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Nostoc A genus of filamentous cyanobacteria (section IV), in which the filaments have a distinctly beaded appearance under the microscope. The filaments typically aggregate to form gelatinous colonies. They are capable of nitrogen fixation. They are found in aquatic and terrestrial habitats. Nostoc punctiliforme occurs in symbiotic association with the plant Gunnera, living in the leaf bases where it fixes atmospheric nitrogen. Nostoc is also found in coralloid root nodules in cycads, as the phycobiont in certain lichens (e.g. the dog lichen, Peltigera canina), and in lichen cephalodia. Some species of Nostoc are used as food in China and Japan.
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nostoc a micro-organism composed of beaded filaments which aggregate to form a gelatinous mass, growing in water and damp places and able to fix nitrogen from the atmosphere. It was formerly believed to be an emanation from the stars, and the name was invented by Paracelsus.