CLIMATE AND PLANT GROWTH. 



67 



far as physical factors in the locality here dealt with are concerned, 

 it appeared pertinent to determine whether or not the water require- 

 ment is also correlated with the factors mentioned. The value of data 

 showing the relative water requirement of different plant species has 

 been demonstrated by numerous investigators. The climatic factors 

 that chiefly affect the rate of dissipation of water through transpira- 

 tion, on the other hand, have until very recently received relatively 

 little attention, although such researches would appear to be of pro- 

 found economic importance. If it were known, for instance, that in 

 a region of limited rainfall the evaporation was largely responsible 

 for the high transpirational demand and consequently the high water 

 requirement of a given plant, habitats might be selected where vegeta- 

 tion, natural barriers, or other features would afford protection 

 against excessive evaporation. Likewise, if temperature were the 

 factor determining the water requirement, cool north and east slopes, 

 or possibly partially shaded sites might be selected at least at lower 

 elevations and failure of crop production thus avoided. 



In order to determine the relation of water requirement of these 

 plants to evaporation and temperature in the type stations, the water 

 used per unit of dry matter by wheat, peas, and brome grass, through 

 practically the entire growing season, was divided by the evaporation 

 for the corresponding period. Tabular and graphic presentation of 

 the results follows. 



TABLE 19. Effect of temperature and evaporation on water requirements of 

 plants grown in type stations. 



As has been shown in previous graphs, the plrysiological tempera- 

 ture efficiency is highest in the oak-brush type, the curve dropping 

 in practically a straight line to the central and highest types. The 

 quotients of the values derived by dividing the water requirement 

 per unit weight of dry matter for the respective types by the evapo- 

 ration for each period when platted (fig. 37) are likewise 



