Aquatic or subaquatic perennial herbs with stems free-floating or rooted in the ground. Lvs usually alternate, sometimes opposite on flowering shoots; petiole sheathing at base. Infl. a fascicle, involucrate head, or simple paniculate cyme. Fls ☿, actinomorphic, 5-merous. Calyx deeply 5-lobed. Corolla 5-lobed, usually deeply so; lobes ± valvate in bud, with margins wing-like, and sometimes fimbriate or crested. Stamens 5, inserted on the sinuses between the corolla lobes or on the corolla tube; anthers sagittate, versatile. Ovary superior or semi-inferior, 1-locular, with 2 parietal placentae; style 1, 2-fid. Ovules few to numerous. Fr. capsular or indehiscent. Seeds endospermic, sometimes winged.