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Nostoc is reproduced only by vegetative and asexual methods. The sexual reproduction is completely absent.
Nostoc reproduces vegetatively by the following methods;
Old trichome becomes very large and irregular due to which it gets to break up into short fragments. These short fragments of trichome divide vegetative cells and develop into new trichome.
Hormogones are the short fragment of trichomes. Hormogones are developed in the region of heterocyst. Then they came out of the trichome due to some movement. They divide vegetative cells and developed heterocyst and again get surrounded by mucilage sheath. In this way, new trichome is formed.
Nostoc reproduces asexually by following methods;
In unfavourable conditions, vegetative cell stores enough food materials and get enlarged in size. They secrete thick wall around and undergo into resting period. In return of favourable condition, akinetes germinate and give rise to new trichome.
In some species of Nostoc, heterocyst cell divides transversely and form (2-4) celled hormogones. These hormogones come out by bursting the thick wall of heterocyst and germination occurs to give rise to new trichome.
In few species of Nostoc, heterocyst cell undergoes irregular cell division and forms thin-walled spores which are called endospores. Endospores come out by bursting thick wall of heterocyst and germination occurs and new trichome is formed.