COMPOSITiB 



139 



nial or biennial herbs with alternate priokly leaves and large heads 

 of reddish-purple or rarely white flowers. 



1. C. unduiatum, (Nutl^.) Spreng. Praihie Thistle. 



Biennial, whitish-woolly throughout; stem leafy, 1-3 ft. high; leaves 

 partly clasping, pinnatifid, but rarely pinnately divided, somietimes quite 

 prickly ; outer bracts of the involucre glandular and 

 glutinous on the back and tipped with spreading 

 prickles ; flowers purple or reddish. Prairies, Man.- 

 Alta. 



2. C. muticum, Michx. Swamp Thistle. 

 Biennial, tall, 2-7 ft. high, smoothish, sparingly 



leafy; leaves deeply pinnatifid, armed with weak 

 prickles ; heads large, few ; flowers purple. Swamps 

 and wet prairie, Man. and westward. 



3. C. arvensis, (L.) Scop. Canada Thistle. 

 Perennial, from a deep, extensively branching 



rootstock; stem 'rather slender, 1-3 ft. high, leafy; 



leaves sinuately pinnatifid and very prickly; heads 



numerous; flowers rose-purple, or more rarely 



whitish. A troublesome introduced weed becoming Fig. 83. — Cirsium 



quite common in cultivated ground. arvensis. 



5. ARCTIUM. Burdock. 



Heads fairly large, of perfect tubular flowers; corolla purple or 

 white, the tube 5-eleft; receptacle, flat and bristly; involucre almost 

 globular, the bracts stiff and tipped with hooks; pappus of numer- 

 ous scales. Coarse biennial herbs with large alternate leaves, and 

 heads developing into burs clustered at the ends of the branches. 



1. A. minus, Bernh. Common Burdock. 



Stem branched, 2-5 ft. high; leaves large, cordate, on hollow petioles; 

 spines of the outer bracts spreading, those on the inner erect, not so long as 

 the flower. An unsightly weed, common locally about towns. 



2. A. LIppa, L. Great Burdock. 



Larger, 4-9 ft. high, with broader, often cordate leaves, and all the spines 

 on the bracts of the involucre spreading. Waste places about towns. 



6. BIGELOWIA. Ratless Goldbnrod. 



Heads discoid, small, few-flowered, rayless; involucre rather lonff, 

 the bracts more or less keeled and arranged in several rows ; corolla- 



