182 BOTANICAL GAZETTE [September 



Evaporating power of the air has not been subject to geographic- 

 statistical treatment, since there are no data. It was beyond the 

 scope of the present study to have attempted instrumental investi- 

 gation on a scale large enough to be of value. There seems to be 

 little doubt that, as a geographic factor in regions of continental 

 climate, evaporating power of the air is of about the same indicator 

 value as rainfall. It varies geographically about as does rainfall, 

 in inverse ratio, since evaporating power is, in large measure, a 

 function of rainfall. This same inverse ratio seems to hold in 

 seasonal distribution as well as geographically. This may be seen 

 from the graphs of Weaver (20), and from data obtained by 

 Cooper in a study of chaparral in California. 



As a local factor evaporation is separately treated in the dis- 

 cussion of local distribution of vegetation. 



For further discussion of the climatology of Colorado in rela- 

 tion to vegetation the reader is referred to the articles of Shantz 

 (15, 16), Ramaley (12), and Robbins (13, 14). Data may be 

 had from the bulletins of the United States Weather Bureau, 

 Colorado College, the Agricultural Experiment Station at Fort 

 Collins, and the Bureau of Plant Industry. 



Local distribution of vegetation 



Physical factors. — Local physical conditions affecting plant 

 distribution are those concerned with substratum and soil; with 

 topography, especially local position with respect to surroundings, 

 and slope of surface, as regards both steepness and direction of 

 exposure; and with local variation in atmospheric conditions, as 

 controlled primarily by topography. The variability of these 

 factors within the region is great, and their interactions are com- 

 plex. Descriptions of the soil, topography, atmospheric condi- 

 tions, etc., of the different parts of the region are scattered through 

 both systematic and regional sections of this study, and a lengthy 

 discussion at this point would be out of place. A few references 

 to other parts, and certain incidental comments, may here be made. 



The character of the substratum, and some of its influences in 

 determining soil conditions and topography, are indicated in the 



