Occurrence, thallus structure and reproduction of nostoc
Nostoc
systematic position
Class - Cyanophyceae
Order - Nostocales
Family - Nostocaceae
Occurrence:
Species of nostoc occur in freshwater as well as in terrestrial habitats. The freshwater species form macroscopic colonies on the surface of water which vary from few millimeters to 8 cm or more in diameter.
Thallus Structure
The trichomes of nostoc are uniseriate, usually contorted and twisted in various ways. An important feature is the presence of prominent constrictions between the adjacent cells which give a characteristics moniliform appearance to the trichome. Each trichome is enveloped in a gelatinous sheath which is transparent, hyaline or coloured. Usually many trichomes aggregate together, and their gelatinous envelopes dissolve to form colonies of various shapes and sizes.
The trichomes of nostoc are characterised 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 nodule.
Cell structure: The cells are spherical or rounded and exhibit typical cyanophycean cell structure. The cell wall is mucopolymeric. The protoplast is diffrentiated into the peripheral chromoplasm and the central, centroplasm or nucleoplasm. The chromoplasm has pigments, cynophycin protein granules and cyanophycean starch. The central colourless centroplasm is the nuclear region of the cell. Unlike eukaryotic cells, it lacks nuclear membrane and nucleolus. This type of nucleus is called incipient nucleus.
Reproduction:
Nostoc reproduces only vegetatively. Sexual reproduction is completely absent. The vegetative propagation takes place by the following methods.
I) By Fragmentation
The colony may into small fragments due to mechanical, physiological or other factors. Each fragment has the capability to develop into a new colony.
II) 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 divide inside the gelatinous sheath of the parent colony. This results in a large number of trichomes inside the parent colony.
III) By akinetes
During unfavourable period, some cells of the trichome are transformed into resting spore or akinetes. They have an additional three layered coat outside the normal cell wall. The outer layer is variously sculptured. The protoplasm is full of food reserves.
Development of akinetes normally takes place in a mature colony. It is always the vegetative cell adjacent to the heterocyst which is the first to metamorphose into akinete. Subsequently, many vegetative cells are transformed into akinetes. It is not unusual to find all the cells between the two successive heterocyst transformed into akinetes. Metamorphosis of the vegetative cell into akinete is probably stimulated by certain chemical substances secreted by heterocysts.
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.
IV) By heterocysts
In some species of nostoc heterocysts act as resting spores. The protoplasm of the heterocyst of such species becomes functional and germinates to form a new trichome. At the time of germination the protoplast divides by a transverse wall to form two cells. Both these cells divide again to form a celled germling. The thick wall of the heterocyst ruptures at this stage and the germling comes out and develops into a new trichome.
V) By endospore
In some species of nostoc the protoplasm of the heterocyst divides successively to form endospores. Unlike akinetes, the endospores are resting spores. Disintegration of the wall results in their liberation, and the rounded spores later germinate to form new trichomes.
Fusion of Filaments
Although true sexual reproduction and sex organs are absent in nostoc as in other blue greens; yet fusion and anastomosis of trichomes have been observed in N. muscorum. This phenomenon can be compared with somatogamy.
