Jones: Flora of Illinois, 152. Compositae 269 



6. Heads 2-3 cm. broad; stem 1-2.5 m. tall; bracts without prickle- 

 points; wet ground, chiefly in the n. half of 111. Aug. -Sept. Swamp 



Thistle C. muticum Michx. 



1. Heads smaller, 1.5-2 cm. high and 1-1.5 cm. in diameter; flowers dioecious; 

 perennials with spreading rhizomes; leaves glabrous or nearly so. 

 7. Leaves deeply pinnately lobed; fields and waste places; nat. from Eur. 



June-Aug. Canada Thistle C. arvense (L.) Scop. 



7. Leaves entire or slightly lobed; occasional in fields and waste places; nat. 

 from Eur. June-Aug. [C. arvense var. integrifolmm Wimm. & Grab.} 

 C. setonun (Willd.) Bieb. 



51. Carduus L. 



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

 from Eur. June-Sept. 



52. Centaurea L. — Star-thistle 

 1. Bracts of the involucre not spiny. 



2. Lower bracts pectinate or fringed to the middle or below; leaves entire or 



toothed. 



3. 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. 



3. Bracts with abruptly dilated tips; flowers rose-purple; plants perennial. 

 4. Flowers all alike, discoid, not enlarged; bracts regularly pectinate- 

 fringed; fields and roadsides, occasional; adv. from Eur. July-Sept. 



Black Knapweed C. Jiigra L. 



4. Marginal flowers enlarged, showy; bracts irregularly lacerate to entire; 

 fields and roadsides, occasional; adv. from Eur. June-Sept. Brown 



Knapweed C. jacea L. 



2. Lower bracts pectinate only near the dark-colored tip; leaves coarsely den- 

 tate to pinnatifid; flowers rose-purple, the marginal ones radiant; plants 



perennial; waste ground, occasional; adv. from Eur. Aug.-Oct 



C. vochinensis Bernh. 



1. Bracts of the involucre tipped with a rigid spine; plants annual. 



5. Flowers purple; stem wingless; heads sessile; waste places, occas'onal; adv. 



from Eur. June-Oct C. calcitrapa L. 



5. Flowers yellow; stem winged by the decurrent leaf-bases; heads peduncled; 

 waste places, occasional; adv. from Eur. July-Sept. Barnaby's Thistle 

 C. sols tit talis L. 



Tribe 12. Cichorieae 

 53. Serinia Raf. 

 S. oppositijolia (Raf.) Ktze. Moist sandy soil, s. 111. Mar.-Apr. 



54. Tragopogon L. — Oyster Plant. Salsify 

 1. Flowers yellow. 



