PLANTS: Aquatic or terrestrial annual (or facultatively perennial) herbs, generally glabrous, with adventitious roots; monoecious or rarely dioecious. STEMS: delicate, simple or branched, usually 1-4 dm long. LEAVES: opposite, simple, often polymorphic, estipulate, the margins entire; submerged leaves narrow linear, the apices generally emarginate or bifid, l-nerved; terrestrial and floating leaves obovate to oblanceolate to spatulate, 3- to several-nerved; floating leaves crowded at the ends of the branches forming rosettes. INFLORESCENCE: of 1-3 sessile flowers in the axils of the leaves. FLOWERS: hypogynous, inconspicuous, usually subtended by a pair of small, white, falcate or obliquely oval bracteoles; perianth absent. STAMINATE FLOWERS: consisting of a single stamen on a slender elongate filament. PISTILLATE FLOWERS: with 2 united carpels; pistil4-lobed (often keeled or winged and apically indented between the lobes), appearing 4-chambered by a false septum, compressed dorso-ventrally; styles 2, slender, often much longer than the ovary. FRUIT: separating at maturity into 4, l-seeded nutlets. 2n = 6-40. NOTES: Taxonomically and nomenclaturely difficult because most species are polymorphic. REFERENCES: Ricketson, Jon. 1995. Callitrichaceae. J. Ariz. - Nev. Acad. Sci. 29(l): 15.