BSc 1st Year Botany Range of Thallus Organisation Sample Model Practice Question Answer Papers
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BSc 1st Year Botany Range of Thallus Organisation Question Answers
Describe the Vegetative organization in the algal types studied by you.
Describe the structure and range of forms of the thallus in green algae.
Describe the different types of thallus structure in Chlorophyceae.
Differentiate between Siphonaceous and filamentous thallus.
Wrote note on Heterotrichous habit.
Ans. The members of Algae shows a great variation in the range of vegetative structure from primitive microscopic unicellular form to macroscopic multicellular forms. The following are the various types of plant bodies met within this group.(Botany Range of Thallus Organisation)
1. Motile forms
These forms of algae are the simplest in the organization of thallus. The simple types are unicellular and remain motile practically throughout their life time. Chlamydomonas affords an example of the simplest form seen in the green algae. This is spherical, unicellular, uninucleate and biflagellate structure with a prominent cup shaped chloroplast. The cells swim with the help of their flagella.
The motile unicellular algae unite to form colonial from e.g., numbers of orders volovocales. These colonies move in water with the help of flagella of the peripheral cells. Each colony consists of definite number of cell arranged in a specific manner e.g., Pascheriella has 2-4 cells, Gonium has 4-16 cells, Eudorina has 32 cells, Volvox has 500-2000 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 gelationous matrix, Chrysodeadrous, Tetraspora, Chlorococus, Sphaerocystis, etc.
3. Coccoid forms
In certain members of order chloroccacales 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 or unicellular motile forms. The filamentous algae contain filaments which are composed of several cells placed one above the other e.g., Ulothrix, Spirogyra, Oedogonium, Nostoc, Oscillatoria. 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 found within the chaeophorals among the green algae. This consists of two parts (1) Prostrate creeping system (2) Erect projecting system. Stegiocolonium and Fritschiella, both the systems are fairly developed while in others out of these two systems one is better developed, the other being less developed, reduced or completely suppressed e.g., Choleochate, Drapandiopsis.
6. Siphonaceous forms
In this case the unicellular plant body is enlarged to form a non-septate multinucleate sac. The plan of construction of plant body has its limitations. Protosiphon has an unsepatate, 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 10 cm in height with a definite external form.
7. Complex forms
In some plants, like Chara the plant body is highly developed. In this case the plant remains attached to the soil by means of rhizoid. These branches are of limited and unlimited growth, the former bearing sex organs. Apparently the plant looks like a small angiosperm.
8. Advanced type
These forms arise from filamentous forms in due course of time by gradual evolution. These may be uniaxial forms which have one main axis surrounded by the cells of other adjacent branches. These give the rough appearance of pseudoparenchymatous plant body e.g., Batrachospermum.
Some forms may be Multiaxial which have a single central siphons which may or may not be inter connected with each other e.g., Polysiphonia.
9. Parenchymatous type
These forms develop by the rapid division of cells in filamentous habit. These forms shows multicellular with many layered structures e.g., Ulva, Fucus, Sargassum.
Thus, from the above account we see that there is gradual evolution of vegetative structure from unicellular forms to multicellular parenchymatous forms.