Erect, aquatic or marsh herbs with perennial rootstock. Leaves basal; petioles elongated, sheathing but open at the base; principal veins of the lamina parallel with the margins but converging at the apex. Inflorescence a much branched panicle or raceme. Flowers bisexual or unisexual, regular. Sepals 3, green. Petals 3, white or pinkish. Stamens 6–many. Carpels 6–numerous, free, superior. Fruit an achene. 12 gen., temp. and trop., mostly N. hemisphere.