs. 

ii. Monothecous: Anthers have only one lobe with two microsporangia. 

6. Fixation of anthers 

i. Basifixed (Innate): Filament 

is attached to the base of the 

anther, eg. Brassica. 

ii. Adnate: Filament is continued 

from the base to the apex of 

anther, eg. Verbena. 

iii. 

iv. 

Didynamous 

Tetradynamous 

Fig : 3.37. Length of stamens 

Basified Adnate Dorsifixed Versatile 

Dorsifixed: Filament is 

attached to the dorsal side of 

the anther, eg. Citrus. 

Versatile: Anther is attached 

Fig : 3.38. Fixation of anthers 

lightly at its back to the slender 

tip of the filament so that it can swing freely, eg. Grass 

183

Gynoecium 

Gynoecium is the collective term for the innermost central whorl of floral 

appendages. It is considered as the female part of the flower. A unit of gynoecium is 

called carpel. Following technical terms and related with gynoecium. 

1. Number of Carpel 

i. Monocarpellary: Gynoecium consists of a single carpel; eg. Fabaceae 

ii. Bicarpellary: Ovary consists of two carpels; eg. Rubiaceae 

iii. Tricarpellary: Ovary consists of three carpels; eg. Liliaceae 

iv. Tetracarpellary: Ovary comprises of four carpels; eg. Melia 

v. Multicarpellary: Gynoecium consists of many carpels eg. Papaver 

2. Cohesion of Carpels 

i. Apocarpous: Gynoecium made up of two or more carpels which are free; 

eg. Polyalthia. 

ii. Syncarpous: Gynoecium consists of two or more carpels which are fused; 

eg. Hibiscus. 

3. Number of locules 

Depending on the 

number of chambers, the 

ovary may be described 

as unilocular, bilocular, 

trilocular etc. 

Types of Placentation 

In Angiosperms, 

ovules are present inside 

the ovary. Placenta is a 

special type of tissue, 

which connects the ovules 

to the ovary. The mode of 

distribution of placenta 

inside the ovary is known 

as placentation. Some 

important types of 

placentation are as 

follows: 

Axile M arginal Parietal 

Basal 

Superficial 

Fig : 3.39. Types of Placentations 

184

1. Axile Placentation 

This type of placentation is seen in bi- or multi carpellary, syncarpous ovary. 

The carpel walls meet in the centre of the ovary, where the lacenta are formed 

like central column. The ovules are borne at or near the centre on the placenta in 

each locule. eg. Hibiscus. 

2. Marginal Placentation 

It occurs in a monocarpellary, unilocular ovary. The ovules are borne along 

the junction of the two margins of the carpel. eg. Fabaceae 

3. Parietal Placentation 

This type of placentation is found in multi carpellary, syncarpous, unilocular 

ovary. The carpels are fused only by their margins. The placenta bearing ovules 

develop at the places, where the two carpels are fused. eg. Cucumber 

4. Basal Placentation 

It is seen in bicarpellary syncarpous, and unilocular ovary. The placenta develop 

directly on the receptacle, which bears a single ovule at the base of the ovary. eg. 

Asteraceae. 

5. Superficial Placentation 

This type of placentation occurs in a multicarpellary, multiocular ovary. The ovules 

are borne on placentae, which develop all round the inner surface of the partition wall 

eg. Nymphaeaceae 

Description of a flower 

The following technical terms are used in connection with the description of flower. 

1. Floral whorls 

1. Complete: When all the four whorls. (Calyx, Corolla, Androecium, and 

Gynoecium) are present in a flower, it is termed complete. 

2. Incomplete: When one or more whorls are absent the flower is described 

incomplete. 

a. Monochlamydeous: Some flowers have only one accessory whorl and they 

are called Monochlamydeous. 

b. Dichlamydeous: Normally flowers have two outer whorls which are usually 

differentiated into calyx and corolla. Such flowers are known as dichlamydeous. 

c. Achlamydeous: There are a number plants, where the flowers have neither 

calyx nor corolla. Such flowers are described naked or achlamydeous. 

185

2. Sex distribution 

i. Bisexual or Perfect: When both the essential whorls i.e., androecium and 

gynoecium are present in a flower, it is called bisexual or perfect. 

ii. Unisexual or imperfect: A flower having only one of the essential whorls is 

called unisexual or imperfect. The unisexual flowers may be of two types. 

a) Staminate. Male flower with androecium, only 

b) Pistillate. Female flower with gynoecium only 

Monoecious 

If male and female flowers develop in the same plant, it is called Monoecious eg. 

Coconut, Maize etc. 

Dioecious 

If male and female flowers are borne on separate plants, it is termed dioecious eg. 

Palmyrah palm, Papaya, Mulberry etc. 

Polygamous 

If a plant develops three kinds of flowers i.e. staminate, pistillate and bisexual 

flowers, it is called polygamous. eg. Mango, Cashewnut etc. 

3. Floral Symmetry 

The shape, size and arrangement of floral appendages (i.e. Calyx, corolla, 

androecium and gynoecium) around the axis of a flower is called floral symmetry. 

The axis to which the flower is attached is called mother axis. The side of flower 

towards mother axis is called posterior side and the side away from it is called 

anterior side. 

On the basis of floral symmetry there may be following three conditions of a 

flower. 

i. Actinomorphic: A flower with radial symmetry, i.e., the parts of each 

whorl are similar in size and shape. The flower can be divided into two 

equal halves along more than one median longitudinal plane, eg. Hibiscus, 

Solanum, etc. 

ii. 

Zygomorphic: A flower with bilateral symmetry, i.e. the parts of one or 

more whorls are dissimilar. The flower can be divided into two equal halves 

in only one vertical plane, eg. Pisum 

186

iii. Asymmetric: A flower which cannot be divided into two equal halves 

along any vertical plane, eg. Canna 

4. Arrangement of floral organs 

i. Cyclic: The floral parts are arranged in definite whorls around the axis of 

flower, eg. Brassica, Solanum etc. 

ii. Acyclic: The floral parts are arranged in spirals and not in whorls, eg. 

Magnolia 

iii. Spirocyclic: Some of the floral parts are in whorls and others in spirals 

(Hemicyclic), eg. Rose, Ranunculus, etc. 

5. Number of floral parts 

Occurrence of the same number of floral parts in different floral whorls of a 

flower is called isomery. Sometimes, flowers have different number of parts in 

each whorl. This condition is called heteromerous. The isomerous flowers may 

be of the following types:- 

i. Dimerous : Floral parts in two's or multiplies of two 

ii. Trimerous : Floral parts in three's or multiples of three 

iii. Tetramerous : Floral parts in four's or multiples of four 

iv.Pentamerous : Floral parts in five's or multiples of five 

Dicotyledonous flowers are usually tetra, or pentamerous whereas 

monocotyledonous flowers are trimerous or multiples of three. 

Fruit 

The fruit may be defined as a fertilized and developed ovary. Fruits and seeds 

develop from flowers after completion of two processes namely pollination and 

fertilization. After fertilization, the ovary develops into fruit. The ovary wall develops 

into the fruit wall called pericarp and the ovules inside the ovary develop into 

seeds. The branch of horticulture that deals with study of fruits and their cultivation 

is called pomology. 

Fertilization acts as a stimulus for the development of ovary into fruit. But 

there are several cases where ovary may develop into fruit without fertilization. 

This phenomenon of development of fruit without fertilization is called 

parthenocarpy and such fruits are called parthenocarpic fruits. These fruits are 

necessarily seedless. eg. Banana, grapes, pineapple and guava etc. 

187

The fruits are classified into two main categories, - true and false fruits. 

i. True Fruit: The fruit, which is derived from ovary of a flower and not 

associated with any noncarpellary part, is known as true fruit. eg. Tomato, 

Brinjal, Pea, Mango, Banana etc. 

ii. False Fruit: (Pseudocarp) The fruit derived from the ovary along with 

other accessory floral parts is called a false fruit. eg. Apple (edible part 

of the fruit is the fleshy receptacle). 

Structure of fruit 

A fruit consists of two main parts - the seeds and the pericarp or fruit wall. 

