Botany Notes – On Diatom Structure – For W.B.C.S. Examination.
It is a large group of algae consisting of 200 genera and over 10,000 species, out of which 92 genera and about 569 species are reported from India. They are commonly known as Diatoms. The diatoms are the most beautiful microscopic algae due to their structure and sculpturing of their walls.Continue Reading Botany Notes – On Diatom Structure – For W.B.C.S. Examination.
They occur in various habitats like fresh water, saline water and also in terrestrial condition on or within the soil. Sometimes they also occur as epiphytes along with algae, on the leaf of forest trees, mostly in tropical rain forests. Depending on the mode of nutrition they may be photosynthetic autotrophs or photosynthetic symbionts or heterotrophs.
Characteristics of Diatoms:
The important characteristics of the class Bacillariophyceae are:
1. They are commonly unicellular and free- living but some members form colonies of various shapes like filaments, mucilaginous colonies etc.
2. Microscopic cells are of different shapes. They may be oval, spherical, triangular, boat- shaped etc.
3. Plant bodies are either bilateral or radial in symmetry.
4. The cells are surrounded by a rigid cell wall, called frustule, consisting of upper epitheca and lower hypotheca; arranged in the form of a box with its lid.
5. The cell wall is composed of pectic substances impregnated with high amount of siliceous substance.
6. The wall may have secondary structures like spines, bristles etc.
7. Vegetative cells are diploid (2n).
8. The cells generally have many discoid or two large plate-like chromatophores. Some cells possess stellate chromatophore.
9. The photosynthetic pigments are chlorophyll a, chlorophyll c along with xanthophylls like fucoxanthin, diatoxanthin and diadinoxanthin.
10. Reserve food is oil, volutin and crysolaminarin.
11. Some vegetative cells show gliding movement.
12. Motile structure (antherozoid) has single pantonematic flagellum.
13. Vegetative multiplication takes place by cell division, which is very common. Some of the cells become very much reduced in size.
14. They produce characteristic spore, the auxospore which develops to regain the normal size.
15. Sexual reproduction takes place by isogamy and oogamy.
Classification of Diatoms:
Fritsch (1935) and many others followed the classification of Hustedt (1930).
Occurrence of Diatoms:
Diatoms are found in all possible habitats. Commonly they are found in fresh water (Denticula tenuis, Navicula pupula, Meridion circulare, Cymbella ventricosa, Melosira variens, Amorpha ovalis etc.), sea water (Corethron, Biddulphia, Sceletonema, Fragilaria, Tropido- nensis etc.) and soil (Pinnularia, Navicula, Frustulia etc.).
The terrestrial species (Amorpha, Navicula, Pinnularia etc.) are able to withstand desiccation for a long period. Some diatoms (Gomphonima, Cymbella etc.) can grow as epiphyte on other algae (Enteromorpha, Cladophora etc.) and higher plant. Licmophora, a member of diatom, grows endozoically.
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