Aquatic often rather large perennial herbs with creeping, subterranean stolons. Stem simple or branched, leafy at the base, stiff or flaccid, erect or floating, bearing a terminal spike or panicle. Leaves long, linear from a sheathing base. Flowers (♂♀), crowded in separate globose clusters; lower clusters ♀, in or above the axil of a leafy bract, stalked or sessile; higher clusters ♂, bractless or with a small bract. ♂: Perianth actinomorphic, choriphyllous. Tepals 3(-6), spathulate. Stamens 3(-6); filaments free or connate at the base; anthers basifixed, oblong; pollen globose. ♀: Tepals as in ♂ but larger. Ovary 1, exceptionally 2, sessile with a narrow base, unilocular; ovule 1, pendulous; style 1, usually simple, rarely forked; stigma unilateral, short. Fruits densely crowded, sessile with a narrow base, crowned by the style, indehiscent; exocarp spongy, endocarp hard; testa thin; embryo in the middle of the mealy endosperm. Distr. Temperate and colder regions of the N. hemisphere, crossing the tropics in Malaysia over the mountains towards Australia and New Zealand. About 15 species have been distinguished, in Malaysia only one sp. occurs.