The structure and thickness of pericarp varies from fruit to fruit. The pericarp 

consists of three layers - outer epicarp, middle mesocarp and inner endocarp. 

The sweet juicy and edible flesh is the mesocarp, the inner most hard covering is 

the endocarp. These three layers are not easily distinguishable in dry fruits. 

The fruits are usually classified into three groups, namely simple, aggregate 

and multiple or composite fruits. 

Simple fruits 

When a single fruit develops from a single ovary of a single flower, it is called 

simple fruit. The ovary may be monocarpellary or multicarpellary syncarpous. 

On the nature of pericarp, simple fruits are divisible into two types 

i) Fleshy fruits and ii) Dry fruits 

Simple fleshy fruits 

188

In these fruits either the entire pericarp or part of the pericarp is succulent 

and juicy when fully ripe. Normally the fruit wall may be differentiated into three 

layers - an outer epicarp, a middle mesocarp and an inner endocarp. As a 

general rule, the fleshy fruits are indehiscent. 

Fleshy fruits are broadly divided into two kinds, baccate and drupaceous. 

Baccate fruits are fleshy fruits with no hard part except the seeds. Berry is an 

example for the first category while drupe falls under the second type. 

1. Berry: It is a many seeded fruit. Here the epicarp is thin, the mesocarp 

and endocarp remain undifferentiated. They form a pulp in which the 

seeds are embedded. In these fruits, all parts including the epicarp with 

the seeds are edible eg. tomato 

2. Drupe: This is normally a one-seeded fruit. In these fruits the pericarp is 

differentiated into an outer skinny epicarp, a middle fleshy and juicy 

mesocarp and an inner hard and stony endocarp. Drupes are called stone 

fruits because of the stony hard 

Legume 

endocarp. The endocarp encloses 

Follicle 

a single seed. The edible portion, 

of the fruit is the fleshy mesocarp 

eg. mango. In coconut, the 

mesocarp is fibrous, the edible 

part is the endosperm. 

3. Hesperidium: It is a skind of 

baccate fruit that develops from 

a superior multicarpellary and 

syncarpous ovary. The fruit wall 

is differentiated into three layers 

- an outer glandular skin or 

epicarp, a middle fibrous 

Siliqua 

Capsule 

mesocarp, and an inner 

membranous endocarp. The latter 

divides the fruit chamber into a 

number of compartments. The 

seeds arise on axial placentae and 

are covered by juicy hairs or 

outgrowths from the lacentae that 

are edible. 

Fig : 3.41. Dehiscent dry fruits 

189

It is characteristic fruit of the genus Citrus (Fam. Rutaceae) 

4. Pepo: A large fleshy fruit developing from a tricarpellary, syncarpous, 

unilocular and inferior ovary with parietal placentation. The fruit is many 

seeded with pulpy interior; eg. Cucumber, Melon, Bottle gourd etc. 

5. Pome: It is a fleshy and a false fruit or Pseudocarp. It develops from a 

multicarpellary syncarpous inferior ovary in which the receptacle also 

develops along with the ovary to become fleshy and enclosing the true 

fruit. The true fruit containing seeds remains inside. The edible part is 

fleshy thalamus. eg. Apple, Pear etc. 

Simple Dry Fruits 

These fruits have dry pericarp, which is not distinguished into three layers. 

The dry simple fruits are further divided into three typesa) 

Dehiscent 

b) Schizocarpic and 

c) Indehiscent 

a) Dehiscent dry fruits 

1. Legume: A dehiscent dry fruit produced from a monocarpellary, superior 

ovary, which dehisces from both the sutures into two valves. eg. Pea 

2. Follicle: A dehiscent dry fruit produced from a monocarpellary, superior 

ovary, which dehisces from one suture only. eg. Calotropis. 

3. Siliqua: A dehiscent dry fruit produced from a bicarpellary, syncarpous, 

superior, ovary, which is unilocular but appears bilocular due to false septum. 

Fruits dehisce along both the sutures from base to apex and large number 

of seeds remain attached to the false septum called replum. eg. Brassica 

4. Capsule: A dehiscent dry fruit produced from syncarpous, superior or inferior 

ovary which dehisces along two or more lines of suture in various ways. 

i) Septicidal - eg. Aristolochia 

ii) Loculicidal - eg. Gossypium, Abelmoschus 

b) Schizocarpic dry fruits 

1. Lomentum: Fruit is similar to a legume but constricted between the seeds. 

Dehiscing sutures are transverse. The fruit splits into one-seeded indehiscent 

compartments at maturity; eg. Tamarindus, Cassia fistula. 

190

2. Cremocarp: Fruit is produced from a bicarpellary, syncarpous, bilocular 

and inferior ovary. It is a two-seeded fruit which splits longitudinally into 

two indehiscent mericarps which remain attached to a thread-like 

carpophore. eg. Coriandrum 

3. Regma: The fruit 

is produced from a 

bi- or multicarpellary, 

syncarpous and 

superior ovary, it 

breaks up into as 

many segments or 

cocci as there are 

carpels; eg. 

Ricinus. 

c) Indehiscent dry 

fruits 

Lomentum 

Mericarp 

Cremocarp 

Carpophore 

Mericarp 

Fig : 3.42. Schizocarpic dry fruits 

Regma 

1. Achene: A small, 

indehiscent one seeded fruit developing from a monocarpellary ovary and 

in which the pericarp is hard, leathery and remains free from seed coat; eg. 

Mirabilis, Clematis. 

Achene 

Caryopsis Cypsela Nut Samara 

Fig : 3.43. Indehiscent dry fruits 

2. Caryopsis: A small, indehiscent and one seeded fruit developing from a 

monocarpellary ovary and in which the pericarp is fused with the seed coat. 

The seed completely fills the chamber; eg. Paddy, Maize 

3. Cypsela: The fruit is produced from bicarpellary, syncarpous and inferior 

ovary with persistent calyx forming the ‘pappus’. It contains only one seed. 

The pericarp and seed coat remain free; eg. Tridax, Helianthus. 

191

4. Nut: A large, indehiscent, one-seeded fruit that develops from a bi- or multicarpellary 

ovary. The fruit wall becomes hard, stony or woody at maturity; 

etc. Cashew nut 

5. Samara: A dry indehiscent, one-seeded winged fruit developing from 

bicarpellary, syncarpous ovary. The wing is a modified outgrowth of pericarp; 

eg. Acer 

Aggregate fruit 

Polyalthia 

An aggregate fruit develops from a single flower, 

with multicarpellary, apocarpous, superior ovaries and each 

of them develops into simple fruitlets. An aggregate fruit, 

therefore consists of a collection of simple fruits as in 

Polyalthia. The carpels of the flower unite and give rise 

to a single fruit as in annona squamosa. 

Multiple or Composite Fruit 

Fig : 3.44. Aggregate Fruit 

Multiple or composite fruit is formed by all the flowers of a whole inflorescence 

grouped together to give a single big fruit. In a sense, multiple fruits are false 

fruits. 

In Jack, the type of multiple fruit is sorosis. The rachis and all the floral parts 

of the female inflorescence fuse together forming composite fruit. The 

inflorescence axis and the flowers all become fleshy. 

In the centre of the fruit, there is a club-shaped, thick, fleshy central axis, 

which is the inflorescence axis. The edible part of the fruit represents the perianth, 

which is fleshy and juicy. The pericarp is bag-like and Sorosis - Jack 

contains one seed. The spines on the tough rind represent 

the stigmas of the carpel. The sterile or unfertilized flowers, 

occur in the form of numerous, elongated, whitish, flat 

structures in between the edible flakes. 

Sorosis: A multiple fruit that develops from a 

spicateinflorescence. eg. Ananas sativus (Pineapple). 

Pineapple plant is largely cultivated for its fruits. The 

stem is short and leafy and bears a terminal spicate 

inflorescence. After fertilization, the axis and the flowers, 

along with the bracts, become stimulated to grow and unite 

together into a fleshy compound fruit, the ‘Pineapple’. On 

the surface of the fruit, the hexagonal areas represent the 

flowers, and the tips of the floral bracts project out. Usually 

Fig : 3.45. M ultiple Fruit 

192

the flowers are sterile and seeds are rarely formed. The inflorescence axis produces 

a tuft of vegetative leaves, which forms a crown at the top. The vegetative top, if 

cut and planted, establishes itself in the ground and gives rise to a new plant. 

