544 



ZONES AND REGIONS [Pt. Ill, Sect. II 



The district between the river Mississippi and the Rocky Mountains 

 possesses typical grassland climate, which it owes in the first place to the 

 dry winter and the uniformly moist early summer, and, in the second place, 

 to the moderate amount of its rainfall. It may also be added, that the hos- 

 tility of the cold winter to trees is increased by frequent north winds 

 accompanied by severe frost. This district is that of the treeless steppes, 

 known as prairies. 



The plateau between the Rocky Mountains and the Sierra has, for the most 

 part, a desert climate to which the character of its vegetation completely 

 corresponds. 



Finally, the Pacific district, except in its southern desert-like portion, 

 possesses a pronounced woodland climate ; to the moderate precipitations 

 in Central California there correspond the xerophilous woodlands ; to the 

 abundant precipitations of the North with its cold winter correspond the 

 tropophilous high-forests. 



The following tables, borrowed from Hann, give the percentages of the 

 annual rainfall for the Central Atlantic forest district and for the grass- 

 land prairie district (Nebraska and Kansas) of the same latitude (about 

 4 o° N.) :— 



MONTHLY PERCENTAGES OF ANNUAL RAINFALL IN THE CENTRAL 

 ATLANTIC FOREST DISTRICT AND THE GRASSLAND PRAIRIE 

 DISTRICT (NEBRASKA, KANSAS) AT THE SAME LATITUDE 

 (ABOUT 40 N.). 



(After Hann, I, Bd. Ill, p. 292.) 



