228 PIGMENTS 



stances it has' been suggested by Willstatter* that since 

 chlorophyll b (C 55 H 7o O 6 N 4 Mg) contains more oxygen than 

 chlorophyll a (C 56 H 72 O 5 N 4 Mg), the former compound is pro- 

 duced ' by the action of chlorophyll a upon carbon dioxide 

 during assimilation, and that chlorophyll b is then reconverted 

 into chlorophyll a with evolution of oxygen. On the other 

 hand, the molecular formulae of carotin (C 40 H 56 ) and xantho- 

 phyll (C 40 H 56 O 2 ) only differ by two atoms of oxygen, and the 

 close association between the carotinoids and chlorophyll may 

 be explained by assuming that the function of carotin is to 

 reduce chlorophyll b to chlorophyll , being itself oxidized to 

 xanthophyll, and that the latter compound is reconverted by 

 some enzyme into carotin with evolution of oxygen. 



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 

 [eaves which was the ratio between the amount of carbon 

 dioxide assimilated per hour and the weight of chlorophyll 

 concerned in the assimilation. Experiments with norm, 

 autumnal, and etiolated leaves showed that the assimilation 

 not always proportional to the chlorophyll content, which ma} 

 be explained by assuming that some enzyme takes part in the 

 process. The fact that in leaves rich in chlorophyll increas< 

 illumination produces no increased assimilation, whereas a ri< 

 in temperature does, is attributed to the accelerating effect 

 increased temperature upon enzyme action. In the case 

 leaves deficient in chlorophyll, on the other hand, increase 

 temperature has but little effect, whereas such leaves are vei 

 susceptible to increased illumination. The explanation 

 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 



* Willstatter : " Untersachungen iiber Chlorophyll," p. 237. 



f Willstatter and Stoli ; " ger, deut, chem. Gesells.," 1915, 48, 1540. 