Seed structure 

Seed 

Seeds vary greatly in size. They can be as small as those of orchids (about 

two million seeds per gram) or as large as those of coconut. In many plants, the 

seeds are so peculiar that it helps in identification of a species. 

Dicotyledon and Monocotyledon Seeds 

On the basis of number of cotyledons in the seed, angiosperms have been 

divided into two groups: 

1. Monocotyledons having embryo with one cotyledon only, eg. maize, 

rice, wheat and onion. 

2. Dicotyledons having embryo with two cotyledons, eg. pea, gram, bean 

and castor. 

Structure of gram seed 

Gram seed may be taken as an example for the study of the structure of a 

dicot seed. 

193

The gram seeds are brown in colour. They are pointed at one end and round 

at the other end. These are contained in a small fruit called, the pod. The gram 

pod is two or three-seeded. The seeds are attached to the wall of the pod by a 

stalk called the funiculus. When the mature seed is detached, the funiculus leaves 

a scar on the seed called the hilum. Just blow the hilum lies the micropyle in the 

form of a small pore. Water is absorbed through the micropyle during the 

germination of seed. If the soaked seed is squeezed, water is seen to ooze out of 

the micropyle. The seed is covered by the tough seed coat. The seed coat consists 

of two layers, outer brownish testa and the papery white membranous tegmen. 

Micropyle 

Gram Seed - External Structure 

Radicle 

Plumule 

Cotyledon 

Testa 

Fig : 3.46. Structure of Dicot seed 

Radicle 

The function of seed coat is protective. It protects the seed from desiccation, 

mechanical injury and extremes of temperature. It also protects the seed from the 

attack of bacteria, fungi and insects. 

On removing the seed coat, two massive and fleshy cotyledons are seen. 

The two cotyledons are attached laterally to the embryonal axis. The embryonal 

axis projects beyond the cotyledons on either side. The lower pointed end of the 

axis is the radicle which represents the embryonic or rudimentary root. The other 

end is feathery. It is called the plumule. It represents the first apical bud of the 

future plant and develops into the shoot. The plumule is seen only after separating 

the two cotyledons. The portion of the axis between radicle and the point of 

attachment of the cotyledons to the axis is called the hypocotyl and the portion 

between the plumule and the cotyledons is the epicotyl. The axis along with the 

cotyledon constitute the embryo. 

2. Structure of Maize Grain 

The Maize grain can be taken as an example of monocotyledon seed. 

The maize grain is a small one-seeded fruit called the caryopsis. In maize 

grain the seed coat (testa) is fused with the fruit wall (pericarp). Externally, the 

194

maize grain is yellow in colour and somewhat triangular in shape. On one side of 

the grain is a small, opaque, oval and whitish area in which embryo lies embedded. 

A longitudinal section of the seed shows the following structures: 

1. Seed coat: It is formed of a thin layer surrounding the whole grain. This 

layer is made up of seed-coat and pericarp, i.e. fruit wall. 

2. Endosperm: When internally examined, maize grain is found consisting 

of two unequal portions 

Cob of 

divided by a layer called maize grain 

L.S. of grain 

epithelium. The bigger 

portion, the endosperm 

Entire 

Aleurone layer 

seed 

which is yellowish or 

Endosperm 

whitish is the food storage 

tissue of the grain and is 

rich in starch. But its 

Scutellum 

outermost layer contains 

Coleoptile 

only protein and is called 

aleurone layer. On the 

Embryo 

other side of the 

Radicle 

endosperm towards the 

Coleorhiza 

pointed end lies an opaque 

body called embryo. 

Fig : 3. 47. Structure of Maize grain 

3. Embryo: It consists of one large and shield shaped cotyledon. This is 

also known as scutellum in the case of maize and other cereals. The axis of the 

embryo lies embedded in the scutellum. The axis consists of a plumule at the 

upper portion and the radicle at the lower end. Both radicle and plumule are 

enclosed in sheath. The sheath covering the plumule is known as coleoptile and 

that covering the radicle is known as coleorhiza. The cone-shaped coleoptile has 

a pore at the apex through which the first foilage leaf emerges during germination. 

Types of Seed 

Non-endospermic or ex-alubuminous seeds 

In gram, pea and bean the cotyledons are thick and fleshy. They store food 

material for the embryo during germination. Such seeds are known non-endospermic 

or exalubuminous seeds. 

Endospermic or albuminous seeds 

However, in seeds like castor, maize and other cereals, the cotyledons are 

thin and membranous. In such seeds food is stored in the endosperm. Cotyledons 

195

act as absorbing organs. They absorb food from the endosperm and supply it to 

the growing embryo. Such seeds are known as endospermic or albuminous seeds. 


One Mark 


1. The most conspicuous and characteristic structure of Angiosperm is 

a. Flower b. Seeds c. Fruits d. Leaves 

2. The number of whorls present in a bisexual flower is 

a. One b. Three c. Two d. Four 

3. A flower is said to be complete when it has 

a. One whorl b. Three whorls c. Two whorls d. Four whorls 

4. Trimerous Flowers are common among 

a. Dicotrs b. Xerophytes c. Monocots d. Gymnosperms 

5. In deciduous type of calyx, the sepals fall off 

a. As soon as flower opens b. After fertilization 

c. In the bud condition d. All the above 

6. When anthers have two chambers, they are described as 

a. Dioecious b. Dithecous c. Diadelphous d. Dimorphic 

7. Gynoecium with united carpels is termed as 

a. Apocarpous b. Multicarpellary 

c. Syncarpous d. None of the above 

8. The type of placentation seen in cucumber is 

a. Basal b. Parietal c. Axile d. Marginal 

9. Seeds are produced from the 

a. Ovary b. Carpels c. Ovules d. Locules 

10. Seedless Grapes are the 

a. Simple Dry fruits b. Multiple fruits 

c. Aggregate fruits d. Parthenocarpic fruits 

11. Which is the edible portion in berry? 

a. Epicarp b. Endocarp c. Mesocarp d. All the above 

12. Coconut belongs to 

a. Drupe b. Syconium c. Baccate d. Aggregate 

13. The type of fruit seen in Jack is 

a. Multiple fruit b. Syconium c. Sorosis d. Aggregate 

196


1. A special leaf at whose axil the flower develops is called ............. 

2. Thalamus is otherwise called ............. 

3. ............. flower has both androecium and gynoecium 

4. A flower having uniform number of all the floral parts is called ............. 

5. Microsporangia are otherwise called ............. 

6. After fertilization, the ovary becomes ............. 

7. Legume is the characteristic fruit of ............. family. 

8. The edible part of the Jack fruit is ............. 

I. Match the following 

Hypogynyous - Petals of Malvaceae 

Twisted - Superior ovary 

Syngenesious - Stamens attached to petals 

Epipetalous - Anthers united, filaments free 

Basal Placentation - Asteraceae 

Caryopsis - Pericarp 

Unfertilized Ovary - Paddy 

Ovary Wall - True fruit 

Fertilized Ovary - Aggregate fruit 

Apocarpous Ovary - Parthenocarpic fruit 

Two Marks 

1. What are monoecious plants? 

2. Define aestivation. 

3. What is a bisexual flower? 

4. What is a zygomorphic flower? Give Example. 

5. Distinguish between monothecous and dithecous anthers. 

6. What is meant by monadelphous stamens? 

7. Distinguish between apocarpous and syncarpous ovary. 

8. Define fruit. 

9. What are the three groups of fruits? 

10. Define simple fruit. 

11. What are dry dehiscent fruits? 

12. What are the two processes necessary for the development of fruits? 

197

13. Define aggregate fruits. 

14. What is legume? Give an example. 

15. How does a fleshy fruit differ from a dry fruit? 

Five Marks 

1. Explain the hypogynous and epigynous flowers with examples. 

2. Explain different types of calyx. 

3. How the symmetry of a flower is determined? Briefly describe different 

types of symmetry seen in flower. 

