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..
Small aquatic fresh-water mostly annual fleshy herbs, the leaves usually poorly differentiated from the stem, the whole structure commonly resembling the thallus of an alga or hepatic, the small usually perfect flowers devoid of any perianth and subtended by a spathe-like involucre, or in some genera with a 3-5-cleft membranous calyx. Stamens hypogynous, only 2 in the following genus, numerous in some others; filaments united or distinct; anthers 2-celled, the sacs longitudinally dehiscent. Ovary stalked or sessile, 2-3-celled; ovules usually numerous in each cell, anatropous; styles 2 or 3, short. Capsules 2-3-celled, ribbed. Seeds numerous, minute, without endosperm; embryo straight.
About 21 genera and 175 species, mostly in the tropics, only the following North American.