Description from Flora of China
Plants aquatic, forming floating mats; roots present (Azolla) or lacking (Salvinia); stems horizontal, protostelic, vegetative reproduction rapid due to fragmentation. Fronds with exposed upper surface covered with water-repellent papillae, fronds in whorls of 3: two fronds floating, green, lamina orbicular to oblong, small (8-25 mm), third one submersed, finely dissected and rootlike (Salvinia), or fronds alternate, distichous, minute (ca. 1 mm), 2-lobed: one lobe floating, with a cavity near base containing blue-green algae (Anabaena), other lobe submersed, only 1 cell thick (Azolla). Sporangia contained within sporocarps, these inserted on submersed frond and externally uniform (Salvinia) or in pairs with a globose microsporocarp and a smaller ovoid megasporocarp (Azolla); spores of two kinds (plants heterosporous), microspores globose, trilete, megaspores large, spore germination endosporic.
Lin Youxing. 2000. Salviniaceae and Azollaceae. In: Lin Youxing, ed., Fl. Reipubl. Popularis Sin. 6(2): 340-345.
Two genera and ca. 17 species: tropical to temperate regions worldwide; two genera and four species (one introduced) in China.
(Authors: Lin Youxing (林尤兴), Shi Lei (石雷); A. Michele Funston, Michael G. Gilbert)