14 Minnesota Plant Life. 



a terrestrial life, and in the same pond may be found land-plants 

 pushing down to the very water's edge or beyond it until they 

 can go no farther because of the limitations of their structure. 



The tension between the forest and prairie, because it extends 

 so widely, may be called a continental tension. The other ten- 

 sions, between knolls and ravines, between banks and meadows, 

 between beaches and pond edges, may be called minor tensions, 

 but the law of the two cannot be very different. Indeed, there 

 may be gained a fair idea of the fundamental difference between 

 prairie and forest by observation of an area so limited as a road- 



FIG. 6. Island in the Mississippi above St. Paul. The center is occupied by elms while the 

 rim is fringed with willows. An example of a "minor tension." After photograph by 

 Professor W. R. Appleby. 



side or path. The principal difference between the two is the 

 duration of the causes at work. Between the prairie and the 

 forest the tension has been in existence possibly for thousands 

 of years, while between the knoll and the ravine possibly for but 

 a few decades or centuries. As a result there have come to 

 exist in the old warring formations structural peculiarities char- 

 acteristic of each, so that, to the eye of the observer, they pre- 

 sent very different appearances. Where the struggle is of more 

 recent origin and of more limited extent the differences are not 

 so great and, therefore, not so evident. 



Forests of Minnesota and of the world. We cannot well 

 consider forest as it exists in Minnesota apart from the general 

 forest which covers the northern part of the continent. From 



