Fls small solitary and axillary or in spikes or panicles; hermaphrodite or unisexual, epig., regular; calyx-lobes 2-4 or 0; petals free, 8, 4, or 0; stamens 8, 4, or fewer, large for size of fl.; ovary 1- or 2- or 4-celled, us. with as many pend. ovules as styles; styles 1-4, free, stigmas papillose or plumose; fr. dry or succulent, ind. or separating into ind. 1-seeded nutlets. Annual to perennial herbs, sts woody, many aquatic; lvs simple, opp., alt. or whorled, us. notched or lobed, exstipulate. Cosmopolitan but with more representatives in Southern Hemisphere. About 7 genera and 160 spp.

Herbs or subshrubs, mostly hydrophytic and often aquatic; stems with monostelic anatomy. Lvs alternate, opposite or whorled, exstipulate, the submerged lvs often finely dissected. Fls solitary and axillary, or in spikes or panicles, small, ☿ or unisexual, actinomorphic. Calyx tube adnate to ovary; lobes 2, 4 or 0. Petals 2, 4-(8) or 0, valvate or nearly so, reflexing, often caducous. Stamens 2, 4 or 8, antipetalous, epipetalous; anthers basifixed. Ovary inferior, (2)-4-celled; each cell with 1-2 ovules; ovules pendulous from apex. Styles (2)-4, free; stigmas capitate and sometimes fimbriate. Fr. usually a small, often winged, rugose or tuberculate nut, sometimes a schizocarp separating into 1-seeded mericarps (nutlets). Seeds endospermic; embryo straight.