A small order of heterosporous, leptosporangiate ferns (division Polypodiophyta) which float on the surface of the water. The delicate, branching stem becomes as much as 1 in. (2.5 cm) or so long and is provided with small, simple to bifid or more or less dissected leaves. The sporangia are enclosed in specialized appendages of the leaves, called sporocarps. A sporocarp contains numerous microsporangia or a single megasporangium, but not both types. The order contains only a single family, with two widely distributed genera, Salvinia and Azolla, with only about 20 species in all. Azolla is of some interest because of its symbiotic relationship with Anabaena azollae, a nitrogen-fixing blue-green alga which inhabits specialized chambers in the leaves.