Aquatic perennials of fresh or salt water, partly or completely submersed; monoecious or dioecious. Stems long, slender, simple or branched, or 0. Leaves basal or cauline, usually sessile, alternate or whorled, variable in shape and size, submersed or floating. Flowers actinomorphic, commonly unisexual, sometimes bisexual. Inflorescence (or flower) sessile or long-pedunculate, in a tubular bifid spathe or within 2 opposite bracts; male flowers usually many within spathe; female flowers solitary; bisexual flowers solitary or several together. Perianth-segments usually 6 or fewer, in 1-2 whorls, 3 (rarely 2) in each whorl; tube often elongate. Stamens (1)-3-many, anthers 2-celled, opening by longitudinal slits (staminodia in female flowers). Ovary inferior, syncarpous, 1-locular, with 3-6 (rarely more) parietal placentae (rudimentary ovaries in male flowers); style-branches as many as placentae; stigmas entire or bifid; ovules many. Fruit submersed, often fleshy, indehiscent, occasionally rupturing irregularly. Seeds many. Genera c.16, in warmer regions of the world, a few in temperate regions.