40 
Ingvar Jorgensen and Walter Stiles. 
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 
