Submersed aquatic perennials of fresh water, rooting or drifting; dioecious, occasionally hermaphrodite. Stems simple or sparingly dichotomously branched. Leaves numerous, sessile, margins minutely toothed; lowest leaves opposite, middle and upper leaves whorled. Flowers unisexual and bisexual, solitary, long-exserted on pedicel-like extension of perianth-tube, from tubular sessile axillary spathes; male flowers usually breaking off and floating; female flowers reaching to surface of water; sepals 3, green; petals 3, rarely 0, white to purple. Male flowers with 9 stamens, usually 6 in a lower outer ring and 3 inner raised on a fused filament column; female flowers with 3 staminodia alternating with the 3 entire or bifid stigmas; bisexual flowers with 3 stamens, and pistil as in female. Fruit a capsule. Seeds several. Spp. 21, of N. and S. America. Adventive sp. 1.