Greek azo, to dry, + olluo, to kill, implying that this water fern is killed by drying.
Plants minute, floating on calm water or stranded on mud, usually proliferating by fragmentation. Fronds 1.5-6 em long. Stems zigzagging, branching, slender, fragile, bearing numerous simple roots that hang down into water. Pinnae sessile, imbricate, 2-lobed, with an upper green lobe and a submerged, colorless lower lobe; cavities in leaves contain a nitrogenfixing blue-green alga. Sporocarps heterosporous, microsporocarps and megasporocarps borne in leaf axils on undersurfaces of plants.
A genus of about ten species of very small plants distributed worldwide. Species identification is difficult because microscopic characters such as megaspore ornamentation are needed, but most specimens are sterile. Represented in Hawai 'i by a single naturalized species, although further study may result in the recognition of another.
The genus Azolla in the past has been included in the Salviniaceae, but except for the fact that both are heterosporous floating ferns, Azolla and Salvinia are very different.