46 THE SYNTHESIS OF CARBOHYDRATES 



beginning of the process since it is not developed so quickly 

 as the chlorophyll, wherefore the rest of the mechanism must 

 await its appearance. 



The assimilation numbers arrived at by Willstatter and 

 Stoll * are inconstant, which is indicative of there being some 

 other operating factor : if they were constants, strong evidence 

 that chlorophyll was the all-important conditioning factor 

 would be provided. According to Willstatter and Stoll f 

 it is an enzyme which thus limits carbon assimilation. They 

 find that in leaves rich in chlorophyll, increased illumination 

 has but little effect upon assimilation nor is it diminished if 

 the illumination is decreased to one quarter. This indicates 

 that the chlorophyll is present in excess compared with the 

 assimilatory enzyme. The increase in carbon assimilation 

 following an increase in temperature they consider to be due 

 to the stimulation of the enzymatic process. 



In leaves containing little chlorophyll and in yellow varieties 

 the conditions are reversed ; the enzyme here being in excess, 

 increased temperature has little effect in stimulating assimi- 

 lation. On the other hand, increased illumination has a very 

 marked effect. 



The remarkable phenomena accompanying autumnal 

 changes in leaves are due to the fact that either the chloro- 

 phyll suffers more than the enzyme, resulting in increase of 

 assimilation number, or conversely the enzyme suffers most, 

 in which case the assimilation number falls. 



The failure to bring about carbon assimilation by means 

 of chlorophyll isolated from the leaf may be attributed, accord- 

 ing to the authors, to the absence of the enzyme. 



This belief in the existence of a controlling enzyme also 

 is shared by Osterhout and Haas $ who experimented with 

 various plants, both fresh water and marine, grown in a culture 

 solution containing gaseous carbon dioxide. The removal of 

 carbon dioxide was followed by the method of determination 

 of the hydrogen ion concentration in the culture medium. It 



* Willstatter and Stoll : " Ber. deut. chem. Gesells.," 1915, 48, 1540. 



t Ibid., p. 1552. 



J Osterhout and A. R. C. Haas : " Proc. Nat. Acad. Sci.," 1918, 4. 8 5- 



