Annual or perennial, marsh or aquatic, sometimes lactiferous herbs, erect, rarely with floating leaves; hermaphrodite, monoecious or dioecious, Leaves all basal, long-petioled, sheathing; laminae linear-lanceolate, hastate, sagittate or cordate. Flowers actinomorphic, usually bisexual, in panicles or racemes; sepals 3, green, persistent; petals 3. deciduous, rarely 0. Stamens (3)-6 or more, hypogynous, free; anthers 2-celled. Carpels numerous, superior, free or rarely connate at base, arranged in a ring or irregularly packed over surface of receptacle; style persistent; ovules 1-several. Fruit a head or whorl of achenes, rarely of follicles. Genera 13, of temperate and tropical regions of N. Hemisphere.