a class of blue-green algae, comprising species of multicellular algae with monostichous (less frequently, polystichous) filamentous thalluses.
The cells in Hormogoneaephyceae are joined together by plasmodesms. The filaments are simple (very rarely, branched) and either single or gathered into bundles or spherical colonies. Sometimes the filaments are adapted to floating slowly in the water and making smooth oscillating movements (Oscillatoria). The filaments of many of the Hormogoneaephyceae have so-called heterocysts, or cells with a thick membrane and without content. Reproduction is accomplished by the hormogonia (multicellular parts of the filament) or, more rarely, by spores, one from each cell, which arise from vegetative cells that grow in volume. Hormogoneaephyceae are found everywhere; they live in soil, in deep water, and at the bottom of bodies of water (both fresh water and saltwater). Some species cause water bloom or accumulate on hydraulic engineering structures; others are capable of atmospheric nitrogen fixation.