4. Describe aggregate fruit with a suitable example. 

5. Describe multiple fruit with a suitable example. 

6. Bring out the essential difference in the structure of a dicot and monocot 

seed by means of labelled diagrams only. 

Ten Marks 

1. Explain the different types of placentation with example. 

2. Given an account of different types of aestivation with example. 

3. Describe the essential organs of a flower. 

4. Explain different types of fleshy fruits with suitable examples. 

5. Describe dry dehiscent fruits with suitable examples. 

6. Describe the structure of Maize grain with the help of diagram. How 

does it differ from a Cicer seed? 

198

IV. GENETICS 

1. Concept of Heredity and Variation 

The children or offsprings closely resemble their parents and to some extent 

their grand parents and great grand parents. Still the offsprings of a set of parents 

differ from each other and from their parents in different degrees. They have 

certain unique characteristics by which we can understand that they belong to the 

same family. The Science that deals with the mechanisms responsible for inheritance 

of similarities and differences in a species is called Genetics. It is a branch of 

biology that encompasses the study of the mechanism of transmission of characters 

from parents to offsprings. The word "genetics" is derived from the Greek word 

"genesis" meaning "to grow" or "to become". 

The Science of Genetics helps us to differentiate between heredity and 

variations and seeks to account for the resemblances and differences due to heredity, 

their source and development. 

Heredity refers to the transmission of characters, resemblances as well as 

differences from one generation to the next. It explains how offsprings in a family 

resemble their parents. 

Variation refers to the differences shown by individuals of the same species 

and also by offsprings (siblings) of the same parents. It explains why offsprings 

eventhough born to the same parents differ from each other. They are similar, but 

not identical (except in identical twins). These similarities and differences are not 

coincidental. 

In brief, genetics is the study of heredity and variation. 

Heredity 

Heredity refers to the transmission of characters from parents to the offsprings. 

In the very early ages though improvement of the races of plants and animals were 

conducted by the Babylonians and Assyrians, it was not known what exactly caused 

the characters to be passed from one generation to the next. 

199

Some early views of heredity 

Many view points were put forward before Mendel to explain the transfer of 

characters to the subsequent generation. 

1. Moist Vapour Theory 

This was put forward by the Greek philosopher Pythagoras who believed that 

each organ of the animal body produced vapours and new organism was formed 

by combination of different organs. 

2. Fluid Theory 

This was propounded by Aristotle who was of the view that both male and 

female produced semen and when these mix the female semen which is not so 

pure provided the inert substance for the formation of the embryo and the male 

semen gave form and vitality to the embryo. 

3. Preformation Theories 

Anton Von Leeuwenhoeck observed human sperms for the first time. This 

theory according to Swammerdam (1679) postulates that the sex cells either the 

sperm or egg contained within itself the entire organism in a miniature form called 

"homunculus". Development was only an increase in 

size of the miniature. This theory was supported by 

Malpighi (1673), Delepatius (1694) and Roux (1800). 

4. Particulate Theory 

French biologist Maupertius propounded that the body 

of each parent gave rise to minute particles for 

reproduction which blend together to form the offspring. 

5. Pangenesis 

This theory proposed by Aristotle 

(384-322 B.C.) holds that the animal body produces minute 

bodies called gemmules or pangenes which were carried 

by blood to the reproductive organs. Here the pengenes 

from two parents blend to give rise to a new individual. 

This theory prevailed for many centuries and was 

accepted by great biologists such as Charles Darwin (1809 - 1882). 

Evidences against the Blending Theory 

The individual, according to the views of the Pre-Mendelian era represents 

the mixture of characters of both parents. This was the blending theory. Under 

200 

Fig : 4.2 Homunculus as per 

Preform ation Theory

this concept the progeny of a black and white animal would uniformly be grey. The 

progeny from further crossing the hybrids would all remain grey as the characters 

once blended can never be separated again. But however in daily life it is seen that 

children of black and white parents may be dark, fair or of a intermediate complexion. 

So also their children may be dark or fair. 

Pattern of inheritance shown by atavism is also against blending inheritance. 

In atavism, the grandchildren may exhibit a feature of an earlier generation not 

seen in the parents. The traits of sex (male or female) do not blend in unisexual 

organisms. 

Basic features of Inheritance / Heredity 

The Swedish taxonomist Carolus Linnaeus and two german plant breeders 

Kolreuter and Gaertner performed artificial cross pollination in plants and 

obtained hybrids. Kolreuter was able to obtain evidence to show the inherited 

traits remained discrete without blending. Though his results were similar to that 

of Mendel, he was not able to interpret them correctly. 

Mendel's great contribution was that he replaced the blending theory with 

the particulate theory. Mendel first presented his findings in 1865, but they were 

not accepted then and remained unknown for many years. Their rediscovery in 

1900 by de Vries of Holland, Carl Correns of Germany and Tschermak of 

Austria independently, led to the beginning of modern genetics. 

Few important characteristics of inheritance are: 

i. Every trait has two alternative forms. 

ii. One alternative form is more commonly expressed than the other. 

iii. Any alternative form can remain unexpressed for many years. 

iv. Hidden character may reappear in original form. 

v. Characters or traits are expressed due to discrete particulate matter and so 

do not get blended or modified. 


One mark 

Choose the correct Answer 

1. Moist vapour theory was given by 

a. Aristotle b.Pythagoras c. Delepatius d. Darwin 

2. Blending theory was replaced by particulate theory of 

a. Kolreuter b.Gaertner c. Mendel d.Darwin 

201

3. The grand children may exhibit a feature of an earlier generation not seen in 

parents. This is called 

a. Homunculus b. Pangenesis c. Atavism d. Blending 


1. Polydactyly is the example for .............. 

2. A group of ramets is called a .............. 

Two marks 

1. Define Heredity / Variation / Homunculus / Parthenogenesis / Pangenes 

Five Marks 

1. Explain the significance of variation 

2. Give the early views of heredity 

3. What are basic features of inheritance 

Ten Marks 

1. Write an essay on the different types of variations 

202

2. Mendel's Laws of Inheritance 

Introduction 

Gregor Johann Mendel was an Austrian monk, who was the first to explain 

the mechanism of transmission of characters from the parents to the offsprings. 

He maintained that there were particles called factors, which carried the traits to 

the subsequent generation. This holds good even today and since he is the pioneer 

of modern Genetics, he is called The Father of Genetics. 

Biography of Mendel 

Gregor Johann Mendel was born in 1822 to a family of poor farmers in Silisian, 

a village in Heizendorf which is now a part of Czechoslovakia. After finishing 

his high school, at the age of 18, he entered the Augustinian monastery at Brunn 

203

as a priest. From here he went to the University of Vienna for training in Physics, 

Mathematics and Natural Sciences. Here he was influenced by two scientists 

Franz Unger (a plant physiologist) and Christian Doppler (the physicist who 

discovered Doppler effect) and he himself became interested in hybridisation 

experiments. 

Mendel returned to the monastery in 1854, and continued to work as a priest 

and teacher in the high school. In his spare time, he started his famous experiments 

on garden pea plant (Pisum sativum) which assumes great historic importance. 

He conducted his experiments in the monastery garden for about nine years from 

1856 to 1865. 

The findings of Mendel and his laws were published in the journal Annual 

Proceedings of the Natural History Society of Brunn, in 1865. The paper was 

entitled Experiments in Plant Hybridisation. But his work was not accepted or 

lauded by the scientific world at that time because 

i. The journal was obscure. 

ii. His concept was far ahead of his time. 

iii. The scientists were busy with the controversy over Darwin's Theory of 

Origin of species and 

iv. Mendel not being very sure of his findings lacked an aggressive approach. 

Later in the year 1900, three scientists Carl Correns of Germany, Hugo de 

Vries of Holland and Tshermak of Austria independently rediscovered Mendel's 

findings and brought to light the ingenuity of father Mendel. To recognise his 

work, it was named as Mendel's Laws and Mendelism. 

Mendel's Experiments 

Mendel conducted cross breeding experiments in the garden pea plant (Pisum 

sativum). 

He crossed two pea plants with contrasting character traits considering one 

character at a time. The resulting hybrids were crossed with each other. The data 

of many crosses were pooled together and the results were analysed carefully. 

