Glabrous, perennial, monoecious or polygamous, rarely dioecious, aquatic herbs, usually submerged but sometimes growing on mud. Stipules 0, but stipule-like outgrowths may occur at the leaf bases. Submerged leaves in whorls of 3-6, pinnatisect into undivided segments; aerial leaves whorled, opposite or alternate. Flowers mostly sessile, 1 or 2 in the axil of a leaf or bract (aquaticum) or in an emergent spike (spicatum), bisexual or unisexual. Calyx of 4 lobes (male flowers) or minute (female flowers). Petals 2 or 4 in males flowers, minute or 0 in female. Fruit separating into 4 (or fewer by abortion) 1-seeded nutlets.