706 



WEED FLORA OF IOWA 



Fig. 542. Tickle Grass (Panicum capillare). A common grass, probably orig- 

 inally found in clearings, has rapidly spread to cultivated ground. 

 (Lamson-Scribner, U. S. Dept. of Agr.) 



have apparently become strongly aggressive under changed condi- 

 tions. These are some of the instances which may show that pre- 

 dominance is not in consequence of change of country and intro- 

 duction to new soil. 



In the interesting paper of Claypole the author argues that the 

 abundance of European weeds in North America is because Euro- 

 pean plants are more plastic than American plants. He says: But 



a weed possessing a plastic nature — one capable of being moulded 

 by and to its new surroundings — ere long adapts itself, if the 

 change is not too great or sudden, to its new situation, takes a new 

 lease of life, and continues in the strictest sense a weed. 



Is it not possible that some such cause as this may lie underneath 

 the facts we detailed in the earlier part of this paper? The true and 

 full explanation of the transfer of European species to America 



