Assimilation in Relation to Radiant Energy. 147 



exact or even approximate. It suggests, however, that in some of 

 Brown and Escombe's experiments at any rate, radiant energy was 

 in excess, and some other factor was limiting the rate of assimilation. 

 In a set of experiments they performed in which the proportion of 

 the full radiant energy of the sun utilised in assimilation was 

 compared with that proportion of it so utilised when it was reduced 

 to a fraction of the full energy by means of rotating sectors placed 

 above the leaf, it was always found that reducing the intensity of 

 illumination increased the proportion of energy used in assimilation. 

 This is exactly what one would expect, as in the experimental 

 arrangement of Brown and Escombe, the full intensity of radiant 

 energy falling on the leaf would be likely to be in excess of that 

 required for the carbon dioxide supply. 



Similarly, in Puriewitsch's experiments, although unfortunately 

 no data whatever are given in regard to temperature, it seems likely 

 that the radiant energy was not limiting the rate of assimilation. 

 The carbon dioxide supply was low, namely that of the atmosphere. 

 The intensity of radiation was, on the other hand, in all cases 

 moderately high. Under these conditions we should expect that 

 carbon dioxide supply would be the limiting factor and that the 

 variations in the total radiant energy in the different experiments 

 would be without influence on the rate of assimilation. Consequently 

 we should expect that the proportion of the sun energy used in 

 assimilation would vary inversely with the intensity of illumination. 

 Such, however, is not by any means the case, and it is clear we 

 must look for other factors of which no data are given, to explain 

 the results obtained. Puriewitsch does indeed point out that his 

 numbers show that the rate of assimilation falls off with time (cf. 

 Blackman's time factor) but this will not explain his results com- 

 pletely. We have here a particular instance of that lack of correl- 

 ation of effort to which we have referred in the first section of this 

 chapter, for if Puriewitsch had taken cognizance of Blackman's 

 researches, his experiments might have yielded results of much 

 greater significance. It is only fair to Puriewitsch to point out that 

 he regards his experiments as preliminary. 



E. Assimilation in Relation to Radiant Energy of Different 



Wave-Lengths. 



On this subject no satisfactory work has so far been performed, 

 although it has been a favourite subject for investigation for more than 

 a century. On the one hand in no case is the method employed for the 



