146 



Carbon Assimilation. 



Table XLV. 



Percentage of Radiant Energy Incident on the Leaf used in 

 Assimilation (Data from Puriewitsch). 



*The values marked with an asterisk were actually observed. The 

 remaining values in this column were obtained by calculation. 



have been possible to correct. 



We may now attempt to correlate the results of Brown and 

 Escombe and of Puriewitsch with those of Blackman on light as a 

 factor in assimilation. It will be recalled that on Blackman's view of 

 limiting factors, if we commence with a very low light intensity, 

 increase in light (radiant energy) will result in a proportionate 

 increase in assimilation until some other factor, such as carbon 

 dioxide supply, is limiting the rate. The curve connecting the light 

 intensity and the rate of assimilation will be of the form already 

 shown in Fig. 5. As regards the proportion of the radiant energy 

 used in assimilation, this should remain constant on Blackman's 

 view so long as light is the limiting factor, for the rate of assimilation, 

 and consequently the energy used for it, is directly proportional to 

 the intensity of the light. But when the light is increased so that 

 some other factor is limiting the rate of assimilation, then if that 

 factor remains constant, increase in light intensity will result in a 

 decrease in the percentage of radiant energy used in assimilation. 



In many of Brown and Escombe's experiments the intensity of 

 illumination is roughly inversely proportional to the proportion of 

 the energy used in assimilation, but this relation is not by any means 



