Thallus structure:
The trichomes of nostoc are uniseriate. An important feature is the presence of prominent constriction between the adjacent cells. Each trichome is enveloped in a gelatinous sheath which is transparent, hyaline Or coloured. Usually many trichomes aggregate together together, and their gelatinous envelope dissolve to form colonies of various shapes and sizes. The trichomes of nostoc are characterized by the presence of heterocysts which are intercalary Or occasionally terminal, usually solitary and of much the same size and shape as the vegetative cell. The intercalary heterocysts have two polar nodules and the terminal ones have only one polar nodule.
Cell structure:
The cells are spherical or rounded and exhibit typical cyanophycean cell structure. The cell wall is mucopolymeric, protoplast is differentiated into the peripheral chromoplasm and the central, centroplasm Or nucleoplasm. The chromoplasm has pigments, cynophycin protein granules and cynophycean starch. The central colourless centroolasm is the nuclear region of the cell unlike eukaryotic cells, it lacks nuclear membrane and nucleolus.
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| Cell structure |
Reproduction in Nostoc:
Nostoc reproduces only vegetatively, sexual reproduction completely absent.
1) By fragmentation:
The colony may break into small fragments due to mechanical, physiological or other factors. Each fragment has the capability to develop into a new colony.
2) By hormogonia:
The trichome breaks into small segments due to the degeneration of intercalary vegetative cells or because of the presence of intercalary heterocysts. Multicellular fragments so formed are called hormogonia. They come out of the gelatinous sheath of the colony, grow rapidly and form new colonies very often, the hormogones fail to come out of the parent colony and decide inside the gelatinous sheath of the parent colony. This results in a large number of trichomes inside the parent colony.
3) By Akinetes:
During unfavourable period, some cells of the trichome are transfered into resting spores or akinetes. They have an additional three layered coat outside the normal cellwall. The protoplasm is densly filled. Development of akinetes normally takes place in a mature colony. It is always the vegetative cell adjuscent to the heterocyst. Metamorphosis of the vegetative cell into akinetes is probably stimulated by certain chemical substances secreted by heterocyst cells. The akinetes have tremendous resistance for cold and drought during favourable conditions, the protoplasm becomes active and breaks the thick outer wall and forms a new trichome.
4) By Heterocysts:
In some species of nostoc heterocyst act as resting spores. The protoplasm of the heterocyst of such species becomes functional and germinate to form a new trichome. At the time of germination the protoplast decides by transverse wall to form 2 cells and 2-4 cells called germling. The thick wall of the heterocyst ruptures at this stage and the germling comes out and develops into a new trichome.
5) By Endospore:
In some species of nostoc the protoplasm of the heterocyst decides successively to form endospores. Unlike akinetes, the endospores are thin walled and hence cannot be regarded as resting spores. Disintegration of the wall results in their liberation, and the rounded spores later germinate to form new trichomes.
6) Fusion of filaments:
Although true sexual reproduction and sex organs are absent in nostoc as in other algae. The fusion and anastomosis of trichomes have been observed. This phenomenon can be compared with somatogamy.
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| Reproduction by Harmogonia and Heterocyst |
