A GUIDE TO THE WILD FLOWERS 



267 



cut foliage, and rarely exceeding 5 ft. in height is also very- 

 common. Both shed immense clouds of pollen that are the 

 cause of much hay fever. 



819. Leaves alternate or basal. 



Leaves or flower heads not prickly no. 825 



Leaves or flower heads, or both, prickly Thistles no. 820 



820. THISTLES. CIRSIUM. 



The thistles are mostly tall prickly herbs of open fields 

 or waste places. Leaves alternate, prickly on the margins, 

 ribs or leaf-stalks, or in all these places ; more or less cut, 

 toothed, or wavy-margined. Flower heads large, the bracts 

 beneath each head prickly in some species, merely bristly 

 and not prickly in others. Of several species, those below 

 may be separated thus: 

 Leaves densely white or brown-hairy on the under side 



Leaves brown-hairy beneath ; flower heads prickly 



Common Thistle no. 821 



Leaves white-hairy beneath ; flower heads merely bristly 



Field Thistle no. 822 



Leaves mostly green both sides, if paler beneath, not densely hairy 



Flower heads 1-4 in. wide, few, yellow . . Yellow thistle no. 823 



Flower heads less than i in. wide, numerous, purplish-white . . 



Canada Thistle no. 824 



