704 WEED FLORA OF IOWA 



of European descent, so too it is never without its plant population 

 of European weeds. 



Dr. Asa Gray, who discussed the subject of weeds from a phi- 

 losophical standpoint, said: 



In the United States, and perhaps in most parts of the world, 

 a large majority of the weeds are introduced plants, brought into 

 the country directly or indirectly by man. Some, such as dande- 

 lion, yarrow, and probably the common plantain and the common 

 purslane, are importations as weeds, although the species naturally 

 occupy some part of the country. 



Why weeds are so pertinacious and aggressive is too large and 

 loose a question; for any herb whatever when successfully ag- 

 gressive becomes a weed ; and the reasons of predominance may be 

 almost as diverse as the weeds themselves. But we may inquire 

 whether weeds have any common characteristic which may give them 

 advantage, and why the greater part of the weeds of the United 

 States, and probably of similar temperate countries, should be 

 foreigners. 



As to the second question, this is strikingly the case throughout 

 the Atlantic side of temperate North America, in which the weeds 

 have mainly come from Europe ; but it is not so, or hardly so, west 

 of the Mississippi in the region of prairies and plains. So that the 

 answer we are accustomed to give must be to a great extent the 

 true one, namely, that, as the district here in which weeds from 

 the Old World prevail was naturally forest-clad there were few 

 of its native herbs which, if they could bear the exposure at all, 

 were capable of competition on cleared land with emigrants from 

 the Old World. It may be said that these same European weeds 

 here prepotent had survived and adapted themselves to the change 

 from forest to cleared land in Europe, and therefore our forest- 

 bred herbs might have done the same thing here. But in the 

 first place the change must have been far more sudden here than 

 in Europe; and in the next place we suppose that most of the 

 herbs in question never were indigenous to the originally forest- 

 covered regions of the Old World; but rather, as western and 

 northern Europe became agricultural and pastoral, these plants 

 came with the husbandmen and the flocks, or followed them, from 

 the woodless or sparsely wooded regions farther east where they 

 originated. This, however, will not hold for some of them, such as 

 dandelion, yarrow, and ox-eye daisy. It may be said that our 

 weeds might have come to a considerable extent from the border- 

 ing, more open districts on the west and south. But there was 

 little opportunity until recently, as the settlement of the country 

 began on the eastern border; yet a certain number of our weeds 

 appear to have been thus derived; for instance, Mollugo verticiL 

 lata, Erigeron canadense, Xanthium, Ambrosia artemisiaefolia, 

 Verbena hastata, V. urticifolia, etc., Veronica peregrina, Solanum 



