Herbs annual, aquatic. Stem prostrate, slender, rooting at nodes. Leaves opposite or verticillate, shortly petiolate; leaf blade small, margin usually entire. Flowers usually only 1 per node, very small. Sepals 2-4, united at base, membranous, apex subacute. Petals 2-4, longer than sepals, apex obtuse. Stamens 1 or 2 × as many as petals. Ovary globose, compressed, 2-4-loculed, apex truncate; ovules numerous; styles 2-4; stigmas capitate. Capsule membranous, 2-4-septicidal. Seeds straight, curved, or horseshoe-shaped, surface reticulate-striate with hexagonal or elliptic pits.

About 25 species: tropical, subtropical, and temperate regions; three species in China.