CENTAUREA, in botany, a genus of the natural order Compositae, containing between four and five hundred species, and of wide distribution, but with its principal centre in the Mediterranean region. The plants are herbs with entire or cut often spiny-toothed leaves, and ovoid or globose involucres surrounding a number of tubular, oblique or two-lipped florets, the outer of which are usually larger and neuter, the inner bisexual. Four species are native in Britain. C. nigra is knapweed, common in meadows and pastureland; C. Cyanus is the bluebottle or cornflower, a well-known cornfield weed; C. Calcitrapa is star-thistle, a rare plant, found in dry waste places in the south of England, and characterized by the rose-purple flower-heads enveloped by involucral bracts which end in a long, stiff spine. Besides cornflower, a few other species are worth growing as garden plants; they are readily grown in ordinary soil:—C. Cineraria, a half-hardy perennial, native of Italy, is remarkable for its white downy foliage; C. babylonica (Levant) has large downy leaves and a tall spike of small yellow flowers; C. dealbata (Caucasus) is a low-growing plant with larger rose-coloured heads; C. macrocephala (Caucasus) has large yellow heads; C. montana (Pyrenees) large handsome blue heads; and C. ragusina (S.E. Europe) beautiful silver-haired leaves and yellow flowers.