Weak aquatic, amphibious or terrestrial herbs, commonly annual (but probably dependent on permanence of water), usually monoecious, glabrous (in Victoria) or with small peltate hairs on stems and leaves; stems floating, prostrate or weakly ascending, rooting at lower nodes. Leaves opposite, usually entire, clustered towards tips of branches, often dimorphic in aquatic species; glandular scales present in leaf-axils; stipules absent. Flowers unisexual, axillary, solitary or paired, if paired then usually one of each sex; bracteoles usually 2, small, membranous; sepals and petals absent; male flowers consisting of 1 stamen, anther 2-celled, reniform; female flowers consisting of 2 fused carpels, becoming 4-celled by ingrowth of septa, ovules 1 per cell, styles 2, free, filiform, papillose. Fruit small, c. circular in outline, dividing into 4 single-seeded indehiscent mericarps; mericarps usually winged or keeled on the outer edge.
c. 75 species, cosmopolitan with 9 species in Australia; 6 native, 3 naturalised.
Mature fruit are necessary for the accurate identification of species.