Nostoc Vaucher ex Bornet et Flahault
 
 
Nostoc filaments are made up of spherical or barrel-shaped cells of uniform size that are blue-green or olive green in color. The bent, kinked, or coiled filaments are long, isopolar, and held together by firm mucilage. The colonies may be microscopic or macroscopic, smooth or warty in texture, loose or dense, and spherical, flat, gelatinous, or in irregular mats. Spherical colonies are usually marble-sized, but may be as large as 30-50 cm in diameter. The mucilaginous sheaths are firm and wide, yellow, brown, or black in color, and are most easily viewed in young colonies.
 
The heterocysts (arrow, top right image) are solitary, barrel-shaped or spherical, and may be intercalary or located at the ends of the trichomes. The akinetes are ellipsoidal and only slightly larger than the vegetative cells. The akinetes of Nostoc are usually located halfway between the heterocysts. This differs from Anabaena, where the akinetes are normally adjacent to the heterocysts.
Mucilage sheaths are clearly visible on spheres (left) and individual filaments (right) stained with India Ink.