Annual or rarely perennial glabrous herbs, aquatic or in marshy places. Stems terete or 4-gonous, striate, often rooting at the nodes. Leaves decussate or verticillate, rarely alternate, sessile or subsessile. Flowers small, actinomorphic, homostylous or heterostylous, 3–6-merous, axillary, solitary, or in terminal racemes or spikes, very rarely in axillary umbels; bracteoles 2, rarely absent. Calyx-tube semiglobose, campanulate or urceolate-tubular, scarious, very rarely herbaceous, the lobes triangular, alternating with ± developed appendages or these often absent; nectariferous ring present at base. Petals persistent, sometimes caducous or absent, equal or unequal. Stamens 1-6, opposite to the sepals. Ovary sessile or substipitate, incompletely 2–4-locular (dissepiments interrupted above the placentas); ovules ± numerous, small; style absent or ± developed, simple. Capsule septicidally 2–4-valved, cartilaginous, the walls horizontally striate (under the microscope). Seeds small, concave-convex.