ARNICA, a genus of plants belonging to the family Com positae, and containing 5o species, mostly north-west American, of which the most important is Arnica montana, a perennial herb of the mountains and uplands in northern and central Europe. 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 sim ple 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 a yellow crystalline substance with an acrid taste. The tincture prepared from it has a popular reputation in the treatment of bruises and sprains.
Typical of the various far northern species is (A. angusti folia), with very narrow leaves, is met with in Arctic Asia and America. The heads of flowers are 2 to 21 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 fruit is brown and hairy, and crowned by a tuft of stiffish hairs. The plant was introduced into England in the 18th cent.
In North America arnicas are most numerous in the Rocky Mountain region from Colorado northward but there are also many in the Sierra Nevada and Cascade ranges. Conspicuous species may be observed in Rocky Mountain, Glacier, Yosemite, Mount Rainier and other national parks of the western United States and Canada. Four arnicas occur in the northeastern United States and adjacent Canada, mostly on the higher mountains.