Glabrous, aquatic herbs with fibrous roots, over wintering by winter buds or rhizomes. Leaves 2-ranked, alternate or opposite, alike or dimorphic (submerged leaves sessile or petiolate, usually translucent; floating leaves petiolate, usually opaque, lamina dilated). Stipular sheaths free, axillary, or adnate to the leaf base. Inflorescence an axillary or terminal, peduncled, ovoid or cylindrical spike, lax or with flowers irregularly whorled, submerged or emergent, enclosed by stipular sheaths when young. Flowers ebracteate, sessile, tetramerous, small, bisexual, hypogynous. Perianth 4-segmented (sometimes the segments have been treated as outgrowth from the connectives of the anthers). Stamens 4, basally adnate to the perianth segments; anthers 2-locular, extrorse, sessile. Carpels 4 (rarely 3-1), apocarpous, 1-celled with one campylotropous ovule; stigma sessile or subsessile. Fruitlet wall opening with a lid of the dorsal area. Seeds lacking endosperm; hypocotyl massive; seed coat free from fruitlet wall; embryo uniform or spiral.