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, includes A. filiculoides Lam., A. Rubra
R.Br. (considered by some authors as a A. filiculoides variety)
, A. caroliniana Willd., A. microphylla Kaulf. and
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.
Rhizosperma comprises A. nilotica Decne, A.
pinnata R.Br. and A. imbricata (Roxb. ex Griff) Nak. (considered
by some authors as a A. pinnata ).