Monoecious, dioecious or hermaphrodite, submerged or rarely floating, freshwater or marine, annual or perennial herbs. Roots mostly simple, adventitious, rarely with root hairs. Stems corm-like or elongate, rhizomatous, stoloniferous or erect, simple or dichotomously branched. Leaves radical, or spirally arranged, or in whorls along the stem, or distichous or rarely opposite, sessile or petiolate, sometimes sheathing at the base; venation parallel or only midrib present. Stipules rarely present. Nodal scales (squamulae intravaginales) often present, situated in leaf-axils. Flowers unisexual or bisexual, 1–many, mostly regular, arranged in a spathe, the spathe axillary, sessile or pedunculate and composed of 2 wholly or partly connate, rarely free, bracts. Perianth-tube (hypanthium) often present in ♀ and hermaphrodite flowers, exerted from or near the apex of the ovary, usually extending to carry perianth to water surface. Perianth segments 3 or 6, the latter differentiated into sepals and petals; sepals free, usually green or whitish, often reflexed; petals free, often showy, sometimes reflexed; stamens 2–many, in 1 or more whorls; anthers 2–4-thecous, basifixed, dorsally or latrorsely, rarely introrsely or extrorsely dehiscent; filaments filiform or flattened, linear or club-shaped in outline, rarely 0; staminodes often present in ♀ flowers, rarely so in ♂ flowers, in the latter the staminodes differentiated from the inner whorl of stamens; ovary inferior, composed of 2–15 connate carpels, 1(–3)-locular; placentation parietal, rarely basal; ovules few to many, anatropus or orthotropus; styles 2–15; stigmas 2–15, entire or 2-lobed. Fruit a capsule, sometimes beaked from remains of the perianth-tube, dehiscent or opening by decay of the pericarp. Seeds usually minute; embryo straight with inconspicuous or conspicuous plumule; endosperm 0.
Fresh-water or salt-water herbs, partly or wholly submerged; roots sometimes floating. Leaves radical or cauline, alternate to whorled. Flowers hermaphrodite or unisexual, arranged in a tubular spathe or within two opposite bracts, females solitary; peduncle sometimes spirally twisted in fruit. Perianth-segments free, 1-2-seriate, 3 in each series, the outer often green, valvate, the inner petaloid. Stamens numerous to 2. Male flowers with rudimentary ovary. Staminodes usually present in the female flower. Ovary inferior, 1-locular, with parietal placentas sometimes protruding nearly to the middle of the ovary. Ovules numerous. Fruit rupturing irregularly. Seeds numerous, without endosperm.
Notes: Advanced relatives of the preceding family, but with the ovary inferior.
Habit: Freshwater or marine aquatic herbs, sometimes emergent, stems stoloniferous or erect, corms often present when stems stoloniferous. Leaves basal or cauline, alternate, spiral, opposite or whorled, simple, sessile or petiolate; stipules sometimes present; basal sheath sometimes present; blades linear or expanded; aerenchyma present in abaxial part of leaf (Hydrocharis L. and Limnobium Rich.), leaf margins often toothed. Inflorescences often complex, of 1-many flowers, sessile in leaf axils or terminal on long scapes, subtended by 1-2 bracts, the scapes occasionally with up to 10 longitudinal ridges or wings. Flowers actinomorphic, bisexual (often cleistogamous) or unisexual (plants dioecious), sessile or pedicellate (staminate flowers) or mostly sessile (pistillate flowers), the pedicels often long and filiform in staminate flowers (the elongated hypanthium often gives impression of pedicel in pistillate flowers), staminate flowers oftern abscising in bud; perianth often fused to form hypanthium; sepals (2)3; petals (absent or 2)3; androecium of 2-41 stamens (when present), the stamens in several whorls of mostly 3, the filaments distinct or united, the anthers basifixed or dorsifixed; gynoecium syncarpous, the ovary inferior (if present), the carpels 3-20+, the locules 1 or falsely 6-9, the styles 1-9, sometimes bifid, the stigmas linear, dry, papillose; placentation laminar. Fruits a capsule or berry -like, dehiscing irregularly or breaking up at maturity. Seeds 3-numerous, ellipsoid to cylindric or fusiform.