CHLOROPHYLL 315 



phyll, and that the latter compound is reconverted by some 

 enzyme into carotin with evolution of oxygen. 



In this connection Baly and Davies * suggest that in view 

 of the fact that the ratio of xanthophyll to carotin tends to 

 increase during photosynthesis, the slow recovery of the photo- 

 synthetic ability of the leaf after intense illumination, for 

 example, is due to the slow reduction of xanthophyll to 

 carotin. 



Quantitative measurements of the relation between the 

 amount of carbon dioxide assimilated and the weight of chloro- 

 phyll concerned have been made by Willstatter and Stoll.f A 

 regular stream of air containing a known amount of carbon di- 

 oxide was passed over from 5 to 20 grams of leaves contained in 

 a small illuminated glass vessel immersed in a constant tempera- 

 ture water bath. By estimating the amount of carbon dioxide 

 in the issuing gas and the amount of chlorophyll in the leaves, 

 they determined the so-called assimilation number for different 

 leaves which was the ratio between the amount of carbon 

 dioxide assimilated per hour and the weight of chlorophyll 

 concerned in the assimilation. Experiments with normal, 

 autumnal, and etiolated leaves showed that the assimilation is 

 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 



* Baly and Davies : " Proc. Roy. Soc," A., 1927, 116, 219. 



I Willstatter and StoU : " Ber. deut. chem. Gesells.," 1915, 48, 1540. 



