Nos"toc (?), n. [F.] (Bot.) A genus of algæ. The plants are composed of moniliform cells imbedded in a gelatinous substance.

&fist; Nostoc commune is found on the ground, and is ordinarily not seen; but after a rain it swells up into a conspicuous jellylike mass, whish was formerly supposed to have fallen from the sky, whence the popular names, fallen star and star jelly. Also called witches' butter.