Perennial or rarely annual herbs, mainly aquatic, with alternate or verticillate leaves, the submerged ones often pectinate-pinnatifid Flowers perfect, or monoecious, or dioecious, axillary, in interrupted spikes, solitary or clustered. Calyx-tube adnate to the ovary, its limb entire or 2-4-lobed. Petals small, 2-4, or none. Stamens 1-8 Ovary ovoid-oblong, or short-cylindric, 2-8-ribbed or winged, 1-4-celled; styles 1-4; stigmas papillose or plumose. Fruit a nutlet, or drupe, compressed, angular, ribbed or winged, indehiscent, of 2-4 1-seeded carpels. Endosperm fleshy; cotyledons minute.