Nostoc

Nostoc, a genus of blue-green Alga; (q.v.), the type of the order Nostocaceae and of the sub-class Nostochineae. Of the numerous species a few swim freely in fresh or brackish water; but most are terrestrial, growing on wet rocks or even on comparatively dry soil or within the tissues of other plants, such as the Rhizocarp Azolla, Anthoceros and other Hepaticaa (q.v.) and duckweed. In most species one or more interwoven filaments, made up of minute round cells, are enclosed in a gelatinous mass, known as the thallus or frond, which is formed by the coalescence of sheaths originally enclosing each filament. These masses are green, blue, or violet, hardened externally in the terrestrial species, and sometimes an inch or more across. Nostoc commune, appearing on gravel walks, after a shower, where unnoticed before, is sometimes known as "Falling stars." Nostocaceae. [Nostoc]