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WEED FLORA OP IOWA 



ows, but also in com and grain fields. It grows in patches like the 

 C5anada thistle. These patches increase in size from year to year. 

 It was described as a doubtful perennial by early botanical writers 

 and so far as I have been able to determine, it is a perennial It 

 may be exterminated by thorough cultivation, plowing well and 

 then following with the cultivator. 



Prairie Thistle (CirsivMi ddscolor (Muhl.) Spreng.). 



Description. — ^Tall, branching, leafy biennial, 5-7 ft. high, with 

 heads larger than in Canada thistle; stem striate, slightly hirsute; 

 leaves radical, 12-14 in. long, deeply pinnatifid, the divisions fre- 

 quently divided, pricMy-toothed, the upper surface smoothish, and 

 the lower white ; woolly single heads terminating the branches, with 



Fia. 221. Prairie or Wood Thistle (.Cirsium discolor). Common border of 



woods, etc. 

 (Photographed by Colbum.) 



