WEED MIGRATION 707 



should at the same time explain the absence of American species 

 from Europe. But the partial causes already alluded to fail to do 

 this. There is a residual effect for which they do not account. 

 May it not be true that the plants of the European flora possess 

 more of this plasticity, are less unyielding in their constitution, can 

 adapt themselves more readily to new surroundings, and thus secure 

 their continuance in the New World 1 And may it not be the lack 

 of this plasticity in the American flora which incapacitates it for 

 securing a foothold and obtaining a living in the different condi- 

 tions of the New World? Under the care of the gardener they 

 grow and embellish the gardens and conservatories of Europe, but 

 without this care they speedily fail and die. 



Dr. Gray in a very friendly criticism of this paper remarked 

 that, ' ' So far as we know, the greater plasticity of European as com- 

 pared with American plants is purely hypothetical. More plastic, 

 would mean of greater variability, which, if true, might be deter- 

 mined by observation. Because Europe once had more species or 

 types in common with North America than it now has, it does not 

 seem to follow that the former has 'a younger plant-life,' or that 

 its existing plants are more recent than those of the American flora. 

 And as already intimated, so refined an hypothesis is hardly neces- 

 sary for the probable explanation of the predominance of Old World 

 weeds in the Atlantic United States." 



It is interesting to note the large number of plants which are con- 

 tinually being added to the flora of Europe. Bitter in his paper on 

 the adventive flora of Bremen notes that out of the 2,492 plants 

 listed by Gfareke in his flora of Germany, 230 are adventive ; Hock, 

 who published a paper on the plants of North Germany, lists 54 

 as weedy and ruderal. 



The region embraced in North Germany probably contains as 

 many exotic weeds as Iowa. For our purpose let us compare foreign 

 weeds of Iowa with the native plants. Hitchcock, in his catalogue 

 of this plants of Ames and vicinity, lists 740 plants, of which 86 are 

 introduced and of these about one-third have not become permia- 

 nently established. 



Dr. Charles Mohr in his paper on Plant Life in Alabama, states 

 that "fully one-sixth of the plants enumerated in the catalogue of 

 the Alabama flora as growing without cultivation are immigrants 

 from other regions, and but few of them are native in the more dis- 

 tant parts of the continent north of Mexico. They are mostly from 

 the warmer temperate, subtropical, and tropical regions of the Old 

 World. Those of widest distribution and which have gained the 



