Type species: Nostoc commune Vaucher ex Bornet et Flahault 1888. - More than 200 described species, from which about 40 are recognizable according to present knowledge. Taxonomic revision not yet finished. Probably two subgenera.
Descriptions:
Komárek (1992): Thallous; thallus micro- or macroscopic, gelatinous, amorph or spherical, later irregularly spherical, lobate, smooth or warty on the surface, filamentous or forming flat gelatinous or "paper-like" (when dry) colonies, usually with distinct periderm on the colonial surface. Filaments within colony irregularly coiled and loosely or densely agglomerated, sometimes more gathered in peripheral layer; sheaths around trichomes present, but visible usually only in the periphery of colony or in young colonies, wide, fine mucilaginous, confluent with colonial mucilage, sometimes yellowish-brown. Trichomes isopolar, of the same width along the whole length, apical cells morphologically not different from other cells; cells cylindrical, barrel-shaped up to almost spherical (forming moniliform trichomes; variability of cell-size and -shape sometimes distinct in one and the same species). Heterocytes solitary, develop in trichomes terminally or interca- lary. (their frequency or absence is dependent on nitrogen metabolism); trichomes in principle metameric. Akinetes arise apoheterocytic, oval, little larger than cells; almost all cells between heterocytes change suc- cesively in akinetes towards heterocytes. Nostoc has special life (vegetation) cycle, during which forms several special and characteristic stages (Mollenhauer 1986, Lazaroff 1972, Komárek & Anagnostidis 1989). From several described subgenera are probably acceptable typical Nostoc and Amorphonostoc, which differ in morphology of colonies (Amorphonostoc has not periderm) and by a type of life cycle, which is more simple in Amorphonostoc.
Genotype differences, molecular data: