Azolla  is an aquatic heterosporic fern, triangular or polygonal in shape. It floats on the water surface in ponds, ditches and paddy fields of tropical and warm temperate regions. Sometimes it gives the appearance of a green-reddish carpet, floating over the water. This genus is the only one of the Azollaceae family and has six to eight recognised species, belonging to two subgenera:

Azolla filiculoides

Azolla filiculoidesAzolla pinnata

Azolla pinnata
The plant has leaves with two lobes. In the dorsal lobe there is an ovoid cavity which contains a endosymbiotic community. This community is composed of two types of prokaryotic organisms: one species of nitrogen-fixing filamentous cyanobacteria - Anabaena azollae  Strasb and a variety of bacteria that some are identified as Arthrobacter  sp. and associated with other showing the presence of nitrogenase. As a consequence of this association, in some regions of the globe (Africa and Asia), Azolla  is used as a nitrogen biofertilizer, namely in rice fields.