Alisma sect._Caldesia (Parl.) Hook. f.
Annual or perennial, fixed floating hydrophytes. Leaves petiolate, floating, broadly elliptic to ovate, truncate to deeply cordate at the base, obtuse. Inflorescence erect, pyramidal panicle consisting of 3, verticillately arranged branches and flowers, scape longer than leaves. Flowers bisexual, white, pedicellate; brac¬teate. Thalamus minute. Sepals 3, patent or reflexed. Petals 3, white. Stamens 6, rarely up to 11, filaments filiform, anthers oblong, basifixed. Carpels (2-) 6-9 (-20), almost terete, beakless, crowded; ovaries with ventral, usually persistent styles. Achenes drupaceous, with smooth, ridged or warted exocarp and sclerenchymatous endocarp. Seeds oblong.
A small genus of 4 species; mainly distributed in the Old World tropics. It is closely allied to Alisma from which it differs in its crowded, definite carpels and the sclerenchymatous endocarp of the achenes.