WEED MIGRATION' 



701 



Pig. 540A Fig- 540B 



Fig. 540. A. Charlock (Brassica arvensis). B. Black Mustard (B. nigra). 

 Immigrants from western Asia brought to the United States by way of 

 Europe ; early colonists. 



(Dewey, U. S. Dept. of Agr.) 



Warming and Vahl, in Oecology of Plants, speaking of the pam- 

 pas in South America and the European plants found there, say : 



The pampas occupy the vast, rockless, alluvial, South American 

 plains that stretch from the Atlantic coast to the Andes, and from 

 Patagonia to the forests of Paraguay and Brazil. The boundless, 

 level or somewhat undulating, uniform, treeless surface is clothed 

 with perennial grasses and herbs, like "a shoreless sea of grasses 

 on whose horizon the eye finds no resting point, save where the 

 sun rises and sinks". The genera represented are Melica, Stipa, 

 Aristida, Andropogon, Pappophorum, Panicum, and Paspalum. 

 Between the grasses grow numbers of herbs belonging to various 

 families; these include Verbena, Portulaca, Apocynaceae, Com- 

 positae, Eryngium, and others. Curiously enough, there are' very 

 numerous European species, which have succeeded in exterminating 

 the inland vegetation for miles and include not only such thistle- 

 like Compositae as Cynara cardunculus, Silybum marianum and 

 Lappa, but also Lolium perenne, Hordeum murinum, H. secalinum, 



