Submerged freshwater, dioecious herb, attached to substrate. Leaves 3(–5) per whorl (basal leaves opposite), mostly very crowded at distal ends of branches or less frequently well-spaced along the branch, sessile, (0.5–)0.6–1.2(–1.7) cm long, elliptic to narrowly ovate or linear-elliptic, glabrous, green, 1–veined; margin translucent, usually minutely toothed with fine dark-tipped teeth, sometimes entire; apex obtuse to acute. Flowers basally subtended by united floral bracts, solitary, each with a thread-like hypanthium; perianth of 2 whorls of 3 segments each, the outer whorl translucent, the inner white. Male flower with hypanthium usually breaking to allow distal part of flower to float to the water surface; stamens 9. Female flower with hypanthium 2–15(–30) cm long; staminodes 3, opposite inner perianth parts; ovary enclosed in base of hypanthium, unilocular; style as long as hypanthium; stigmas 3, broad and 2-lobed at apex. Seeds c. 4.5 mm long.
1 species from temperate America, naturalised in Australia, New Zealand, British Isles and Europe.