PLANT GEOGRAPHY OF THE UNITED STATES 489 



It is evident that the rainfall increases southward along the 

 Atlantic coast, but that on the Pacific coast it diminishes south- 

 ward. Passing from either coast inland, one finds the rainfall 

 diminishing until it reaches a minimum in the Eocky Mountain 

 region. 



467. Plant geography of the United States. All of the con- 

 tinuous territory of the United States l lies in the north tem- 

 perate zone. There is material for volumes in the discussion of 



FIG. 373. Annual rainfall of the United States 



Darkest shade, over 80 inches ; lighter vertical lines, from 40 inches to 80 inches ; 

 horizontal lines, from 20 inches to 40 inches ; hlank, from 10 inches to 20 inches ; 

 dotted, less than 10 inches. After W. M. Davis 



the distribution of plants over our territory in this continent 

 alone, but it is possible to sum up a brief outline of the matter 

 in a few pages. Excluding the floras of many single mountains 

 and mountain ranges, the land surface of the country may for 

 botanical purposes be divided into four great areas, as follows : 

 The forest region. This occupies the eastern and central 

 portion of the United States. It is bounded on the west by an 



i That is, not counting in Alaska, our West Indian possessions, the 

 Hawaiian Islands, or the Philippines. 



