10 THE MONTANA EXPERIMENT STATION. 



commercial operations, is the chief agent in providing conditions 

 suitable for weed growth. 



The occupation of a country by nomadic tribes or a pastoral 

 population essentially disturbs the previously existing balance of 

 native vegetation in that region. The native species are killed 

 about the temporary camps and habitations and the pasturage of 

 flocks and herds tends to reduce or even exterminate many of the 

 more nutritious forage plants and to introduce others, which take 

 the place of those destroyed. A large proportion of the weeds of 

 the Plains probably owe their wide distribution to the Indian and 

 the buffalo, and the stockgr owing industry has merely continued 

 and extended the conditions previously prevailing. 



The change in the flora following the settlement of a country 

 by an agricultural population is relatively much greater, owing to 

 the increase of population and the extent of the changes produced 

 by cultivation, travel and commerce which facilitate the intro- 

 duction of many foreign species. 



Thus the weeds of any particular region are of two kinds 

 native (or indigenous) and introduced, the latter coming in from 

 other regions adjacent or remote. It is often desirable to separate 

 these two groups, as it is manifestly impossible to prevent the 

 introduction of species already a component part of our flora, 

 while the foreign species may be prevented from securing a foot- 

 hold, exterminated, or confined to certain limited sections already 

 invaded. 



In the older and more densely populated states it is often difficult 

 to distinguish the introduced plants from the native species without 

 long study and careful comparison over an extended area, and sys- 

 tematic botanists are often lax in this discrimination. In a new state 

 like Montana, the problem is greatly simplified because of the 

 sparsity of settlement and because the sources of weed infection 

 are relatively few and easily traced. Even here many introduc- 

 tions pass as native species by reason of their abundance in 

 certain sections and our uncertainty as to their natural distribu- 

 tion, yet there are certain rules by which we may form a fairly 

 correct judgment as to whether a given species is native or intro- 

 duced. In general, other conditions being the same, we may infer 

 that a species is introduced if— 



