PRESIDENTIAL ADDRESS SECTION C. 57 



the existence of any direct relation between chlorophyll content 

 and rate of assimilation while temperature is the external 

 limiting factor. 



Comparison with yellow varieties of leaves rendered the 

 absence of such a relation strikingly evident. These leaves 

 contain a low proportion of chlorophyll. Nevertheless their rate 

 of assimilation did not fall far short of that of normal green 

 leaves of the same species, and their assimilation numbers w^ere 

 therefore high, ranging in extreme cases up to 140. This means 

 that in hght of the intensity of direct sunlight the small amount 

 of chlorophyll present was sufficient for the assimilation of nearly 

 as much COo as was assimilated by the much more abundant 

 chlorophyll of the green leaves. Willstatter and StoU infer that 

 in the latter there is far more chlorophyll than is needed, even 

 for the enhanced rate of assimilation which obtained in their 

 experiments wdth a practically unlimited supply of COo. It 

 follows that some other slower agent is also concerned, and, 

 since temperature was the external limiting factor, that the rate 

 of action of this agent is dependent upon temperature. As most 

 ordinary chemical reactions, including those brought about by 

 enzymes, show a similar dependence upon temperature, 

 Willstatter and Stoll call this part of the assimilation mechanism 

 the " enzymic factor." The stage of the process for which it is 

 responsible is not photochemical, and it must therefore not be 

 confused with the factor which developed later than chlorophyll 

 in Miss Irving 's experiments. 



In the case of the yellow leaves light proved to be the ex- 

 ternal limiting factor even when its intensity was raised above 

 that of sunlight. The enzymic factor is therefore not w^orking up 

 to its full capacity. As the light diminishes in intensity the rate 

 of assimilation diminishes. There are also indications in some 

 of the experiments of a rough proportionality between rate of 

 assimilation and chlorophyll content; but in view of Brigg's 

 ■demonstration of a second photochemical agent we are left in 

 doubt as to whether in any particular case it is this or chloro- 

 phyll which is determining the rate. Nevertheless it is on the 

 whole clear that the higher intensity of light required by yellow 

 leaves to enable them to assimilate as rapidly as green leaves is 

 correlated with their lower chlorophyll content; and we may 

 suppose that under certain conditions the photochemical stage is 

 detemiined jointly by light and chlorophyll. 



The demonstration of the plurality of internal agents raises 

 the question of their respective roles. Here, however, we are 

 still very much in the dark. Willstatter and Stoll have not yet 

 taken Brigg's second photochemical factor into consideration, but 

 they have made some interesting contributions towards a solution 

 of the general problem and the theory they put fonvard is of 

 sufficient interest to warrant a brief statement of it. As it is 

 based upon the chemical composition and properties of chlorophyll 

 an outline of their main conclusions in this direction* may first 

 be given. 



* Untersuchungen iiber Chlorophyll. Berlin, 191.3. 



