Tall perennial herbs from creeping rhizomes. Stems simple, submerged at base, cylindrical, jointless. Lvs. mostly radical, elongate-linear, alternate, rather thick and spongy. Fls. unisexual, anemophilous, very numerous, crowded in a terminal elongate spike, the male above, the female below. Perianth of many slender, jointed threads. Staminate fls. with 2-5 stamens; fils. free or connate; anthers linear, basifixed. Fertile female fls. with 1-celled stipitate ovary, narrowed into a style with narrow or ligulate stigma. Many female fls. sterile and with abortive terminal ovaries and perianth hairs beneath. Fr. dry, tardily dehiscent. Seed striate, with mealy endosperm. One genus of about 10 species of fresh water and marshy places, temp. and trop. regions. (Munz, Flora So. Calif. 1012).