PRESIDENTIAL ADDRESS SECTION C. 55 



proportional to the product of their concentrations. Increasing 

 the concentration of either substance increases this rate. It can 

 also be increased further by raising the temperature. Clearly the 

 principle of limiting factors is not applicable to simple direct 

 reactions of this type, between substances in solution. That it 

 does hold for assimilation is in itself a demonstration of the com- 

 plexity of the process. It is at least probable that to each of 

 the external limiting factors coiTcsponds a distinct stage of the 

 process which is dependent upon it. The rate of the process as 

 a whole is in reality determined by the rate of the slowest 

 contributory stage, as has been pointed out by Briggs.* 



If the process is complex, it is to be expected that the in- 

 ternal mechanism which carries out the process is also complex. 

 One factor in this mechanism is obviously the green pigment 

 ■chlorophyll, in the absence of which there is no assimilation of 

 CO2. It has long ago been inferred, from the sensitiveness of 

 the process to poisons and other influences that lower the tone of 

 the organism, that the protoplasmic stroma of the chloroplast also 

 plays a part. 



The first direct evidence that chlorophyll is not the sole agent 

 was furnished by Miss Irving 's experiments, carried out in 

 Blackman's laboratory, with etiolated leaves! . The leaves used 

 in her investigation turned green many hours before they began 

 to assimilate. Some other essential part of the assimilation 

 mechanism must therefore have lagged behind the chlorophyll 

 in its development. 



Willstatter and Stoll| failed to obtain the same results when 

 they attempted to repeat Miss Irving's experiments, but they 

 liave recently been confirmed and extended by Briggs, again in 

 Blackman's laboratory. Briggs has shown that the second factor 

 develops gradually in the dark so that whereas a leaf a few days 

 old will turn green without at once being able to assimilate, an 

 older one will begin to assimilate as soon as chlorophyll appears. 

 It is a puzzhng fact that, while the leaf is deficient in this factor, 

 the rate of assimilation is limited by the intensity of hght. The 

 activity of this factor is therefore dependent upon light and ife 

 must belong to the photochemical part of the mechanism. § 



We must now turn to Willstatter's contributions to our 

 knowledge. His experimental investigations on the assimilation 

 of carbon dioxide in collaboration with Stoll, which were pub- 

 lished in book form in 1918, || are a sequel to his fundamental 



* Eoy. Soc. Proc, B. 91, 1922. p. 249. 



t Annals of Botany, 24, 1910, p. 805. 



J Loc. cit. p. 127 et seq. 



§ The analogy of a photographic plate i.s usehil in this connection. Light 

 brings about a photochemical reaction the rate of which depends upon 

 the intensity of the light but is little influenced by temperature. 

 Subsequent development on the other hand takes place in the dark^ — • 

 i.e., it is not a photochemical process — and its rate is greatly influenced 

 by changes of temperature. 



II Loc. cit. 



