CENTAUREA  , in

botany, a genus of the natural order Cornpositae, 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 .