8 



Plant Physiology 



extending from the Atlantic westward to the region of little 

 precipitation. 



Agriculture and commerce have already encroached to 

 an enormous extent upon the natural domain of both the 

 native forest and grazing lands ; but in mountainous 

 regions and towards the northern limit of vigorous growth 



FIG. 2. Merriam's Life Zones of the United States; Boreal (1), Transi- 

 tion (2), Upper and Middle Austral (3), Lower Austral (4), Gulf strip 

 of Lower Austral (5) , and Tropical (6) . Dotted parts of the Austral zones 

 indicate humid divisions. [After Cockerell, in Bailey's Cyclo. Agr.] 



most small crops become less profitable, and the forest will, 

 through many generations, at least, form the natural 

 boundary line separating agriculture from the Arctic zone. 

 In addition, of course, forests will continue to thrive in the 

 agricultural region where permitted by man. 



Furthermore, in taking a bird's-eye view of the forests 

 of the United States there is noticed a more or less striking 

 limitation in range, hence in general adaptability, of many 



