COM POSIT 'A E (THISTLE FAMILY) 



60 ARCTIUM. 



i a Heads 1-2 in. broad; inner bracts of involucre as long 



or longer than the flowers. .GREAT BURDOCK. (A. Idppa.) 

 b Heads V2-% in. broad; inner bracts of involucre usually 



shorter than the flowers. .COMMON BURDOCK. (A. minus.) 



61 CARDUUS. 



1 a Flowers yellow ....YELLOW THISTLE. (C. spinosissimus.) 

 b Flowers not yellow, usually purple or white 2 



2 a Upper branches winged by spiny segments; pappus not 



plumose CURLED THISTLE. (C. crispus.) 



b Upper branches not winged as in a; pappus usually 



plumose 3 



3 a Heads i in. or less broad, several in a corymb 



CANADA THISTLE. (C. arvensis.) 

 b Heads 1-2 in. broad, usually solitary at the ends of the 



branches 4 



4 a Bracts of involucre without prickles, more or less 



cottony SWAMP THISTLE. (C. muticus.) 



b Bracts of involucre tipped with bristles or prickles .... 5 



5 a Leaves green on both sides, not woolly beneath 



PASTURE THISTLE. (C, odoratus.) 

 b Leaves more or less woolly beneath 6 



6 a Leaves, excepting sometimes the lowest, not pinnatifid, 



merely toothed with bristly tips 



TALL THISTLE. (C. altissimtts.) 

 b All leaves more or less pinnatifid, not merely toothed. . 7 



7 a Lobes of leaves triangular-lanceolate; all bracts of in- 



volucre tipped with prickles 



BULL THISTLE. (C. lanceolatus.) 



b Lobes of leaves linear or linear-lanceolate; inner bracts 

 of involucre not tipped with bristles; prickles of outer 

 bracts long and slender .... FIELD THISTLE. (C. discolor.) 



62 ONOP6RDON COTTON THISTLE. (O. acdnthium.) 



63 CENTAUREA. 



1 a Plant very woolly, cultivated as bedding plant 



WOOLLY DUSTY MILLER. (C. cineraria*) 

 b Plants not very woolly, especially when old, not culti- 

 vated as in a 2 



2 a Outer bracts of involucre tipped with stout, spreading 



spines STAR THISTLE. (C. calcitrapa.) 



b Outer bracts of involucre not tipped with spines 3 



3 a Heads Vz-i in. broad; pappus wanting 



KNAPWEED. (C. nlgra.) 

 b Heads 1-3 in. broad; pappus of short bristles 4 



239 



