PLANT GEOGRAPHY OF THE UNITED STATES 489 



It is evident that the rainfall increases southward along the 



o 



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

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

 diminishing until it reaches a minimum in the Rocky Mountain 



regon. 



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

 tinuous territory of the United States 1 lies in the north tern- 



t- 



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



FIG. 373. Annual rainfall of the United States 



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

 horizontal tines, from 20 inches to 40 inches ; blank, 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 t<> 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 



O : 



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 



1 That is, not counting in Alaska, our West Indian possessions. tin- 

 Hawaiian Islands, or tin- Philippines. 



