Perennial water or marsh plants usually with stout, creeping rhizomes. Leaves floating, submerged or aerial; floating leaves peltate or cordate, usually long-petioled; submerged leaves simple, thin and translucent. Flowers solitary, terminal, generally floating, bisexual, actinomorphic, hypogynous to epigynous, the parts variously arranged. Perianth usually differentiated into 3–6 green sepals and 3 to many petals which may pass gradually into the usually numerous stamens with introrse anthers and are either hypogynous or inserted at various heights on the wall of the more or less inferior ovary. Carpels 8 or more, either free, but then sunk in the receptacle, or united and superior or united into a many-celled ovary and inferior. Ovules 1–many, inserted over the inner walls of the carpels. Fruit a group of achenes sunk in the receptacle or a spongy capsule dehiscing regularly or irregularly by the swelling of internal mucilage; seeds often with arils, with or without endosperm, sometimes with perisperm as well as endosperm.

Contains about 3 genera with some 75 species, cosmopolitan.

Nymphaea L. nom. conserv.

Castalia Salisb.

Perennial herbs with stout rhizomes. Leaves more or less subrotund, rarely peltate, with stipules. Flowers usually floating, often large and showy. Sepals (3–)4(–5), green, hypogynous. Petals numerous, inserted at successively higher levels on the side of the half-inferior ovary, the outermost much longer than the sepals, the innermost shorter and narrower and grading into the stamens. Stamens numerous, the outer with broad, petaloid filaments, all inserted towards the top of the ovary. Nectaries absent. Ovary syncarpous, many-celled with numerous ovules in each cell, concave above with a boss at the centre from which the stigmatic surfaces radiate. Fruit a spongy, berry-like capsule, splitting by the swelling of internal mucilage to release the seeds, which float because of the air-containing aril and seed wall.