64 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



The curves (figs. 33, 34, and 35) representing leaf expansion, in 

 some instances at least, follow the temperature curves, but in gen- 

 eral slope in opposite direction to the curves portraying the evapora- 

 tion rate. Exception to the latter occurs in the case of wheat in the 

 aspen-fir type ; but data already presented have shown that evapora- 

 tion is not a limiting factor for this plant in this type. Also it will 

 be seen that growth toward the end of the season does not in all cases 

 bear an inverse relation to evaporation. As an explanation of this 

 fact it should be stated that the leaf growth is rapidly declining at 

 that time, because of the approach of maturity of the plants. On 

 the other hand, there appears to be no consistent correlation between 

 growth increments and temperatures. The agreements that do occur 

 also show a direct correlation between growth and evaporation, and 

 hence it may be concluded that the temperature and growth relations 

 recorded are more or less incidental. 



Considering the graphs more in detail, in the case of figure 33 the 

 leaf increment curves of both species in practically all instances have 

 concavities opposite in direction to those of the curve representing 

 the evaporation. It should be stated that no growth data were ob- 

 tained for brome grass in the second period of measurements, and 

 therefore the slope of the line at that point is not in agreement with 

 the growth curve for wheat, nor is it in opposition to the slope of 

 the evaporation curve. In the case of wheat, the leaf expansion 

 curve and the evaporation values platted for the first three periods 

 show inverse relation. Between the fourth and fifth periods there is 

 a slight disagreement in these values, but between the fifth and sixth 

 periods the leaf measurement and evaporation curves again show 

 opposite trend. The leaf increment curve for brome grass, on the 

 other hand, is in inverse proportion to the evaporation in all in- 

 stances. 



As would be expected, where evaporation is unusually high, as in 

 the oak-brush type, temperature in general is high, and these factors 

 usually run more or less parallel. This being the case, it is hardly 

 to be expected that the growth increments, would follow the physio- 

 logical temperature indices. As is well known, the growth rate in- 

 creases with increase in temperature up to the optimum requirement 

 of a given species. In the oak-brush type there is reason to believe 

 that the temperature not uncommonly exceeds the optimum require- 

 ments of the species observed. The lack of correlation between 

 growth rate and temperature in the oak-brush type, then, would 

 seem to strengthen the evidence that the evaporation in that asso- 

 ciation is the determining factor in the rate of elongation as well as 

 ultimate expansion of the leaf. 



In figure 34, in which similar data are given for the aspen-fir 

 type, wheat, except in one instance follows the evaporation curve, 



