CONSTITUTION OF CHLOROPHYLL 231 



not always proportional to the chlorophyll content, which may 

 be explained by assuming that some enzyme takes part in the 

 process. The fact that in leaves rich in chlorophyll increased 

 illumination produces no increased assimilation, whereas a rise 

 in temperature does, is attributed to the accelerating effect of 

 increased temperature upon enzyme action. In the case of 

 leaves deficient in chlorophyll, on the other hand, increase of 

 temperature has but little effect, whereas such leaves are very 

 susceptible to increased illumination. The explanation in 

 this case is that there is more than sufficient enzyme for the 

 chlorophyll, but that the greatest assimilative effect can only 

 be attained when all the chlorophyll is exerting its maximum 

 activity. Attempts to bring about assimilation with chloro- 

 phyll outside the leaf failed, presumably owing to the absence 

 of this enzyme. The removal of epidermis from the under 

 surface of leaves had no deterrent effect on assimilation, but a 

 slight pressure applied to the leaves brought assimilation to a 

 complete standstill. 



THE CONSTITUTION OF CHLOROPHYLL. 



As already stated, chlorophyll was first isolated from its 

 accompanying yellow pigments, the carotinoids, by Willstatter 

 and Hug in 191 1, and in the following year it was shown by 

 Willstatter and Isler that the chlorophyll so obtained was not 

 a single substance, but a mixture of two distinct substances, 

 chlorophyll a and chlorophyll b, in the proportion roughly of 

 three molecules of the former to one of the latter. The con- 

 stitutions provisionally assigned to these two substances are 

 given by the following formula; : — 



/COOCH.. yCOOCHj 



C:„H.«N,Mg f COOC,„H3« C,,H,30,N,Mg/ 



iV 



NH 



Chlorophyll a Chlorophyll b 



from which it may be seen that they are both esters of methyl 

 and phytyl alcohol (CaoHgj.OH), and that the former contains 

 what is known as a lactam grouping. 



