This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Shrubs, rarely spiny. Leaves usually 3-foliolate, sometimes 1-foliolate, or wanting. Calyx more or less two-lipped. A considerable genus, nearly all the species from the Mediterranean region. The origin of the name is obscure.
Fig. 68. Cytisus scoparius. (1/2 nat. size.)
1. C. scoparius (fig. 68), syn. Genista, Spartium and Sarothdmnus. Common Broom. - This is common throughout Britain, varying from 2 to 6 feet in height according to situation, etc. The slender branches are silky, hairy, and angular, and the flowers bright yellow, in May or June. The most remarkable characteristic of this species is the spirally coiled style. There are several varieties, including a white and a double-flowered variety.
2. C. purpureus. - A glabrous shrub about 3 feet high with slender ascending or pendulous branches and 3-foliolate leaves. Leaflets obovate. Flowers lateral, usually two together, dull purple, rosy purple, or white. This species, crossed with the Common Laburnum, gave rise to the remarkable hybrid alluded to above; but writers are by no means agreed as to the origin of it. Some assert that it is a graft-hybrid, while others incline to the more probable opinion that it is of sexual origin.
3. C. nigricans. - A pretty shrub about 3 feet high with small 3-foliolate leaves and terminal erect racemes of yellow flowers. This is one of the most ornamental of this group, bearing racemes of flowers about 6 inches long in June and July. It is a native of Austria.
Amongst others in cultivation are : C. capitatus, a very hairy species with the flowers in a dense terminal head; C. argenteus, with silvery foliage and axillary yellow flowers; and C. sessilifolius, with sessile leaves and terminal yellow flowers.