i 9 i 9 ] VESTAL— PHYTOGEOGRAPHY OF COLORADO 183 



account of the sedimentary area. The contrast between the granite 

 soil of the foothills and the soil of sedimentary origin lying just out- 

 side, with its slight selective action on flora and vegetation, has 

 also been noted. Other mentions of soils, especially as regards soil 

 texture, are scattered. 



Topography is systematically treated for particular regions 

 and smaller areas by dividing each type of topographic complex 

 into topographic areas or habitats ; with each type is correlated the 

 particular plant community or the several communities which 

 accompany it. In the regional section will be found similar analyses 

 of the cuesta, high mesa, mesa-terrace, and flood-plain complexes. 

 Particular physical factors controlled by local position and by 

 slope are mentioned in a former article (18). 



Atmospheric factors vary locally in this region to a probably not 

 very great extent, but even slight differences may be critical, 

 as has been found by Cooper in the California chaparral. The 

 factor of greatest influence upon plant life, and the one most readily 

 measured, is the evaporating ppwer of the air, the value of which 

 represents the resultant of several contributing factors. Local 

 distribution of evaporating power is believed to be controlled 

 primarily by differences in topography, and secondarily by differ- 

 ences in vegetation-cover. That is to say, flatness of the land 

 surface makes for comparative uniformity of exposure to wind 

 and sun; hilliness causes diversity of exposure. Local water or 

 wet-soil surfaces may lower evaporating power by contributing 

 much water vapor to the air. Topography thus determines the 

 original local distribution of evaporating power. This original 

 local distribution is modified by vegetation-cover. In flat country 

 the uniformity is changed. Low and open vegetation lowers evap- 

 orating power at the ground surface only slightly, but mesophytic 

 closed forest lowers it very greatly (Gates 3,4). In hilly country in 

 not too humid climates the originally protected ravines and shaded 

 or wind-sheltered slopes may develop mesophytic vegetation which 

 still further lowers evaporation, while the originally exposed slopes 

 and summits usually remain xerophytic. Thus, in the mountain- 

 front region here considered, primary environmental differences 