Reasons for Mendel's Success 

A combination of luck, foresight and the aptitude of Maths all contributed to 

the success of Mendel's experiments. 

Selection of Material 

He chose the pea plant as it was advantageous for experimental work in many 

respects such as : 

204

1. It is a naturally self-fertilizing 

plant and so it is very easy to 

raise pure-breeding individuals. 

2. It has a short life span as it is an 

annual and so it is possible to 

follow several generations. 

3. It is easy to cross-pollinate the 

pea plant. 

4. It has deeply defined contrasting 

characters. 

5. The flowers are all bisexual. 

Therefore the pea plant proved to 

be an ideal experimental plant for 

Mendel. 

Method of Work 

Stamen 

1. He conducted hybridisation experiments in true breeding parents (individuals 

which produce the same type of offsprings for any number of generations 

when selfed). 

2. Mendel worked with seven pairs of contrasting character traits and he 

considered one pair of contrasting character traits at a time. 

3. He carried out his experiments to the second and third generations. 

4. He maintained a clear statistical record of his work. 

Pistil 

Fig : 4.3. L.S. of Pisum sativum flower 

Table 4.1. Contrasting characters of traits chosen by Mendel 

S.No. Character Dominant Recessive 

1. Seed shape Round Wrinkled 

2. Cotyledon colour Yellow Green 

3. Seed coat colour Grey White Grey White 

4. Pod shape Inflated Constricted 

5. Pod Colour Green Yellow 

6. Position of Pod or Flower Axillary Terminal 

7. Stem length (Height) Tall Dwarf 

Crossing Techniques 

Since garden pea is self pollinating, great care was taken to see that 

205

a. The parents were 

emasculated to 

prevent self 

pollination. 

b. The anthers were 

collected from male 

parent and dusted 

onto the female 

parent and the stigma 

was bagged. 

c. The seeds were 

collected separately in 

marked bottles. 

d. Reciprocal crosses, 

(by interchanging the 

male and female 

parents) were 

conducted to prove 

that there was no 

change in the ratio of 

the offsprings i.e. sex 

has no influence on 

inheritance. 

The plants used as 

parents represent the parental 

generation designated as P. 

The resulting progeny from 

crossing of parents is called 

first filial generation 

designated as F 1 

and progeny 

resulting from selfing the F 1 

plants was called second filial 

generation, denoted as F 2 

. 

Tall stem 

Crosses involving inheritance of only one pair of contrasting characters are 

called monohybrid crosses and those involving 2 pairs of contrasting characters 

are called dihybrid crosses. 

206 

Dominant 

Green pod 

Inflated pod 

Grey seed coat 

Term inal 

flower 

A xillary 

flower 

Yellow 

cotyledon 

Green 

cotyledon 

Recessive 

Dwarf stem 

Yellow pod 

Constricted pod 

W hite seed coat 

Wrinkled seed 

Round seed 

Fig : 4.4. Contrasting characters of traits chosen by Mendel

Monohybrid Cross 

(Experiments with garden pea for a single pair of contrasting characters) 

P TT tt 

Tall x Dwarf 

Gametes T t 

F 1 

Tt Monohybrid 

Tall (Selfing) 

Gametes T t 

F 2 

T t 

T TT Tt 

t Tt tt 

TT -1 Homozygous tall 

Tt -2 Heterozygous tall 

tt -1 Dwarf 

Tall : Dwarf = 3:1 

Mendel's Explanation of Monohybrid Cross 

Parental Generation : Mendel selected a pure breeding tall plant and a pure 

breeding dwarf plant as parents (Homozygous). 

F 1 

Generation : He crossed the parents and from the seeds obtained he raised 

the first filial generation. Here the plants were all tall and were called 

monohybrids. 

F 2 

Generation: Mendel allowed selfing of the F 1 

monohybrids and he obtained 

Tall and dwarf plants respectively in the ratio of 3:1. The actual number of 

tall and dwarf plants obtained by Mendel were 787 tall and 277 dwarf. The 

ratio of 3:1 is called Phenotypic ratio as it is based on external appearance 

of offsprings. 

F 3 

Generation: By selfing the F 2 

offsprings Mendel obtained the F 3 

generation. 

He found that 

1. The F 2 

dwarf plants always bred true generation after generation whether 

self or cross pollinated. 

207

2. Of the F 2 

tall plants one third bred true for tallness. The rest two thirds 

produced tall and dwarf in the ratio of 3:1. This meant that the F 2 

generation 

consisted of 3 types of plants. 

i. Tall homozygous (pure) - 25% 

ii. Tall heterozygous - 50% 

iii. Dwarf homozygous (pure) - 25% 

Thus based on the constitution of factors the ratio of a Monohybrid cross is 

1:2:1 which is called the genotypic ratio. 

Mendel's Interpretation and Explanation 

During Mendel's time structure of chromosomes or the role of meiosis was 

not known. So he concluded that the inheritance of characters is by particles called 

hereditary units or factors. 

He explained the results of Monohybrid cross by making certain presumptions. 

i. Tallness and dwarfness are determined by a pair of contrasting factors 

(now called as genes). A tall plant possesses a pair of determiners 

(represented by T-taking the first letter of the dominant character) and a 

plant is dwarf because it possesses determiners for dwarfness (represented 

as t). These determiners occur in pairs and may be alike as in pure breeding 

tall parents (TT) and dwarf parents (tt). This is referred to as homozygous. 

They may be unlike as in the monohybrid (Tt) which is referred to as 

heterozygous. 

ii. The two factors making up a pair of contrasting characters are called alleles 

or allelomorphs. One member of each pair is contributed by one parent. 

iii. When two factors for alternative expression of a trait are brought together 

by fertilization only one expresses itself, (tallness) masking the expression 

of the other (dwarfness). The character which expresses itself is called 

dominant and that which is masked is called the recessive character. 

iv. The factors are always pure and when gametes are formed, the unit factors 

segregate so that each gamete gets one of the two alternative factors. It 

means that factors for tallness (T) and dwarfness (t) are separate entities 

and in a gamete either T or t is present. When F 1 

hybrids are selfed the two 

entities separate and then unite independently forming tall and dwarf plants. 

208

Dihybrid Cross 

(Cross involving two pairs of contrasting characters) 

Mendel also experimentally studied the segregation and transmission of two 

pairs of contrasting characters at a time. This was called the Dihybrid cross or 

Two-factor cross. He took up the round and wrinkled characters of seed coat 

along with yellow and green colour of seeds. 

Mendel found that a cross between round, yellow and wrinkled green seeds 

(P) produced only round yellow seeds in the F 1 

generation, but in the F 2 

generation 

4 types of combinations appeared of which two were different from that of the 

parental combinations, in the following ratio. 

These are : 

Round Yellow - 9 Parental 

Round Green - 3 

New combination 

Wrinkled Yellow - 3 

Wrinkled Green - 1 Parental 

Therefore the offsprings of the F 2 

generation in a dihybrid cross were produced 

is a ratio of 9:3:3:1. This ratio is called the dihybrid ratio. 

