Rhizosperma Meyen.

Plants usually very small, floating. Stems prostrate or erect, very short and slender, easily broken, green, branching pinnate or falsely dichotomous, normally decumbent and floating on water surface, up to 5 cm above water surface when in shallow water or in crowded situations. Fronds sessile, alternate, in 2 rows along upper side of stem, usually overlapping, divided into dorsal (floating) lobe and ventral (submersed) lobe; dorsal lobe oblong or ovate, slightly concave abaxially at middle, densely papillose adaxially, fleshy, with a cavity near base containing blue-green algae (Anabaena), green, yellow, or red; ventral lobe shell-like, membranous, tightly imbricate, transparent, colorless, or reddish and slightly thickened near base, sometimes resembling ventral lobe when stem is erect and emergent from water. Sporocarp usually in pairs [or 4 together], at base of lateral branches; megasporocarp located under microsporocarp, oblong or ovoid, containing 1 megasporangium producing 1 functional megaspore; megaspore topped with conic structure (indusium) covering 3-9 colorless spongelike floats and an Anabaena colony; microsporocarps globose or peach-shaped, large, 4-6 × size of megasporocarp, umbonate at apex, wall thin and transparent, containing many microsporangia, each microsporangium containing 32 or 64 microspores, embedded in 5-8 colorless massulae, covered with various attachments according to species. x = 22.

The color of the lamina can change from green to yellow or red dependent on environmental conditions (e.g., high temperature, lack of nutrients, salinity). The blue-green algae fix atmospheric nitrogen.

About seven species: tropical to temperate regions worldwide; two species (one introduced) in China.