Aquatic herbs or undershrubs. Leaves opposite or verticellate, simple. Stipules present. Flowers axillary, solitary, glomerate or fascicled, actinomorphic or zygomorphic, hermaphrodite. Sepals 3—5, free or shortly connate, imbricate, persistent. Petals as many as the sepals, free, imbricate, persistent. Stamens as many or twice as many as the sepals, persistent; the filaments often dilated; anthers dorsifixed, 2-celled, longitudinally dehiscent. Ovary superior, 3—5- celled, 3—5-carpelled; styles 3—5, free, short, persistent; stigmas capitate. Ovules numerous in 2 or more rows on each of the axillary placentas. Fruit a small septicidal capsule. Seeds many, small. Endosperm scanty or wanting. Embryo straight or curved. About 40 species in 2 genera in temperate, tropical and subtropical regions.