708 WEED FLORA OF IOWA 



firmest foothold are wanderers following civilized man in his con- 

 quest of the wilderness. Originally children of the open plain, ex- 

 posed to the extremes of heat, cold, drought, and excessive rain, these 

 plants necessarily acquire the widest elasticity in adapting them- 

 selves to new surroundings and possess the greatest power of re- 

 sisting adverse conditions. ' ' 



THE DIRECTION OF WEED MIGRATION. 



Mr. L. H. Dewey, who made a study of 200 North American 

 weeds and their migration, says: "A study of the origin of weeds 

 now in this country will impress one with the largeness of the num- 

 ber that have been introduced from Europe in comparison with the 

 number of native species or of species received from other direc- 

 tions." In the list of 200 weeds of the United States published in 

 the Year Book for 1895, 108 species are of foreign origin, while 92 

 are native. Of the 108 introduced species, 64 are native in Europe 

 and 30 are ascribed to the Old World in general, only 2 Asiatic 

 species in the list having established themselves as weeds in this 

 country without being first distributed in Europe. Africa and 

 Australia are not represented among our weeds, while Central and 

 South America have contributed only 12 or 15 important species, 

 most of which are confined to the Gulf states. A list of the plants 

 of Michigan published in 1892 contains 1,604 indigenous species, of 

 which 22 are recognized as injurious weeds, and 142 species intro- 

 duced from Europe, of which 57 have become troublesome weeds. 



A list of Kansas weeds enumerating 209 species contains 129 na- 

 tive species, 42 introduced from Europe, and 38 from all other 

 sources. Eighteen species native in the states east of the Mississippi 

 river have been introduced into Kansas in opposition to the pre- 

 vailing winds and the direction of the drainage, while only 3 species 

 are mentioned which have come from the Rocky Mountain region 

 with both of these natural forces in their favor. 



In an article on the weeds of California 110 species are mentioned 

 as troublesome in that state. Of these, 53 are native, 43 are intro- 

 duced from Europe, 5 are from the eastern United States, 3 from 

 Central and South America, and only 2 from Asia. Even in the 

 states bordering the Gulf of Mexico the number of weeds introduced 

 from Europe in cultivated land equals or exceeds those from Mexico 

 and South America. Canada thistle, bur clover, and skunkweed 

 have been taken from California to Australia, where they quickly 

 become naturalized and are now rapidly spreading. 



The weeds which have followed civilization in America are, 

 shepherd's purse, dandelion, sow thistle, stinging nettle, mallow, 

 plantain, chickweed, St. John's- wort, yarrow, toadflax and purs- 

 lane. Manasseh Cutler, in 1783, reported 66 species, among them 

 buttercup, chicory and daisy. Dr. Bigelow, in Florula Bostoniensis, 



