A specialized family of free-floating, aquatic ferns of uncertain affinities. The branched stems bear roots, and 2-lobed, overlapping leaves. Sori bear either microsporangia or megasporangia. The megasporangia are each provided with floats; microspores are numerous and grouped together into ‘massulae’ which bear barbed hairs (glochidia) that hook on to the megaspores. The leaves have a basal cavity inhabited by a cyanobacterium (Anabaena azollae) which fixes nitrogen. There is 1 genus, Azolla, with 6 species, of tropical and warm temperate distribution in America and Asia. They are not native in Europe, but are naturalized there.
Subjects: Plant Sciences and Forestry.