A. Alsinastrum: aquatic, dark-green, much-branched, entirely floating under water; leaves numerous, opposite or in whorls of 3-4, sessile, linear-oblong, transparent; female flowers, the only ones known in this country, sessile in the upper axils, the slender perianth-tube elongated, so as to reach the surface of the water, where it terminates in 3-6 small spreading segments. - Ponds, canals, and slow streams, in which it spreads so rapidly as to choke up the channels; recently introduced from North America. Fl. July to September.