Any of the cyanobacteria that make up the genus Nostoc. The cells are arranged in beadlike chains grouped together in a gelatinous mass. Ranging from microscopic to walnut-sized, nostoc masses may be found on soil and floating in still water. A special thick-walled cell has the ability to withstand drying for long periods of time. After 70 years of dry storage, cells of one species can germinate into filaments when moistened. Like most other cyanobacteria, nostocs contain two pigments and are capable of nitrogen fixation.