(Hydrilla verticillata), submerged aquatic plant that is the sole member of the genus Hydrilla in the frog's-bit family (Hydrocharitaceae). Hydrilla is native to central Europe, Asia, and Australia and was brought to North America in the 1950s, where it has become a troublesome aquatic weed in lakes, streams, and reservoirs. It has long, slender stems that grow to several feet in length and form tangled carpets of vegetation beneath the water surface. Each stem bears 3 to 8 lance-shaped or oblong leaves that are 0.6–0.8 cm (0.25–0.3 inch) long and are borne in whorls. The flowers are tiny, white, and unisexual. Hydrilla is a fast-growing, hardy weed that has invaded numerous bodies of freshwater in the southern United States, destroying fish populations and clogging the intake valves of power plants and water installations.