Herbs, perennial, in marshes or aquatic, with creeping rhizomes. Leaves alternate, erect, distichous, linear, usually spongy, margin entire, sheathed at base. Flowers unisexual, minute, numerous, densely crowded in a cylindric spike with lower part female and upper part male; bracts leaflike. Perianth absent. Male flowers consisting of 1-3 stamens usually connate at base of filaments, surrounded by hairs; anthers 2-thecous, basifixed, longitudinally dehiscent; filaments short; pollen grains in monads or tetrads. Female flowers: ovary 1-loculed, on a long capillary stalk with many fine hairs or bracteoles at base; styles capillary; stigmas broadened or spatulate; ovule 1; sterile ovary without style. Fruit minute, falling off together with stalk.

About 16 species: tropical and temperate regions; 12 species (three endemic) in China.