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.

Fls in spikes on long or short axillary scapes; bracts small, each subtending a single sessile fl. Sepals 4, ± keeled, anterior and posterior often dissimilar; corolla scarious, persistent, with short tube and 4 lobes; stamens 4, alternating with corolla-lobes, filaments long, anthers versatile, dehiscing longitudinally; ovary superior, bilocular, ovules 1 to many in each loculus, style simple with long stigma; fr. us. a circumscissle capsule, rarely an ind. nutlet. Seeds hard, often mucilaginous when wet; embryo erect. Annual or perennial herbs or subshrubs. Lvs mostly linear or broad, exstipulate, sheathing at base, entire or toothed, spiral, alt. or opp., often rosulate. 3 genera, about 200 spp., mainly temperate.

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.

Herbs or occasionally subshrubs. Lvs usually all radical, sometimes cauline, alternate or opposite, simple, sometimes reduced, exstipulate, generally sheathing at base. Fls usually in spikes, small, actinomorphic, usually ☿, bracteate. Calyx herbaceous or scarious; sepals 4, often connate at base, persistent. Corolla gamopetalous; segments (3)-4, imbricate, scarious. Stamens (1)-4, epipetalous or inserted at base of perianth; anthers versatile, 2-locular. Ovary superior, 1-4-locular; style 1; ovules 1-many; placentation axile or basal. Fr. a circumscissile capsule or hard indehiscent nut. Seed endospermic; embryo straight.