​MARE’S-TAIL, in botany, the popular name for an aquatic herb known botanically as Hippuris vulgaris (natural order Haloragaceae). It grows on margins of lakes, ponds and similar localities, and has a submerged stout creeping rootstock from which spring many-jointed cylindrical stems bearing numerous narrow leaves close-set in whorls. The minute greenish flowers are borne in the leaf-axils. Like many freshwater plants it has a wide distribution, occurring in arctic and temperate regions in the northern hemisphere and reappearing in antarctic South America.