Annual or perennial, monoecious or dioecious, mostly submerged aquatic herbs in fresh or saline waters. Stem corm-like or slender, stoloniferous or not. Leaves exstipulate, sessile or petiolate, radical or cauline and spirally whorled, opposite or alternate, submerged, rarely partly emergent or floating, variously shaped; veins ± parallel, straight or curved. Flowers solitary or clustered, enclosed at first by a bifid sessile or long stalked pair of opposite united and spathe-like or free bracts, bisexual or unisexual, when unisexual then males usually more than 1, females solitary, actinomorphic or occasionally ± zygomorphic, mostly 3-merous. Sepals 3, free, often green, valvate. Petals 3 or absent, membranous, showy or minute and inconspicuous, imbricate, or con¬volute. Stamens (1-) 2 to numerous, uni or multiseriate, inner ones rarely staminodial, filaments ± slender free or united in pairs, rarely absent; anthers dithecous, dehiscing by parallel longitudinal slits; 1 or more pistillodes usually present in male flowers. Ovary inferior, composed of 2-15 fused carpels, linear, lancecolate or ovate, generally protruding into a filiform beak or rostrum at the apex, unilocular with parietal placentation and numerous orthotropous to anatropous, erect to pendulous, bitegmic ovules; styles as many as carpels, entire or 2-lobed. Fruit ripening under water, linear-lanceolate, ovate to globose, dry or pulpy, many-seeded, dehiscing irregularly. Seeds fusiform, elliptic, ovate or globose, glabrous, warty or spinose, exalbuminous; embryo upright.