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From The Collaborative International Dictionary of English v.0.48:
Nostoc \Nos"toc\, prop. n. [F.] (Bot.) A genus of algae. The plants are composed of moniliform cells imbedded in a gelatinous substance. [1913 Webster] Note: Nostoc commune is found on the ground, and is ordinarily not seen; but after a rain it swells up into a conspicuous jellylike mass, which was formerly supposed to have fallen from the sky, whence the popular names, fallen star and star jelly. Also called witches' butter. [1913 Webster]