Stout, monoecious, rhizomatous perennial herbs rooted in mud. Stems erect, simple, without nodes, terminated by dense cylindric flower spikes. Leaves distichous, simple, linear, ± glaucous, entire, ± all basal, with long, open, but tightly clasping sheathing bases; the free part of the leaves usually twisted. Flowers unisexual, very numerous, densely crowded in a terminal spike, the male flowers in the upper part, the female flowers below. Male flowers subtended by scales or bracteoles; perianth 0 or of 3-6 small scales. Female flowers with or without bracteoles; perianth of slender hairs; abortive female flowers often present. Ovary superior, 1-locular with a solitary ovule. Fruit ellipsoid or subcylindric.