This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol2", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Aquatic herbs, with opposite or verticillate pinnatifid submerged leaves, and clustered rhombic-ovate dentate floating ones with inflated petioles. Flowers perfect, axillary, solitary, short-peduncled. Calyx-tube short, the limb 4-parted, the lobes persistent. Petals 4, sessile, inserted on the perigynous disk. Stamens 4, inserted with the petals; filaments subulate-filiform. Ovary 2-celled, conic above; style subulate; ovule 1 in each cavity. Fruit coriaceous or bony, large, 2-4-spinose or swollen at the middle, 1-celled, 1-seeded, indehiscent. Seed inverted.
A single genus of 3 species, natives of Europe, tropical Asia and Africa.