DESCRIPTIVE MANUAL 379 



teeth with weak prickles; rather large heads; involucre 1-1% in. 

 long; bracts with broad glandular back, the inner with a some- 

 what attenuated colorless tip. 



Distribution. — Common in borders of woods and in fields. From 

 Iowa to South Dakota and Kansas. In Story, Emmet, Kossuth, 

 Marshall, Boone, Linn, Clinton, "Webster and Carroll counties in 

 prairie meadows. 



Extermination. — This biennial is readily destroyed by cutting 

 the plants off below the surface of the ground. When left to 

 flower it dies but in meadows where cut off above the surface of the 

 ground it acts like a perennial. 



Canada Thistle (Cirsiam arvense (L.) Scop.). 



Description. — Smooth perennial, spreading by roots and root- 

 stocks, 1-3 ft. high, corymbosely branched at the top ; stem smooth ; 

 leaves lanceolate, sessile, and deeply pinnatifid, lobes and margins 

 of leaf with spiny teeth; heads small, %-l in. high, bracts ap- 

 pressed, the outer with a broad base, inner narrow, all with an 

 acute, never spiny, tip; somewhat arachnoid flowers purple, di- 

 oecious; in staminate plant, flowers exserted with abortive pistils, 

 in pistillate less so, scarcely exceeding the bracts; tube of the 

 corolla 6 lines long; stamens with abortive anthers, anther tips 

 acute, filaments minutely pubescent; young achene pubescent; all 

 of the bristles of the pappus plumose; trichomes simple, long, 

 floccose. 



Distribution.— This European weed is widely distributed in Can- 

 ada to the Pacific coast; found in Iowa in many counties, more 

 common in northern counties than in the southern. It is more or 

 less abundant in Hardin, Pocahontas, Clinton, and Worth coun- 

 ties, frequently in clover meadows and in pastures. 



Extermination. — The Canada thistle can be treated with sodium 

 arsenite. No other chemicals, so far as our experiments extend, 

 will entirely destroy this weed. Carbolic acid only partially de- 

 stroys the roots and the plants shoot up again from below the point 

 of injury, but by repeating the process the Canada thistle can ulti- 

 mately be exterminated. A good method of eradicating the weed 

 is to plow shallow and cultivate frequently during the summer. 

 The roots of the Canada thistle extend deeply down into the soil, 

 hence for this reason deep cultivation will be of no avail. After 