In F 2 

generation, since all the four characters assorted out independent of the 

others, he said that a pair of contrasting characters behave independently of the 

other pair. i.e. seed colour is independent of seed coat. At the time of gamete 

formation in the F 1 

dihybrid, genes for round or wrinkled character of seed coat 

assorted independently of the yellow or green colour of seed coat. As a result 4 

types of gametes with two old and two combinations were formed namely RY, 

Ry, rY and ry. These 4 types of gametes on random mating produced 16 offsprings 

in the ratio of 9:3:3:1. The actual number of individuals got by Mendel for each of 

the four classes were 

a. 315 round yellow seeds 

b. 108 round green seeds 

c. 101 wrinkled yellow seeds 

d. 32 wrinkled green seeds 

Mendel represented round character of seed as R and wrinkled as r, and yellow 

character as Y and green character as y. So the dihybrid cross was between parents 

209

having factor constitution as RRYY x rryy. This cross may be represented as 

follows: 

Round Yellow Wrinkled Green 

P RRYY x rryy 

Gametes RY 

ry 

F 1 

RrYy (Dihybrid) 

Round Yellow (selfed) 

Gametes RY Ry rY ry 

F 2 

RY 

Ry 

rY 

ry 

RY Ry rY ry 

RRYy RrYY 

Round Round 

Yellow Yellow 

RRYY 

Round 

yellow 

RRYy 

Round 

Yellow 

RyYY 

Round 

Yellow 

RrYy 

Round 

Yellow 

RRyy 

Round 

Green 

RrYy 

Round 

Yellow 

Rryy 

Round 

Green 

RrYy 

Round 

Yellow 

rrYY 

Wrinkled 

Yellow 

rrYy 

Wrinkled 

Yellow 

RrYy 

Round 

Yellow 

Rryy 

Round 

Green 

rrYy 

Wrinkled 

Yellow 

rryy 

Wrinkled 

Green 

Laws of Mendel 

Based on his experiments of monohybrid and dihybrid cross, Mendel proposed 

three important laws which are now called Mendel's Laws of Heredity. 

i. Law of dominance and recessiveness 

ii. Law of segregation or Law of purity of gametes 

iii. Law of independent assortment 

i. Law of Dominance 

The law of dominance and recessiveness states : "When two homozygous 

individuals with one or more sets of contrasting characters are crossed, the 

210

characters that appear in the F 1 

hybrid are dominant and those that do not appear 

in F 1 

are recessive characters". 

ii. Law of Segregation or Law of Purity of Gametes 

The Law of segregation states that "When a pair of contrasting factors or 

genes or allelomorphs are brought together in a heterozygote or hybrid, the two 

members of the allelic pair remain together without mixing and when gametes are 

formed the two separate out, so that only one enters each gamete". 

This law though it was a conception originally and propagated by Mendel, 

now it has been confirmed by cytological studies. Dominance or no dominance 

segregation holds good for all cases. 

iii. Law of independent Assortment 

Law of independent assortment states : "In case of inheritance of two or more 

pairs of characters simultaneously, the factors or genes of one pair assort out 

independently of the other pairs". 

Mendel gave this law based on his dihybrid cross experiment. Here the total 

number of individuals in F 2 

will be sixteen which occur in a ratio of 9:3:3:1 where 

two parental classes and two new combinations will be produced. 

Back cross and Test Cross 

In Mendelian inheritance F 2 

offsprings are obtained by selfing the hybrid, but 

if the F 1 

hybrid is crossed to any of the pure breeding parents it is called a back 

cross. If the hybrid is crossed to the dominant parent, all the F 2 

offsprings will 

show dominant character. 

If the hybrid is crossed to recessive parent, dominant and recessive phenotypes 

with appear in equal proportions as shown, which is called a test cross. 

Monohybrid Back Cross 

F 1 

Tt x TT 

(Back cross) 

Gametes T t T 

F 2 

T t 

T TT Tt All are tall 

211

Monohybrid Test Cross 

F 1 

Tt x tt (Test cross) 

Gametes T t t 

F 2 

T t 

t Tt tt 

Tall : Dwarf = 1: 1 

Dihybrid Test Cross 

In a dihybrid test cross the four types of phenotypes are obtained in equal 

proportions as shown. The test cross is used to determine whether segregation of 

alleles has taken place and to ascertain if hybrid is homozygous or heterozygous. 

F 1 

RrYy x rryy 

dihybird 

recessive parent 

Gametes RY Ry rY ry ry 

F 2 

RY Ry rY ry 

ry ry 

RrYy 

Round 

Yellow 

Rryy 

Round 

Green 

rrYy 

Wrinkled 

Yellow 

rryy 

Wrinkled 

Green 

RrYy Rryy rrYy rryy 

Round Round Wrinkled Wrinkled 

Yellow Green Yellow Green 

1 : 1 : 1 : 1 


One Mark 


1. The village where Mendel was born is 

a. Heizendors b.Silisian c. Brunn d. Austria 

2. The cross which proves that sex has no influence on inheritance is 

a. Back cross b. Test cross c. Reciprocal cross d. Monohybrid cross 

3. The recessive state for seed coat colour is 

a. Green b. Grey c. Yellow d. White 

212


1. The pairs of contrasting character traits of Mendel are called ................ 

2. The dihybrid test cross ratio is ................ 

Match 

1. Plant height - Wrinkled 

2. Position of flower - Constricted 

3. Colour of pod - Terminal 

4. Seed shape - Dwarf 

5. Pod shape - Yellow 

Two Marks 

1. Name the three scientists who rediscovered Mendel's work 

2. Define true breeding / Monohybrid test cross / Back cross / Alleles / Law of 

purity of gametes / Dihybrid test cross. 

Five Marks 

1. Write short notes on the Life History of Mendel. 

2. Explain the reasons for Mendel's success. 

3. Describe the Monohybrid cross. 

Ten Marks 

1. Write an essay on Mendel's Dihybrid cross. 

2. Give an account of the Laws of Mendel. 

213

3. Chromosomal Basis of Inheritance 

The factors of Mendel were called genes by Johansen 1909, who did not 

know their exact nature and structure. 

Gene Concept 

Sutton introduced the gene concept which was elaborated by the studies of 

Morgan, Bridges and Miller. 

The important features of the gene concept are: 

i. Genes are transmitted from parents to offsprings and are responsible for 

the physical and physiological characteristics of the organism which are 

present inside the nucleus of the cell. 

ii. The genes are present on the chromosome. 

iii. Since the number of genes far exceeds the number of chromosomes, several 

genes are located on each chromosome. In man about 40,000 genes are 

present in 23 pairs of chromosomes. 

iv. The genes are present at a specific position on the chromosome called 

locus. 

v. Genes are arranged on the chromosomes in a linear order like beads on a 

string. 

vi. A single gene may have more than one functional state or form. These 

functional states are refered to as alleles. 

vii. The alleles may be dominant or recessive but sometimes co-dominance or 

incomplete dominance may be seen. 

ix. Genes may undergo sudden heritable changes called mutations, induced 

by chemical and physical factors. 

x. Due to mutation a gene may come to possess more than two alternative 

states and these states of the gene are called multiple alleles. 

xi. Genes undergo duplication by a phenomenon called replication. 

xii. Genes are responsible for the production of proteins called enzymes by 

which they show their expression which brings about a change in the 

organism. 

xiii. A gene is a particular DNA segment which contains the information to 

synthesize one polypeptide chain or one enzyme. The information is 

214

contained as a sequence of nucleotides which is called genetic code. The 

sequence of three nucleotides that code for an aminoacid is called Codon. 

Molecular structure of a gene 

A gene, is made of DNA. The gene may be subdivided into different units 

according to Benzer such as Recon, Muton, Cistron and Operon. 

Recon 

It is that smallest portion of a gene which can undergo crossing over and 

recombination and may be as small as a single nuclecotide pair. 

Muton 

It is the smallest unit of a gene that can undergo mutation and can involve a 

pair of nucleotides. 

Cistron 

It is the functional unit which can synthesize one polypeptide. 

Operon 

It is a group of genes having an operator a structural gene and other genes in 

sequence which all function as a unit. 

Exons and Introns 

In Prokaryotes generally, the genes are continuous segments of DNA occurring 

collinearly without interruption. But in Eukaryotes, the genes on the DNA strand 

have coding regions called exons interrupted by non-coding DNA segments which 

do not carry genetic information called introns. This led to the concept of 

interrupted genes or discontinuous genes. Such genes while producing m-RNA 

will first form a primary transcript which will then cut off the introns to form the 

functional m-RNA and this is called splicing. 

Chromosomal basis of inheritance 

The chromosome basis of inheritance was put forth by Sutton and Boveri 

independently in the year 1902. W.S. Sutton and Theodor Boveri faced and solved 

the problem of drawing a parallel between chromosomes and genes. 

Both had concluded that the genes are contained in chromosomes. Allelic 

genes present in a heterozygote segregate independently because the chromosomes 

carrying these genes segregate when the sex cells are formed. This conclusion of 

Sutton and Boveri was verified extensively by further studies conducted by various 

geneticists and cytologists. 

