MIGRATION OF WEEDS 



our native plants are affected in this way. Common 

 purslane (Portulaca oleracea) has without doubt in some 

 places replaced native forms. Bull thistle (Cirsium 

 lanceolatum) without doubt trespasses on the ground of 

 some of our native species. C. altissimum, C. iowense and 

 C. discolor are undoubtedly affected in this way. These 

 thistles are certainly less abundant where bull thistle 

 occurs. In Manitoba the Canada thistle is much more 

 abundant than th bull thistle. In places it occurs not 

 only in fields, but in the woods. The sow thistle (Son- 



+ C ar ,oJa 1t,;stft 

 , Wafer Hyacinfli 



I w u m i; nea if 



Fig. 33d. Distribution of some weeds. Canada thistle, etc. (C. M. 

 King.) 



chus arvensis) is abundant everywhere in fields. There 

 is little of the annual species (S. oleraceus). 



Most of our common weedy plants are of European 

 origin. This is true for certain parts of the United States 

 only, notably New York, Massachusetts, Ohio, Illinois, 

 and other states that belong to the North Atlantic sec- 

 tion. Yet the weeds of the interior region, the territory 

 embracing our prairies, include a surprising number of na- 

 tive plants that are weedy in their nature. A few of the 



