Nostoc

Nostoc is a genus of fresh water cyanobacteria that forms spherical colonies composed of filaments of moniliform cells in a gelatinous sheath. When on the ground, a Nostoc colony is ordinarily not seen; but after a rain it swells up into a conspicuous jellylike mass, which was once thought to have fallen from the sky, whence the popular names, fallen star and star jelly. It is also called witches’ butter.

It can be found on moist rocks, at the bottom of lakes and springs, and rarely in marine habitats. These bacteria sometimes contain photosynthetic pigments in their cytoplasm to perform photosynthesis. It may also grow symbiotically within the tissues of plants, such as the aquatic fern Azolla (mosquito fern) or hornworts, providing nitrogen to its host.