Herbs, annual or perennial, aquatic, rooted in mud or free-floating. Stems of 2 types, vegetative and flowering; vegetative stems indeterminate, bearing many leaves, glabrous; flowering stems determinate, usually emersed, bearing single leaf, spathe, and terminal inflorescence. Leaves of 2 types, sessile and petiolate; stipule marcescent, apex truncate, associated with petiolate leaves only; sessile leaves submersed or rarely emersed, forming basal rosette or alternate on elongate stem, blade linear to occasionally oblanceolate, base sheathing, margins entire; petiolate leaves floating or emersed, blade cordate, reniform, or ovate. Inflorescences paniculate, spicate, umbellate, or 1-flowered; spathes bractlike, folded or clasping, commonly with acute to caudate extension. Flowers sessile, 3-merous; perianth with tepals connate proximally, yellow, blue, mauve, or white, tube tubular or funnelform, limb 6-lobed; stamens 3 or 6, adnate to perianth; pistils 3-locular, 1 or 3 developing to maturity; placentation parietal or basal; ovules 1-many. Fruits capsular or utriculate. Seeds smooth or with longitudinal wings.

PLANTS: Herbaceous aquatic annuals or perennials, rooted in mud or free-floating. STEMS: of two types: vegetative stems indeterminate, containing many leaves; flowering stems determinate, containing a single leaf, spathe, and terminal infloresence. LEAVES: alternate, with a strongly sheathing base, petiole (excluding sessile leaves), and lamina; stipules marcescent, transparent, truncate at apex, sheathing or absent. INFLORESCENCE: a panicle, spike, or with a solitary flower; spathe bract-like, glabrous, with an acute to caudate apex. FLOWERS: 3-merous, perfect, zygomorphic, sessile; perianth of six petaloid, oblong to oblanceolate tepals, basally fused into a tube; stamens 3 or 6, adnate to the perianth, the anthers rounded to elongate; ovary superior, 3-locular; ovules one to many; stigma entire or 3-lobed with single-celled hairs. FRUIT: a many-seeded capsule with persistent unmodified perianth, or a nutlet with thickened, ridged perianth. Seeds longitudinally winged or smooth. NOTES: 6 genera, ca. 40 spp., throughout the tropics and subtropics.