Aquatic, rootless, submerged, perennial, monoecious herbs; young plants green to olive green, older brown with thin, slender stems. Leaves whorled, 1-4 times dichotomously divided, much crowded towards the apex, stipulate; segments linear or filiform, often minutely toothed at irregular intervals with horny apices; horns usually 2, sometimes a glandular process is present in-between. Flowers minute, axillary, sessile, unisexual. Perianth or involucre 6-15, basally connate, subvalvate, apically terminated by 1-2 spines. Staminate flowers with 8-30 stamens on convex torus; filaments very short, broad, anthers 2-celled, extrorse, linear-oblong; connective projecting, apex truncate or provided with 2 sharp spines. Pistillate flowers greenish to orange-red with 1-carpelled, 1-locular, 1-ovuled, superior ovary; style subulate, stigma one-sided; ovule orthotropous pendulous. Fruit a 1-seeded nut, ellipsoidal or ovoid, with long persistent style and 2 sharp spines near the base, smooth, tubercled, winged or provided with 2-5 lateral spines. Seeds exalbuminous; embryo straight; cotyledons oval, thick.