394 COMPOSITAE. 



sterile, often much larger and conspicuous, the others perfect 

 and fertile; involucre globular, the tegules tipped or margined 

 with spines or scarious appendages; receptacle very bristly; 

 pappus of numerous rigid or sometimes chaffy naked bristles; 

 akenes mostly compressed, attached by one margin just above 

 the base. 



Tegules each tipped with a slender spine; ray-flowers yellow. C. melitensis. 

 Tegules spineless, all more or less fimbriate; ray-flowers red, blue, 



white or violet. C. cyanus. 



Centaurea melitensis L. Annual; herbage pubescent; stems 30-100 cm. 

 high, loosely branched; basal leaves petioled, lyrately lobed, the lobes obtuse or 

 rounded ; cauline leaves sessile, entire or merely toothed ; heads short-peduncled, 

 10-12 mm. in diameter; tegules each tipped with a branched spine. 



Introduced from Europe. Very troublesome as a weed in California where 

 it is called " Napa Thistle." 



Centaurea cyanus L. Bachelor's Button. Stems erect, slender, usually 

 branched, 30-90 cm. high; heads solitary on slender branches; involucre 

 ovoid; tegules in about four series, pale, the middle ones margined with a silvery 

 scarious-toothed border; ray-flowers large, white, blue, pink or violet. 



A common weed in fields and waysides. 



547. SILYBUM. Milk Thistle. 



Coarse prickly herb with pinnately lobed alternate mottled 

 leaves and large solitary heads of purple flowers; involucre 

 subglobose; tegules imbricated in a few series, large, rigid, the 

 spiny tips spreading; receptacle flat, bristly; flowers all alike, 

 tubular, perfect, fertile; akenes glabrous; pappus bristles num- 

 erous, flattish, scabrous or barbellate, united at base and falling 

 together. 



Silybum marianum (L.) Gaertn. Glabrous or slightly tomentose; stems 

 60-120 cm. high; leaves blotched with white, oblong-lanceolate, the lower ones 

 unequally lobed, spinose-dentate, the cauline strongly clasping at base; heads 

 globose, 5-6 cm. broad; flowers purple: pappus bristles white. 



In waste places, adventive from Europe. 



548. CIRSIUM. Thistle. 



Biennial or perennial herbs; leaves alternate, sessile, mostly 

 pinnatifid and prickly; heads usually large, terminal, many- 

 flowered; flowers all tubular, perfect and all alike, rarely im- 

 perfectly dioecious; involucre ovoid or spherical; tegules im- 

 bricated, in many rows, the tips scarious or prickly; receptacle 

 thickly clothed with soft bristles or hairs; pappus of numerous 

 bristles united into a ring at the base, plumose, deciduous; 

 akenes oblong, flattish, not ribbed. 



Dioecious; heads 1-1.5 cm. in diameter; perennial by spread- 

 ing roots. C. arvense. 

 Hermaphrodite; heads larger; biennials. 



