174 BOTANICAL GAZETTE [September 



season is in general 2-3 weeks in advance of that of Denver, 14 

 miles from the mountains. 



The divide between Platte and Arkansas drainage, 'which should 

 be considered in connection with the mountain-front area, has a 

 mean temperature and frostless period intermediate between those 

 of mountain-front and foothills areas, as it is intermediate in alti- 

 tude and in vegetation. 



The plains have a slightly lower mean temperature and shorter 

 season without frost than the mountain-front area; the tempera- 

 ture of the dry plains at some distance from the mountains 

 approaches that of the mountain-front more closely than that of 

 the plains adjoining it. This difference is accompanied by a 

 floristic one. Temperature extremes are greatest in the plains, a 

 condition inimical to growth of woody plants. 



The plains, mountain-front, and foothills in northern Colorado 

 ("northern area") are cooler than those to the south, but the north- 

 south differences in temperature and length of growing season due 

 to latitude are of much smaller range and influence upon 

 vegetation than the east-west differences due to altitude and 

 changes of topographic character. 



For purposes of comparison table III includes rainfall data for 

 the higher parts of the mountains bordering the foothills on the 

 west (montane zone) , and for the plains of eastern Colorado border- 

 ing the region studied on the east. Annual rainfall is higher to the 

 west, increasing with elevation, and higher also in the eastern 

 plains, as a part of the gradual geographic increase of rainfall from 

 the dry belt of the Great Plains eastward through the prairie region 

 to the border of the eastern deciduous forest region. The eastern 

 plains mark the transition from short-grass plains to the taller 

 prairie-grass vegetation of the prairie, and are known in Colorado 

 as "the rain belt." The driest part of the plains region lies between 

 the rain belt and the plains near the mountains, in a zone distant 

 from the mountains about 18-25 miles, and of a breadth 30-60 

 miles. It is narrowed on the west by the elevation of the Platte- 

 Arkansas divide, and extends farther eastward in the Arkansas 

 River Valley. It extends only a little way north into Wyoming 

 and apparently is much narrowed on the west in extreme southern 



