Aquatic or marshy, erect perennial monoecious herbs. Rhizome creeping, submerged, scaly; aerial stem erect, with or without nodes. Leaves radical or cauline, elongated. Inflorescence a terminal, cylindrical, superposed spike; male above and female below, parts separated or sometimes united. Flowers numerous, densely crowded. Male flowers consisting of 1-3 stamens and surrounded by one or more hairs, some representing the perianth; anthers basifixed, linear. Female flowers usually intermixed with clavate pistillodes, covered with hairs and bracts or bracts absent; ovary superior, 1-locular, stipitate; stigma linear or lanceolate. Fruit a follicle, dehiscing longitudinally by a slit. Seed striate.