Discuss Nostoc As An Example Of Cyanobacteria?


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Habitat and occurrence
Nostoc is common in both terrestrial and sub aerial habitats. It is widely distributed in alkaline soils and on damp rocks and cliffs. The Nostoc forms a jelly like mass in which numerous filaments (trichomes) are embedded.

The cells are joined to form trichomes, which are unbranched and appear moniliform. Individual cells are mostly spherical but some times barrel shaped or cylindrical.
All cells in trichome are mostly similar in structure but at interval are found slightly large, round, light yellowish thick walled cells known as heterocycts. Trichome mostly breaks near heterocyst and forms hormogonia. So heterocysts help in fragmentation.

Sexual reproduction is absent. It reproduces asexually by formation of hormognia. Hormogonia are formed when filament breaks at different points into smaller pieces. This is due to death and decay of an ordinary cell or the heterocyst may serve as a breaking point. Reproduction can also be due to akinetes formation. Akinetes are thick walled, enlarged vegetative cells, which accumulate food and become resting cells. On the onset of favorable conditions they form normal vegetative cells.

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