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Plants perennial, of fresh waters. Rhizomes and stolons absent. Erect stems rooted in substrate, branched or unbranched, elongate. Leaves cauline, whorled, 3--7 at each node, or leaves opposite at 4+ proximalmost nodes, submersed, sessile; blade linear to linear-lanceolate, apex acute; midvein without lacunae along side(s), blade uniform in color throughout;; abaxial surfacely without prickles or aerenchyma; intravaginal squamules entire. Inflorescences solitary, sessile; spathes not winged. Flowers unisexual, staminate and pistillate on different plants, or rarely bisexual, usually projected to surface of water by elongate floral tube base, sessile; petals white. Staminate flowers: filaments distinct or 3 inner connate ½ their length; anthers oval; pollen in monads or tetrads. Pistillate flowers: ovary 1-locular; styles 3, not 2-fid. Fruits ovoid to lance-ellipsoid, smooth, dehiscing irregularly. Seeds cylindric to fusiform, glabrous to hirsute.
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.
Freshwater, dioecious, monoecious or bisexual, attached, perennial, stoloniferous, with unbranched roots. Leaves submerged, cauline, distributed ±evenly along stem, mostly in whorls of 3 (–8), sessile, not differentiated into blade and sheath, with midrib; margins entire or serrulate. Inflorescences mostly 1-flowered, spatheate. Flowers bisexual or unisexual, with perianth of 2 almost equal whorls each with 3 membranous segments. Male flowers usually separating early and becoming free-floating; stamens 3–9. Female flowers floating; hypanthium thread-like, carrying flower to water surface; staminodes 3; ovary unilocular; styles 3. Fruit capsular. Seeds 1–5.
