176 BOTANICAL GAZETTE [September 



Colorado and northeastern New Mexico by the lava-capped plateaus 

 which there extend eastward from the mountains. 



As for the mountain-front and adjacent foothills and plains, 

 the first two average about the same, the mountain-front receiving 

 slightly more rainfall in the north and at the divide than the foot- 

 hills. This may perhaps be due to the fact that in the northern 

 part of the state, as at Boulder, the rain usually comes with east and 

 northeast winds; and since the change of elevation is greatest at 

 the mountain-front, more precipitation might occur there than in 

 the foothills beyond. At Boulder the more mesophytic forms of 

 vegetation occur more frequently and in larger areas in the sedi- 

 mentary rocks of the mountain-front than in the granite foothills 

 half a mile or a mile inside the foothills. In the southern part of the 

 state the mountain-front is drier than the foothills, as a rule. The 

 plains near the mountains receive almost 2 inches less rain, on 

 the average, than foothills and mountain-front, and the dry 

 plains to the east nearly another 2 inches less. 



The "northern area" (foothills, mountain-front, and adjacent 

 plains) receives on the average about 1 inch greater rainfall than 

 the "southern area." Coupled with the higher temperature and 

 greater evaporation, this results in a considerably more xerophytic 

 vegetation south of the Platte-Arkansas divide. There is little 

 difference in the plains, but at the mountain-front, with a difference 

 of 1 .57 inches, the vegetation to the south is markedly drier. 



Cooper finds, in the chaparral region of California, that very 

 slight differences in the original physical conditions of north and 

 south slopes result in very marked differences in vegetation. The 

 same principle seems to apply, in perhaps a smaller degree, in a 

 semi-arid region like the Colorado mountain-front. It appears that 

 differences in rainfall from place to place, or from month to month, 

 although small in absolute amount, can be critical in their influence 

 upon vegetation distribution. The slight differences appear to 

 represent marginal values above or below a critical point. The 

 difference in vegetation in two areas, moreover, is not necessarily 

 the result of climatic difference, but is a resultant of differences in 

 soil, topography, geographic position, and vegetational history, in 

 addition to climate. It should not be surprising, therefore, that 



