230 PIGMENTS 



during assimilation, and that chlorophyll /; is then reconverted 

 into chlorophyll a with evolution of oxygen. On the other 

 hand, the molecular formulae of carotin {Q^^Yi^^ and xantho- 

 phyll (QoHjgOg) 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 a, being itself oxidized to 

 xanthophyll, and that the latter compound is reconverted by 

 some enzyme into carotin with evolution of oxygen. 



With a view to throwing some light on the mechanism 

 of photosynthesis. Wager * has studied the decomposition of 

 chlorophyll on exposure to oxygen both in the light and in 

 the dark, with the result that he finds that the process is not 

 catalytic. Oxygen is absorbed and aldehydes are formed, and 

 it is suggested that the sugars produced during assimilation 

 are not formed directly from carbon dioxide and water but 

 by the polymerization of aldehydes produced in this way, 

 Warner -j- has also found that formaldehyde is produced when 

 chlorophyll is exposed to sunlight or electric light in air ; 

 since this substance is produced both in the presence and in 

 the absence of carbon dioxide, it would appear that the latter 

 plays no part in the production of formaldehyde by photo- 

 synthesis outside the plant, and that the formaldehyde is in 

 reality an oxidation product of the chlorophyll. 



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. \ 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 



* Wager: " Proc. Roy. Soc," 1914, [B], 87, 386. 



+ Warner: id., 378. 



X W illstatter and Stoll : " Bar. deut. chem. Gesells.," 1915, 48, 15^0. 



