HOREHOUND (0. Eng. harhune, Ger. Andorn, Fr. marrube)  . Common or white horehound, Marrubium vulgare, of the natural order Labiatae, is a perennial herb with a short stout rootstock, and thick stems, about i ft. in height, which, as well as their numerous branches, are coated with a white or hoary felt—whence the popular name of the plant . The leaves have long petioles, and are roundish or rhombic-ovate, with a bluntly toothed margin, much wrinkled, white and woolly below and pale green and downy above; the flowers are sessile, in dense whorls or clusters, small and dull-white, with a ro-toothed calyx and the upper lobe of the corolla long and bifid . The plant occurs in Europe, North Africa and West Asia to North-West India, and has been naturalized in parts of America . In Britain, where it is found generally on sandy or dry chalky ground, it is far from common . White horehound contains a volatile oil, resin, a crystallizable bitter principle termed marrubiin and other substances, and has a net unpleasant aromatic odour, and a persistent bitter taste . Formerly it was official in British pharmacopoeias; and the infusion, syrup or confection of horehound has long been in popular repute for the treatment of a host of dissimilar affections . Black horehound, Ballota nigra, is a hairy perennial herb, belonging to the same order, of foetid odour, is 2 to 3 ft. in height, and has stalked, roundish-ovate, toothed leaves and numerous flowers, in dense axillary clusters, with a green or purplish calyx, and a pale red-purple corolla . It occurs in Europe, North Africa and West Asia, and in Britain south of the Forth and Clyde, and has been introduced into North America .