Aquatic or subaquatic perennial herbs with linear, verticillate, exstipulate leaves. Flowers minute, sessile, solitary axillary, mostly bisexual or polygamous. Perianth none or inconspicuous round the top of the ovary. Androecium of 1 stamen, anterior. Ovary inferior, mono-carpellary, 1-celled, 1-ovuled; ovule solitary, apical, pendulous, anatropous with single integument; style 1, filiform, stigmatic down one side; stigma papillose. Fruit a 1-seeded achene.