NOSTOC
Common Indian Species: Nostoc commune, N. rivulare, N. calcicola, N. hatei and N. sphaericum.
Systematic Position
Sub-division : Algae
Class : Myxophyceae or Cyanobacteria
Order : Nostocales
Family : Nostocaceae
Genus : Nostoc
Common Occurrence: It occurs commonly in fresh water pools, ponds, ditches, etc., in the form of balls. Some species occur epiphytically while others endophytically (as in coralloid roots of Cycas and thalli of Anthoceros). It is one of the common associates of lichens.
Work to be done
To study (i) External features of the thallus, (ii) A single cell, and (iii) Reproductive structures.
External Morphology
1. Thalli are present in the form of colony.
2. Ball-like colony is enveloped by a gelatinous sheath.
3. Balls are greenish to bluish-green in colour.
4. Each colony contains thousands of straight or twisted filaments or trichomes.
5. Each trichome is surrounded by its individual sheath and is called the filament.
6. A trichome is contorted and consists of many cells arranged in a beaded manner.
7. Each cell is somewhat cylindrical or spherical in shape.
8. In filaments there are present some large, spherical or cylindrical, colourless, empty cells called heterocytes.
4. Heterocyata are generally intercalary but in the young conditions they may be terminal.
10. Two polar nodules are present in each heterocyte.
11. Some cells of the filament become enlarged and filled with the food material. These thick-walled cells are called akinetes. Akinetes are generally present in chain.
A Single Cell
1. Each cell is surrounded by a cellulose cell wall .
2 Protoplast shows typical Myxophycean structure, ie., inner colourless centroplasm and outer pigmented chromoplasm. In the chromoplasm are present pigments, proteinaceous cyanophycin granules and cyanophycin starch granules while in the centroplasm is present the incipient nucleus.
Reproductive Structures
1. Sexual reproduction is totally absent. Plant reproduces only by vegetative means, i.e., by hormogones and akinetes.
2. Hormogones are the small segments of trichome which separate generally at the place of heterocyte.
3. On separation, hormogones develop into new trichomes.
4. Akinetes are thick-walled large cells, filled with reserve food material. All the cells between two heterocysts generally change into akinetes.
5. In some species, c.g. Nostoc commune, heterocyst becomes functional and germinates to form new trichome.
Read more :- Comments on Spirulina in English
Post a Comment