34 THE SYNTHESIS OF CARBOHYDRATES 



assimilation. Irving concludes that this factor controls the 

 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 (cf. p. 48). 



In leaves containing little chlorophyll and in yellow varie- 

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

 excess, increased temperature has little effect in stimulating 

 assimilation. 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. Also they ob- 

 served that carbon assimilation continued in leaves from which 

 the lower epidermis had been removed whilst a slight pres- 

 sure applied to the leaves brought the process to a standstill. 

 This is explained on the assumption that the enzyme is situated 

 at the surface of contact between the chloroplast and the plasma. 



This belief in the existence of a controlling enzyme also is 

 shared by Osterhout and Haasj who experimented with 



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



t/rf., p. 1552. 



J Osterhout and Haas : "Proc. Nat. Acad. Sci.," 1918,4, 85. 



