CLIMATE AND PLANT GROWTH. 65 



hence to some extent that of the temperature; while in the case of 

 brome grass the leaf expansion curve slopes opposite to the evapora- 

 tion curve. In the central type, the evaporation being less than in 

 the lowest and the highest types (fig. 8) , in no instance has proved a 

 limiting factor for the type in question so far as concerns the physio- 

 logical activities of wheat. In the case of brome grass, however, the 

 contrary is true. The reason for the difference in the response of 

 these species is not obvious. 



In the spruce-fir type figure 35 shows that the leaf increment 

 values in the case of brome grass are in inverse proportion in every 

 instance to those of evaporation. 1 Hence, between the first and third 

 periods, when the highest rate of evaporation occurs, is recorded the 

 lowest rate of leaf expansion for the entire period ; and between the 

 fourth and sixth periods, which marks the lowest rate of evapora- 

 tion, by far the highest growth rate is recorded. The rates of leaf 

 expansion in wheat likewise show inverse relation to evaporation, 

 though less pronounced than the leaf-expansion rates of brome grass, 

 with the exception of the next to the last period. This disagreement, 

 however, is explained by the fact that the wheat specimens were 

 approaching maturity and the growth rate was, therefore, begin- 

 ning to decline. 



As in previous instances, the curve showing the growth rate of 

 wheat in the spruce-fir type seems to follow, in a general way, the 

 daily temperature curve, but this is evidently more or less inci- 

 dental. In order to determine these relations more definitely, aver- 

 ages of daily leaf increment of wheat and of temperature and evapo- 

 ration were computed for the season as a whole, the results of which 

 are platted in figure 36. From these curves it is evident that leaf 

 increment of wheat is in inverse proportion to evaporation, no obvi- 

 ous relation to temperature being shown. This relation is also found 

 to hold in the case of brome grass. 



To sum up the facts regarding the relation of leaf expansion to 

 evaporation and temperature : The daily rate of growth of the species 

 studied, as well as the total leaf surface produced, varies inversely 

 as the evaporation, except in the case of the daily rate for wheat at 

 the middle station. Evaporation in the aspen-fir type is lower than 

 in the types immediately above and below. As a limiting factor the 

 evaporation may be declared transitional in a sense that is, it 

 may determine growth rate periodically or seasonally in one species, 

 but not distinctly so in another. Since temperature and evaporation 

 are admittedly more or -less interrelated, it is difficult to separate 



1 As previously shown, the high evaporation in the spruce-fir type is chiefly accounted 

 for by high wind movement. 



56866 18 Bull. 700 5 