215

In order to accept this conclusion we must be able to understand the behaviour 

of chromosomes in the light of Mendel's assumption. 

i) Individuality of Chromosomes :Every organisms has a fixed number of 

chromosomes. The nuclei of gametes contain haploid (n) and those of 

zygotes have double the number or diploid (2n) number of chromosomes. 

ii) Meiosis : At the time of meiosis, for the formation of gametes, the pairs of 

chromosomes of the diploid sets undergo pairing. 

iii) The chromosomes of each pair segregate independently of every other pair 

during their distribution into gametes. This is similar to Mendel's law of 

independent assortment in the segregation of factors. 

iv) During the fusion of haploid gametes, the homologous chromosomes from 

two parents are brought together to form the diploid zygote. Accordingly 

Mendel had maintained that maternal and paternal characters mix up in 

the progeny. 

v) The chromosomes maintain the structure and uniqueness during the life 

time of the individual whether observable or not. Mendel had also 

demonstrated that the characters are never lost though they are not expressed 

in a particular generation. 

From these points it is evident that a clear parallelism exists between Mendel's 

factors and chromosomes and so there is a firm basis for Mendel's Laws of heredity 

in the behaviour of chromosomes during meiosis and fertilisation and therefore 

the Chromosomal Theory of Inheritance has been proposed. 

Postulates of the Chromosomal Theory of Inheritance 

i. The factors described by Mendel are the genes which are the actual physical 

units of heredity. 

ii. The genes are present on chromosomes in a linear order. 

iii. Each organism has a fixed number of chromosomes which occur in two 

sets referred to as diploid (2n). A pair of similar chromosomes constitute 

the homologous pair. 

iv. Of this, one set is received from the male parent (paternal) and the other 

from the female parent (maternal). 

v. The maternal and paternal chromosomes are contributed by the egg and 

sperm respectively during zygote formation. But only sperm nucleus is 

involved proving that chromosomes are present within the nucleus. 

vi. The chromosomes and therefore the genes segregate and assort 

independently at the time of gamete formation as explained in Mendel's 

law of segregation and Law of Independent Assortment. 

216

Physical and Chemical Basis of Heredity 

Physical Basis 

Gregor Johann Mendel put forward in 1866 that particles called germinal 

units or factors controlled heredity. These were present in both the somatic cells 

as well as the germinal cells. Though he was not able to actually see these particles, 

he did explain the pattern of inheritance of genetic characters. It was the gamete 

which carried these factors to the next generation and so gametes form the physical 

basis of heredity. 

Chemical Basis 

Now it is known that genes control heredity and these are definite segments 

of chromosomes and so are particulate bodies. The genes travel from one generation 

to the next carrying the traits and since gene is composed of DNA and protein, the 

DNA part functions as the chemical basis of heredity. 


One Mark 

Choose the correct Answer 

1. The smallest unit of the gene which codes for an amino acid is 

a. Cistron b. Muton c. Recon d. Codon 

2. The functional unit of a gene which can synthesize one polypeptide is called 

a. Codon b. Cistron c. Muton d. Recon 

3. The gene is present at a specific position on the chromosome called 

a. Locus b.Nucleotide c. Nucleoside d. Allele 

4. The chromosomal basis of inheritance was given by 

a. Schleiden & Schwann b. Sutton & Boveri 

c. Singer & Nicholson d. Morgan & Bridges 

Two Marks 

1. Define : Exon / Intron / Splicing / Codon 

Five Marks 

1. Explain the molecular structure of a gene 

2. Give an account of the postulates of the chromosome theory of inheritance 

Ten Marks 

1. List the important feature of the gene concept. 

2. Draw a parallel between Mendel's factors and chromosomes and explain the 

chromosomal theory of inheritance. 

217

4. Intermediate Inheritance 

(Incomplete Dominance) 

From Mendel's experiments it had been established that when two alleles are 

brought together from two different pure breeding parents, one of them completely 

dominates over the other manifesting itself in the hybrid. Researches by many 

investigators revealed that in a number of living organisms complete dominance 

was absent, the hybrid exhibited an intermediate character as both the genes of 

the allelomorphic pair showed partial expression. 

Thus in incomplete dominance or partial dominance or intermediate 

inheritance or blending inheritance the F1 hybrid does not resemble either of 

the parents. A very good example for this is the 4 `O' clock plant Mirabilis jalapa 

studied by Correns in 1906. A similar condition is seen in Antirrhinum majus. 

In Mirabilis jalapa, there are two distinctive types of flower colours namely 

the red and the white. Both the types are true breeding. When a pure-red flowered 

(r 1 

r 1 

) variety is crossed with a pure white flowered (r 2 

r 2 

) one, the F1 hybrids produce 

pink flower, a character which is intermediate between red and white coloured 

flowers of the parental generation. This is because neither red flower colour nor 

white is completely dominant over the other. When F1 hybrids were selfed red, 

pink and white flowered varieties were obtained respectively in the ratio of 1:2:1. 

This is the phenotypic ratio. Here the genotypic ratio is also 1:2:1, producing one 

homozygous red, two heterozygous pink and one homozygous white. The red and 

the white varieties breed true on self fertilisation of the F 2 

individuals but the pink 

varieties on selfing once again produce a phenotypic ratio of 1:2:1 proving the 

law of purity of gametes. 

Since neither of the parents is completely dominant over the other, the symbol 

for Red parent is r 1 

r 1 

and the white parent r 2 

r 2 

, and so naturally the genotype of the 

hybrid is r 1 

r 2 

. 

It has been observed in the given example that there is blending of phenotypes 

- not genotypes and the alleles of the genes are discrete or particulate. They appear 

blended in F 1 

but have separated out in F 2 

generation. 

Incomplete dominance is also called blending inheritance because both the 

characters of the parental plants are mixed to give an intermediate character which 

is different from that of the parents. But only the characters are mixed with each 

218

other and not the alleles. In Mirabilis the r 1 

r 1 

always produces red coloured flowers 

and r 2 

r 2 

produces white coloured flowers, when they are combined, the intermediate 

colour namely pink is produced. Because of this it is described as blending 

inheritance. 

Red White 

P r 1 

r 1 

x r 2 

r 2 

Gametes r 1 

r 2 

F 1 

r 1 

r 2 

(Selfed) 

Pink 

Gametes r 1 

r 2 

F 2 

r 1 

r 2 

r 1 

r 1 

r 1 

r 1 

r 2 

r 2 

r 1 

r 2 

r 2 

r 2 

Red : Pink : White 

1 : 2 : 1 

r 1 

r 1 

: r 1 

r 2 

: r 2 

r 2 


One Mark 


1. Incomplete dominance is also called 

a. Intermediate inheritance b. Blending inheritance 

c. Partial dominance d. All the above 

2. The phenomenon of intermediate inheritance is observed in 

a. Lathyrus b. Antirrhinum c. Cucurbita d. Maize 

3. The phenotypic ratio of incomplete dominance is 

a.1:2:1 b.3:1 c.9:3:3:1 d.1:1 

Two Marks 

1. Define : Incomplete dominance 

Five Marks 

1. Why is intermediate dominance also called blending inheritance? 

Ten Marks 

1. Explain intermediate inheritance in the 4' 0' clock plant. 

219

5. Epistasis 

The pioneer work of Gregor Johann Mendel seemed to imply that every 

character was determined by a single factor or determiner or in other words a pair 

of genes influenced one trait. Later work by geneticists led to the idea that a 

character need not necessarily be due to the action of a single factor but may also 

be due to the action of several factors. These hereditary units or factors are now 

known as genes. 

Gene Interaction 

The genes interacting to affect a single trait, if present on different 

chromosomes will show independent assortment and no interference between the 

effects of different genes. The condition where one pair of genes reverses or inhibits 

the effect of another pair of genes by causing the modification of the normal 

phenotype is called gene interaction. 

Types 

Gene interaction is of two types 

1. Allelic or intragenic interaction 

This kind of interaction occurs between alleles of the same gene pair as in the 

case of incomplete dominance, co dominance and multiple allelism. 

2. Non-allelic or intergenic interactions 

These interactions occur between alleles of different genes present either on 

the same or different chromosome and alter the normal phenotype. Complementary 

gene interaction, supplementary gene interaction, duplicate factors and inhibitory 

factors are examples of intergenic interactions. 

