It is blue green alga of filamentous form. Nostoc is present in ponds, ditches and other pools and also in soil. Few species lie in intercellular cavities of plants like duck weed and root of cycas and are called endophytic. Some lead symbiotic life with a fungus forming a lichen.
Each jelly like mass of Nostoc consists of many slender long and short inter woven filaments which look like chain of beads. Each filament is un-branched and consists of a row of rounded or oval cells like series of beads in a chain. There is often a gelatinous sheath convering each filament in which entangled masses of Nostoc filaments remain embedded. Each cell is differentiated into two regions called chromplasm and an inner colourless region called central body (as in oscillatoria).
The filament increases in length by cell division in one plane only. Some enlarged vegetative cells with thickened walls and transparent contents are seen to occur at frequent intervals and also at the ends are called heterocysts. A pore is present at each pole of the heterocysts maintaining cytoplasmic connection with the adjoining cells. There is one pore in the terminal heterocyst. At a later stage the pore is closed by a button like thickening of the wall called the polar molecule.
Nostoc reproduces vegetatively by fragmentation of the filament and sometimes asexually by resting cells (Spores) called akinetes.
Fragmentation: The filament breaks up into at the junction of the heterocyst and the adjoining cell into a number of shore fragments called hormogonia. Each hormogonium grows in length by cell divisions in one plant only. The heterocyst may other wise be a food storage cell.
Akinetes: Here and there some vegetative cells of the filament may become enlarged and thick walled containing reserve food. These are resting cells (spores) called akinetes. Later they may germinate into Nostoc filaments.