Dahlia, a genus of herbaceous plants belonging to the order Compositae, named after Dr. Dahl, a pupil of Linnaeus. D. variabilis, a native of Mexico, where it grows in sand at an altitude of 5,000 feet, was introduced into Europe in 1784 and again in 1804. It has fasciculate, spindle-shaped, tuberous roots, which were expected to prove edible but are acrid. The plant grows seven or eight feet high in rich loam, but becomes dwarfed in lighter soil. Its foliage is coarse, and the whole plant is glabrous. The head of flowers is surrounded by a double involucre, and has a chaffy receptacle and no pappus. When first introduced the head of florets was flat, the disk yellow, and the ray a dull scarlet; but under cultivation it speedily varied into an infinite number of forms and colours. In many of these the head has become globular, and the corollas of all the florets are alike horn-shaped or funnel-shaped. Fashion has of late returned to "single flowers," i.e. heads with large ray-florets and a distinct disk. The plants can be raised in a frame from seed sown in March, or from cuttings; they require protection from north and east winds, and different varieties are in bloom from June to November.