Give a brief and illustrated account of the range of structure of the thallus in Chlorophyceae.

Ans. The members of Chlorophyceae show a great variation in the range of vegetative structure from primitive microscopic unicellular form to macroscopic multicellular forms. The following are the various type of plant bodies met within this class.

1. Motile forms: These forms of algae are the simplest in the organization of thallus. The simplest type of chlorophyceae are unicellular and remain motile practically throughout their lifetime. Chlamydomonas affords an example of the simplest form seen in the green algae. This is a spherical, unicellular, uninucleate and biflagellate structure with a prominent cupshaped chloroplast. The cells swim with their flagella.

The motile unicellular algae unite forming colonial form e.g. members of order volvocales. These colonies move about in water with the help of flagella of the peripheral cells. Each colony consists of definite number of cells arranged in a specific manner e.g. Paschariella has 2-4 cells. Gonium has 4-16 cells, Eudorina has 32 cells, Volvox has 500 to 20000 cells in each colony.

2. Palmelloid forms: Palmella stage is a temporary phase in the life history of many algae e.g. Chlamydomonas but in certain members of Chlorophyceae, Xanthophyceae. Chrysophyceae etc. this state of existence becomes permanent. The plant body remains embedded in a common gelatinous matrix of reproduction e.g. Chrysodendron, Tetrospora etc.

3. Coccoid forms: In certain members of order chlorococcales the small non-motile cells are held together to form non-motile colonies with either a definite or indefinite number of cells. They are free floating colonies e.g. Pediastrum, Hydrodictyon (water net).

4. Filamentous forms: Some algae have thread like plant body. These threads are known as filament. The filamentous forms have been derived either from palmellate at unicellular motile forms. These filamentous algae contain filaments which are composed of several cells placed one above the other e.g. Ulothrix, Spirogyra, Oedogonium. This is known as unbranched filamentous habit. Sometimes the filaments may be branched e.g. Cladophora. This is known as branched filamentous habit.

5. Heterotrichous forms: This type of thallus met within the chaetophorales among the green algae. This consists of two parts (1) Prostrate creeping system. (2) Erect projecting system. In stigeoclonium both the systems are fairly developed while in other of these two systems is better developed, the other being developed, reduced or completely suppressed e.g. Coleochaete, Draparnaldiopsis.

6. Siphonaceous forms: In this case the unicellular plant body has enlarged to form a non septate multinucleate sac.

The part of construction of plant body has its limitations Protosiphon has an unseptate, unbranched : tubular thallus containing numerous nuclei. In Vaucheria the thallus is branched and contain numerous nuclei. In Caulerpa there is a complex development of coenocytic branches to form a thallus, which may be as high as 10 cms. in height with a definite external form.

7. Complex forms: In some plants, like chara the plant body is highly developed. In the case the plant remains attached to the soil by means of rhizoids. These branches are limited and unlimited growth, the former bearing sex organs. Apparently the plant looks like a small angiosperms.

The above account of the green algae indicates that there is a gradual evolution of vegetative structure from unicellular motile forms to multicellular complex forms.

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