Floating aquatic herbs with dimorphic leaves, submerged ones opposite pinnatifid rootlike, apical ones in a rosette, rhomboid, dentate, with spongy often inflated petiole, arranged in leaf-mosaic; stipules 4-8, minute. Flowers bisexual, small, solitary, axillary, short-pedicelled, 4-merous, white or lilac. Petals imbricate. Disk present. Ovary half-inferior with 1 style and 2-4 persistent sepals turning often to thorns or horns. Fruit mostly 1-celled, 1-seeded, shell bone-hard; thorns after withering often set with barbs at the apex. Seed often producing 2-5 free germ-stalks. Distr. Several species in the Old World, but not known from Australia.