125. CoMPOsiTAE 269 



55. Carduus L. — Musk Thistle 

 C. nutans L. An occasional weed in waste places; introd. from 

 Eur. May-Sept. 



56. Onopordum L. — Scotch Thistle 

 O. acanthium L. Roadsides and waste places, occasional; nat. 

 from Eur.; Champaign and Cook counties. July-Aug. 



57. Centaurea L. — Star Thistle 



1 . Bracts of the involucre not spiny. 



2. Heads large, few or solitary, the involucre 3.5-4.5 cm high; 

 flowers purple, the marginal ones enlarged, sterile; waste 

 ground, occasionally escaped from cult, but not established 

 in 111.; native of western U.S. June-Aug. American Basket 



Flower C. americana Nutt. 



2. Heads smaller, the involucre 0.5-2.5 cm high. 



3. Bracts entire; leaves linear-lanceolate, sessile, the basal pin- 

 natifid; flowers rose; perennial with deep horizontal black 

 roots; occasional in fields and waste places; nat. from the 

 region of the Caucasus. June-Aug. Russian Knapweed 



C. re pens L. 



3. Bracts pectinate. 



4. Lower bracts pectinate or fringed to the middle or below; 



leaves entire or toothed. 



5. Bracts lanceolate, pale, without dilated tips; flowers blue, 



purplish, pink, or white, the marginal ones enlarged; 



plants annual; waste places, escaped from cult.; 



native of Eur. July-Sept. Bachelor's Button 



C. cyanus L. 



5. Bracts with abruptly dilated tips; flowers rose-purple; 

 plants perennial. 

 6. Flowers all alike, discoid, not enlarged; bracts reg- 

 ularly pectinate-fringed; fields and roadsides, oc- 

 casional; adv. from Eur. July-Sept. Black Knap- 

 weed C. nigra L. 



6. Marginal flowers enlarged, showy; bracts irregularly 

 lacerate to entire; fields and roadsides, occasional; 



adv. from Eur. June-Sept. Brown Knapweed 



C. jacea L. 



4. Lower bracts pectinate only near the dark-colored tip. 

 7. Leaves lanceolate, entire or toothed, only the lower ones 

 lyrate; heads about 4 cm in diameter; flowers rose- 

 purple; plants perennial; waste ground, occasional; 



adv. from Eur. Aug.-Oct C. vochinensis Bemh. 



7. All but the uppermost leaves pinnatifid with linear seg- 

 ments; heads 2-2.5 cm broad; flowers white to purple; 

 plants annual or biennial ; roadsides and waste 

 places in the northern counties; nat. from Eur. July- 

 Sept. Spotted Knapweed C. maculosa Lam. 



