Fresh or salt water herbs, partly or wholly submerged. Leaves crowded at the base, or alternate or whorled or opposite and dispersed along elongated stems. Flowers bisexual or unisexual, regular, arranged within a bifid spathe or within 2 opposite bracts; male flowers several together or solitary. Spathes sessile or on long peduncles, sometimes spirally twisted. Perianth segments free when present, in 1 or 2 whorls, 3 in each whorl; the outer ones green, valvate; the inner ones usually petaloid, imbricate. Stamens-1, or 2–numerous. Ovary inferior or superior, 1-locular, with parietal placentas bearing numerous ovules or 1-seeded. Staminodes sometimes present in female flowers and a rudimentary ovary in the males. 18 gen., temp. and trop.