COLEOCHAETE - STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION

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A. CLASSIFICATION:
Division – Chlorophyta
            Class – Chlorophyceae
                        Order – Chaetophorales
                                    Family – Coleochataceae
                                                Genus – Coleochaete
The genus Coleochaete comprises about 10 species. It is a fresh water alga which usually grows as an epiphyte upon other algae such as Vaucheria, Oedogonium, etc. Some of them are also found to grow upon other submerged aquatic angiosperms such as Hydrilla, Potamogeton, Oedogonium, Lemma, Thypa, Trapa, etc. Some species also grow endophytically with the cells of Chara and Nitella. The disc-like species (C. scutata) grow on shells, glass and other substrates.
B. STRUCTURE OF THE VEGETATIVE BODY:
         The structure of Coleochaete shows diverse forms. Some species of Coleochaete have a very much branched filamentous thallus which is distinctly heterotrichous in habit (C. pulvinata). Other species grow as entirely prostrate in which the branches radiating from a common centre and are laterally apposed to form a pseudoparenchymatous disc. However, occasionally erect branches may arise from the prostrate system at short intervals.
In C. irregularis the thallus is distinctly branched, prostrate filaments. In the thalli of C. solute, C. scutata and C. orbicularis, the thalli are more regular and circular.

C. REPRODUCTION: Coleochaete reproduces both by asexual and sexual methods of reproduction.
1. Asexual Reproduction:- It takes place by means of motile asexual spores called zoospores. Zoospores are developed from the terminal or sub-terminal vegetative cells of the thallus. Each zoospore is biflagellate, ovoid or rounded structure. It possesses a single chloroplast and is without eye spot. On maturity the zoospores escape through the opening formed at the apex of a short papilla-like outgrowth.
After liberation each zoospores takes a short period of rest and secretes a wall, restore flagella and produce a one-celled germling.  The germling soon begins to germinate. During germination it divides transversely into two daughter cells. The upper one forms the seta and the lower one divides and redivides, finally producing a discoid thallus.
In heterotrichous form the germinating zoospore first produces a branched prostrate system from which the branches of erect system arise later.
          2. Sexual Reproduction:- Sexual reproduction is of advanced oogamous type followed by the fruit formation. The plants may be monoecious (C. pulvinata) or dioecious (C. scutata). The male sex organs are called antheridia and the female oogonia or carpogonia.
(a) Oogonia:- The female sex organs i.e., oogonia are formed terminally on the marginal cells of the short lateral branch of the projecting filaments.
Each oogonium is a cone shaped or flask shaped structure with a swollen basal portion and a long neck-like extension called trichogyne. Hence oogonia are called carpogonia. The basal portion contains a single nucleus and chloroplast with a pyrenoid. All these combined to form the egg. At maturity the trichogyne disintegrates.
(b) Antheridia:- The antheridia are usually borne terminally in groups on the branches of erect system. The antheridia arise as colourless, bluntly conical outgrowths from the terminal cells of the lateral branches. The protoplast of each antheridium metamorphoses into a single colourless and biflagellate antherozoid or sperm.

(c) Fertilization:- The liberated antherodoids swim towards the opened neck of the oogonium. On reaching the neck, they are attached to the disintegrated trichogyne. Finally, one antherozoid enters the oogonium and fuses with the egg to complete the fertilization. As a result a diploid zygote (2n) is formed.
After fertilization the zygote is retained within the oogonium, secretes a thick wall and increases in size greatly. Under favourable condition, the zygote nucleus (2n) divides meiotically until 16 – 32 daughter nuclei are formed. Simultaneously, the wall formation between the nuclei takes place, which results in the formation of daughter cells.
Each daughter cell is metamorphosed into a single biflagellate zoospore or swarmer. All the swarmers are liberated by breaking of the zygote wall, and after a little period of rest, it develops into a new individual.

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