Fls minute, unisexual, axillary, solitary, or ♂ and ♀ paired; ♂ reduced to 1 stamen with long slender filament; ♀ a 4-lobed ovary, 4-celled by secondary septa; styles 2, elongate, papillose; ovules solitary in each cell, pend. Fr. 4-lobed; lobes ± keeled or winged, separating into 4 compartments at maturity. Annual to perennial glab. herbs, us. aquatic or subaquatic, with very slender stems; lvs opp., us. entire, exstipulate. A single almost cosmopolitan genus with some 20 spp.

Small, monoecious or very rarely dioecious annual or perennial herbs, aquatic or terrestrial, glabrous or sometimes with peltate cauline hairs. Stems slender. Lvs opposite, petiolate, almost always entire, exstipulate. Fls minute, axillary, solitary or rarely 1 ♂ and 1 ♀ together, sessile or pedicellate. Bracteoles 2 or 0, ± horn-like. Calyx and corolla 0. ♂ fls with 1 stamen; filament slender; anthers 2-celled. ♀ fls with subsessile 4-lobed ovary; ovary 4-celled by secondary septa. Styles 2, free, elongated, papillose. Ovules 1 per cell, pendulous from apex. Fr. 4-lobed; lobes ± keeled or winged, schizocarpic and separating at maturity into 4 mericarps. Seeds with fleshy endosperm; embryo straight.