i8o 



BOTANICAL GAZETTE 



[SEPTEMBER 



in fig. 15; they contrast strongly. The northern area is character- 

 ized by the Rocky Mountain foothill type of rainfall, the southern 

 area by the New Mexican type (Ward 19). Both of these types 

 are described as having a single maximum, for the first in May, for 

 the second in July-August. The northern area receives most of 

 its rain from northeasterly winds; the southern area probably 

 from southeasterly winds. The centrally situated Platte-Arkansas 



-Southern Foothills 



-Southern Mountain-Front Southern Plains 



- Northern Area 



-Platte-Arkansas Divide 



Southern Area 



Figs. 15, 16. — Seasonal distribution of rainfall: fig. 15, southern foothills, 

 mountain-front, and plains; fig. 16, northern and southern areas and divide. 



divide receives rain from both directions, and has both maxima, 

 with a higher June rainfall, partly because of its considerable ele- 

 vation. Probably rain is carried from either direction past the 

 divide, producing secondary maxima, in July in the northern area, 

 and in April in the south. 



The abundant rainfall of the divide, especially in June, forms a 

 local rainfall type which is intermediate between that of well 

 watered parts of the foothill zone and that of the eastern or rain- 



