OPTIMUM TEMPERATURE I39 



8° C, at which no evolution of oxygen was observed. The 

 temperature was then allowed to rise slowly, and the evolu- 

 tion of oxygen was found to be initiated at o,°-5 C. The 

 activity increased at first slowly, then more rapidly with 

 further rise of temperature. The rate of increase became 

 uniform in the middle portion of the curve. After the 

 optimum point, which was generally above 30 C. and very 

 sharply defined, the decline of photosynthetic activity was 



Fig. 38. Responses of Mimosa to Testing Stimulus of same 

 intensity through Cyclic Variation of Temperature at 23 , 

 34 , 37 , and back to 34 and 23 



Note maximum response at 34 , which is therefore the optimum 



temperature. 



very rapid. Thus, in a typical case, an activity of 337-5 

 c.mm. at 28°- 3 was increased to 416-3 c.mm. at 30°- 5. 

 Above this it declined to 386-0 c.mm. at 32°- 8. In another 

 case the optimum was found to be at or about 32 C. 



The optimum point is to some extent modified by the 

 influence of the season. In winter the optimum may be as 

 low as 28 C, while in summer it is about 33 C. 



Stronger intensity of light is also found to raise the 

 optimum point. 



The table on the opposite page gives the results obtained 

 with three typical specimens. 



