184 BOTANICAL GAZETTE 



SEPTEMBER 



due to topography may rather thoroughly control vegetation dis- 

 tribution. In such cases the reaction of vegetation-cover upon 

 local evaporation conditions may merely heighten the original 

 topographically determined contrast between protected and ex- 

 posed habitats. Topography governs local vegetation distribution 

 through the mediative influence of a number of physical factors, 

 of which evaporating power is one. Depending as it does upon 

 several other factors, evaporation forms a convenient index of 

 habitat, but is not in itself the basic controlling condition. For 

 these reasons the writer has subordinated the influence of evaporat- 

 ing power. upon local distribution to that of topography. 



The sudden change of elevation at the mountain-front is a 

 topographic condition affecting evaporating power. At many 

 places the hogbacks, mesas, and outer slopes receive no direct 

 sunlight during several hours before sunset, being shaded by the 

 higher slopes immediately to the west. This contributes to 

 the comparative mesophytism of certain mountain-front stations 

 where the descent from foothills to plains is more than ordinarily 

 abrupt. 



Direction of exposure affects local atmospheric conditions and 

 vegetation in many easily observed ways. Cloudiness and showers 

 occur on summer afternoons much more frequently than in the 

 mornings, as Ramaley has noted. East-facing slopes are thus 

 likely to be drier than west-facing slopes (the latter are less fre- 

 quent east of the range-crest). As would be expected, the differ- 

 ence between north- and south-facing slopes is considerable, the 

 latter being more exposed to sun and conditions favoring rapid 

 evaporation, and with sparser, more xerophytic vegetation. In 

 open parts of the foothills where slopes are quite gentle the north- 

 facing slopes are not sufficiently sheltered from sun and wind to 

 differ in vegetation from the south-facing slopes in any marked 

 degree. Steep north slopes, or both sides of steep and narrow 

 ravines which run down to the north, however, are quite meso- 

 phytic. In different parts of so large a territory the combina- 

 tions of contrasting vegetation of north and south slopes would 

 be expected to vary, and a few of them are listed herewith by way 

 of illustration. 