Epistasis 

There are two pairs of independent non-allelic genes affecting a single trait. 

The suppression of the gene on one locus of a chromosome by the gene present 

at some other locus is called epistasis meaning "standing over". The gene which 

is suppressed is called hypostatic and the other is the epistatic or inhibiting gene 

which is also called the suppressing gene. 

220

Epistasis can be of the following types. 

1. Due to recessive gene : Recessive gene a masks the effect of dominant 

gene B. 

2. Due to dominant gene : Dominant gene A masks the effect of the dominant 

gene B. Apart from this, the term epistasis refers to all non-allelic 

interactions involving a pair of genes. Therefore epistasis may be 

responsible for the production of several modified dihybrid ratios as follows: 

1. Duplicate recessive epistasis (9:7) 

2. Dominant epistasis (12:3:1) 

3. Recessive epistasis (9:3:4) 

4. Dominant recessive epistasis (13:3) 

5. Duplicate dominant epistasis (15:1) 

Duplicate Recessive Epistasis 

This type of inheritance is also called complementary gene interaction 

observed in Lathyrus odoratus (Sweet pea) by Bateson and Punnett. Inheritance 

of flower colour was studied. 

When two pure breeding white flowered varieties of sweet pea where crossed, 

the F 1 

hybrids were all purple flowered plants. When the F 1 

hybrids were selfed, 

purple and white flowered varieties were produced respectively in the ratio of 9:7. 

Explanation 

Here two dominant genes C and P interact to produce purple colour. When 

any one of the genes is present in recessive condition, colour is not produced. 

Thus both the genes in the recessive state inhibit the formation of purple colour 

and so this has been referred to as Duplicate recessive epistasis. 

Biochemical explanation for production of Flower colour 

Dominant gene (C) controls the production of a pigment precursor called 

chromogen and the dominant gene (P) is responsible for the production of the 

enzyme which converts the chromogen into the pigment anthocyanin which is 

responsible for the purple colour. 

If gene C is absent there is no formation of chromogen and if gene P is absent 

chromogen does not get converted to anthocyanin. Thus both the genes have to be 

in dominant state for production of purple coloured flowers. 

221

White Flowered White Flowered 

P CCpp X cc PP 

Gametes Cp cP 

F 1 

CcPp (Selfed) 

Purple Flowers 

Cc Pp x Cc Pp 

Gametes CP Cp cP cp 

F 2 

Purple : White 

CP 

Cp 

cP 

cp 

CP Cp cP cp 

CCPP CCPp CcPP CcPp 

Purple Purple Purple Purple 

CCPp CCpp CcPp Ccpp 

Purple White Purple White 

CcPP CcPp ccPP ccPp 

Purple Purple White White 

CcPp Ccpp ccPp ccpp 

Purple White White White 

9 : 7 

Dominant Epistasis - 12:3:1 

This type of interaction was studied by Sinnott in summer squash (Cucurbita 

pepo). 

In Cucurbita pepo there are three common fruit colours white, yellow and 

green. White colour is produced due to the presence of dominant gene W. In the 

absence of W, the dominant gene Y produces yellow fruit colour and the double 

recessive is green. The effect of dominant gene `Y' is masked by dominant gene 

`W' which is the epistatic gene so this is called dominant epistasis. 

When pure breeding white fruited variety is crossed with the double recessive 

green variety, the F1 hybrids are all white. When the hybrids are selfed, white, 

yellow and green fruited plants arise respectively in the ratio of 12:3:1 

222

White Green 

P WWYY X wwyy 

Gametes WY wy 

F 1 

WwYy (Selfed) 

White 

WwYy x Ww Yy 

Gametes WY Wy wY wy 

F 2 

White : Yellow : Green 12:3:1 

WY 

Wy 

wY 

wy 

WY Wy wY wy 

WWYY WWYy WwYY WwYy 

White White White White 

WWYy WWyy WwYy Wwyy 

White White White White 

WwYY WwYy wwYY wwYy 

White White Yellow Yellow 

WwYy Wwyy wwYy wwyy 

White White Yellow Green 

Recessive epistasis - 9:3:4 

In Sorghum the dominant gene (P) is responsible for purple colour which is 

dominant over brown (q). 

When both the dominant genes (P and Q) are brought together either in 

homozygous or heterozygous condition, the purple colour is changed to red. 

A cross between purple (PPqq) and brown (ppQQ) results in plants with red 

colour in F 1 

and when the F 1 

heterozygotes are selfed, three kinds of phenotypic 

classes are produced in the ratio of 9:3:4 (9 Red, 3 Purple and 4 Brown). 

Thus in this example, the gene `p' is epistatic to the other colour genes. 

If the Sorghum is pp, it is brown inspite of other genotypes. The expression 

of the colour genes is masked if pp is present. 

The genes for recessive epistasis are also called supplementary genes because 

the gene P determines the formation of colour. The alleles of the other gene Q and 

q specify the colour. 

223

If the other gene, P_Q_ occurs, the colour of the glume will be red. When 

P_qq genotype is present the colour of the glume will be purple. Likewise if pp 

genotype is present the colour of the glume will be brown. 

P PPqq x ppQQ 

Purple Brown 

Gametes Pq pQ 

F 1 

PpQq(Selfed) 

Red 

Pp Qq x Pp Qq 

Gametes PQ Pq pQ pq 

F 2 

PQ Pq pQ pq 

PQ 

Pq 

PPQQ 

Red 

PPQq 

Red 

PPQq 

Red 

Ppqq 

Purple 

PpQQ 

Red 

PpQq 

Red 

PpQq 

Red 

Ppqq 

Purple 

Red: Purple :Brown 

9 : 3 : 4 

pQ 

PpQQ 

Red 

PpQq 

Red 

ppQq 

Brown 

ppQq 

Brown 

pq 

PpQq 

Red 

Ppqq 

Purple 

ppQq 

Brown 

ppqq 

Brown 

Table Differences : 4.2. Differences Between between Epistasis Epistasis and and Dominance 

Epistasis 

Dominance 

i. This type of gene interaction 

involves two non-allelic pairs 

of genes. 

ii. One pair of genes masks the 

effect of another pair of genes 

iii. Expression of both the 

dominant and recessive 

alleles may be suppressed by 

the epistatic gene 

iv. Number of phenotypes in the 

F 2 generation are reduced 

Only one pair of genes is involved, 

therefore there is no interaction. 

An allele masks the effect of 

another allele of the same gene 

pair 

Expression of a recessive allele is 

masked by the dominant allele 

There is no reduction in the 

number of phenotypes of F 2 

generation 

224


One Mark 


1. Inheritance of flower colour in Lathyrus odoratus was studied by 

a. Morgan & Bridges b. Bateson & Punnett 

c. Sutton & Boveri d. Schleiden & Schwann 

2. The inheritance of fruit colour in Cucurbita pepo gives a ratio of 

a. 13:3 b. 12:3:1 c.9:7 d.9:3:4 

3. A ratio of 15:1 is observed in 

a. Sweet pea b. Cucurbita pepo c. Rice d. Sorghum 

Two Marks 

1. Define : Gene interaction / Epistasis / Duplicate factors 

Five Marks 

1. Explain Duplicate recessive epistasis. 

2. Describe the Inheritance of glume colour in Sorghum. 

3. What is the duplicate factor in rice. 

4. Explain dominant recessive epistasis in the inheritance of leaf colour in rice. 

5. Explain inheritance of fruit colour in Cucurbita pepo. 

6. Differentiate between dominance and epistasis. 

Ten Marks 

1. Write an essay on the various epistatic gene interactions you have studied. 

225

This figure is pertained to Unit 2. 

Chapter 6. Cell Membrane 

boundary lipid 

lipid 

bilayer 

intrinsic 

protein 

lipid 

hydrophobic tail 

hydrophilic head 

intrinsic 

protein 

extrinsic 

proteins 

Fig. 2.11 Fluid-mosaic model of the plasma membrane. Proteins floating in a sea of lipid. 

Some proteins span the lipid bilayer, others are exposed only to one surface or the other. 

1

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