ACCUMULATION OF ENERGY 93 



limiting factor, namely some factor which interferes with the 

 expected increase in rate. The limiting factors are illumination 

 and temperature, as an increase in either of these allows the 

 rate of synthesis to continue rising as the concentration of 

 carbon dioxide increases. 



Effect of Intensity of Illumination. In a similar way it 

 can be shown that with weak illumination the rate of synthesis 

 is proportional to the intensity of the light, but above a 

 certain intensity of light the synthesis does not increase with 

 the increase in illumination. The limiting factors in this case 

 are concentration of carbon dioxide and temperature, as a 

 rise in either of these factors allows photosynthesis to rise to 

 a still greater rate with a further increase in the intensity of 

 light. 



Effect of Temperature. The temperature effect is less simple 

 than the preceding factors. With increasing temperature the 

 rate of reaction is more than doubled by each rise of ten 

 degrees Centigrade. This is well seen at medium temperatures, 

 but at low or high temperatures the rule does not hold. 

 At temperatures near the freezing point photosynthesis 

 practically ceases, so there is a high temperature coefficient, 

 as the temperature rises to temperatures at which photo- 

 synthesis is appreciable. This effect must be due to some 

 process with a high temperature coefficient which cuts short 

 the synthesis at low temperatures. 



Likewise at high temperatures the rate decreases. This 

 decrease is due to a destructive action on the cell colloids. 

 The whole process is paralleled by the action of temperature 

 on enzymes. As the temperature rises the rate of destruction 

 increases, and owing to the time required to produce photo- 

 synthesis the cell is killed before a measurable amount of 

 photosynthesis has occurred. 



Blackman has estimated the initial rate of synthesis in the 

 following way. He measured the rate of photosynthesis at 

 different time intervals, from which he constructed a curve 

 for each temperature, showing the falling off in photosynthesis 

 with the duration of the experiment. By exterpolating back 

 to zero time the initial rate of reaction for each temperature 

 was estimated* (see Fig. 13). 



The initial rates thus obtained showed a fair agreement 

 with what one would expect by calculation from the rate of 

 reaction and the temperature coefficient, each measured at 

 lower temperatures. 



* F. F. Blackman, Ami. of Bot., 1905, vol. 19, p. 281. 



