Vigorous, submersed, aquatic perennials of fresh water; dioecious. Stems ± branched, leafy throughout. Leaves numerous, sessile, minutely toothed, alternate, subopposite or whorled. Spathes solitary, axillary; in male plant enclosing many pedicellate flowers; in female plant enclosing 1-(2-3) sessile flowers; male flowers usually breaking off and floating within a bell formed by recurved perianth-segments; female flowers long-exserted on pedicel-like extension of perianth-tube and reaching surface of water. Perianth of 6 segments. Male flowers; stamens 3, staminodia 3, > stamens, coloured at top and joined forming a sail. Female flowers: staminodia 3, small, stigmas 3, bifid, often brightly coloured. Fruit a beaked capsule, becoming mucilaginous and bursting irregularly. Seeds many. Spp. c. 15, of tropical and S. Africa, Madagascar and India. Adventive sp. 1.