RATE OF PHOTOSYNTHESIS AT DIFFERENT HOURS 163 



continuous rise of temperature ; but this rise was suddenly 

 interrupted, and even reversed after i p.m., though the 

 temperature was still rising. The reason of this decline is 

 to be found in the fact that after i p.m. the optimum 

 temperature had been exceeded, with the result of a fall in 



Fig. 45. The Curves for Light (dotted line), Temperature (thin 

 line), and Resultant Photosynthetic Activity (thick line) 



activity. This was temporarily arrested by the tempera- 

 ture falling to the optimum at 3 p.m. and by a transient 

 increase of light which preceded that hour. But after 3 p.m. 

 both light and temperature were falling, with the result of 

 a very sharp decline in the rate of C0 2 -assimilation. After 

 4 p.m. the fall in the intensity of light was rapid ; at 5 p.m. 

 it had fallen to 56, in place of 68 at midday. The fall of 



