CLIMATE AND PLANT GROWTH. 



61 



for the season at any given station should show an opposite slope to 

 the corresponding graph of leaf-expansion rate. In order to deter- 

 mine whether such a relation exists, the rate of leaf expansion was 

 calculated for the periods for which plant measurements were made, 

 as was also the evaporation rate. 



The values of seasonal march of leaf -growth rate given in Table 

 18, and graphically shown in figures 33, 34, and 35, represent average 

 daily increase in length per leaf for the various culture periods. 

 This quantity was obtained for each period by dividing the total leaf 





28.0 

 SO. 9 



4.7 



* 



'9.4 





JT 



\ 



-50 



\ 



GROWING SEASON' 



Phys/o/og/cd/ fempera+ure 

 efficiency 



Brome- grass 

 --- Wheat 



FIG. 33. Periodic relation between leaf expansion, evaporation, and temperature, oak- 



brush type. 



length produced up to the time of measurement by the total number 

 of leaves, thus giving the average length per leaf. The increase in 

 the average length per leaf of the plants from period to period was 

 then determined by subtracting from each of the average leaf-length 

 values the corresponding value of the preceding period. Since the 

 periods for which measurements were taken varied somewhat in time, 

 these increases, in order to make them comparable, were reduced to 

 daily rates by dividing each increase in average leaf length by the 

 number of days in which the increase took place. The physiological 



