ARNICA  , a genus of

plants belonging to the natural order Compositae, and containing 18 species, mostly north-west American . The most important species is Arnica .montana (mountain tobacco), a perennial herb found in upland meadows in northern and central Europe (but not extending to Britain), and on the mountains of western and central Europe.' A closely allied species (A. angustifolia), with very narrow leaves, is met with in Arctic Asia and America . The heads of flowers are large, 2 to 2 in. across, orange-yellow in colour; and borne on the summit of the stem or branches; the outer ray-flowers are an inch in length . The achenes (fruits) are brown and hairy, and are crowned by a tuft of stiffish hairs (pappus) . The root-stock of A. montana is tough, slender, of a dark brown colour and an inch or two in length . It gives off numerous simple roots from its under side, and shows on its upper side the remains of rosettes of leaves . It yields an essential oil in small quantity, and a resinous matter called arnicin,.C12H2202, a yellow crystal-line substance with an acrid taste . The tincture prepared from it is an old remedy which has a popular reputation in the treatment of bruises and sprains . The plant was introduced into English gardens about the middle of the 18th century, but is not often grown; it is a handsome plant for a rockery .