Mare's-tail

The aquatic plant Hippuris vulgaris or either of two other species of its genus, which constitute the water milfoil family (Hippuridaceae). Mare's-tail grows from submerged, stout rootstocks along the margins of lakes and ponds in temperate regions throughout the world. It resembles the unrelated horsetail (Equisetum species) in having whorls of small, linear leaves