Wild Flowers East of the Rockies 389 



CANADA THISTLE (Cirsium arvense) (EURO- 

 PEAN) is a small flowered, perennial species that has 

 strayed across the ocean and became a pernicious 

 weed. Individual plants are not themselves any 

 more of a pest than are our native thistles but they 

 have a dangerous, latent or potential power, in that 

 they are far more prolific than our native species, 

 due perhaps more to the number of the flowering 

 heads than to any physical qualities of the plant. 



The stem is rather slender, branching and grows 

 from 1 to 3 feet in height. It grows from a perennial, 

 creeping rootstalk that is, as farmers have discover- 

 ed, very difficult to eradicate from the soil. It grows 

 in extensive colonies and, unless strenuous efforts 

 are made to destroy them, they very soon take pos- 

 session of a field to the exclusion of almost every- 

 thing else. 



The leaves, that grow alternately and closely to- 

 gether on the stem, are long, lance-shaped, deeply 

 cut into sharply-prickled lobes. Numerous flower 

 heads, about one inch across, terminate the branches. 

 When in full bloom, the florets vary in color from 

 rose-purple to white; the involucre is almost globu- 

 lar and covered with over-lapping bracts, each with 

 a tiny, sharp, out-turned point. 



All the thistles yield an abundance of nectar and 

 are frequented by bees and butterflies, by one of the 

 latter so persistently that it has been named the 

 Thistle Butterfly or Painted Lady (Pyrameis cardui) ; 

 in fact this butterfly usually begins its career, as a 

 caterpillar, on the thistle and lives chiefly upon its 

 nectar and pollen through life. 



