Aquatic, submerged and floating perennials. Leaves in whorls of 3-6, pectinate or pinnate. Flowers unisexual or hermaphrodite, axillary, alternate or verticillate in usually emergent spikes. Calyx campanulate; lobes 4, triangular. Petals 4, usually scaphoid or absent. Male flower: stamens 6-8. Female flower: ovary 1-4-locular; stigmas plumose or papillose. Fruit schizocarpic, splitting into one-seeded mericarps.