On the Function of Chlorophyll. 



15 



failures, although apparently repeated in exactly the same way. Possibly 

 the stage of bleaching may be of importance, but in any case these results 

 cannot be taken as establishing a fact but rather as suggesting a possi- 

 bility, namely, that a reconstruction of chlorophyll may be possible from 

 the products of its oxidation and from xanthophyll. There is no doubt, 

 however, as to xanthophyll being one of the first decomposition products 

 of chlorophyll, especially in the presence of C0 2 and sunlight. Although 

 not claiming any exact quantitative accuracy for the following equations, 

 they may express the probable process of carbon dioxide assimilation in the 

 plant. 



Stage 1. — Carbon dioxide and water combine with the phytyl base of 

 chlorophyll, forming xanthophyll and oxygen. This will take place slowly in 

 darkness but is accelerated hj light. 



4COOC 2 oH 39 * + 76C0 2 + 34H 2 = 4C 40 H 56 O 2 + 93O 2 . 



Stage 2. — A portion of this oxygen is used in the oxidation of the xantho- 

 phyll into phytyl, hexose sugars and formaldehyde, and the remainder is 

 excreted from the chloroplastid. This takes place in light only. 



4C 40 H 56 O 2 + 42H 2 O + l7O 2 = 4COOC 20 H 3 9 + 8CyE 12 O 6 + 28CH 2 O. 



Stage 3. — The phytyl retakes its place in the chlorophyll molecule, the 

 oxygen remaining (760 2 ), which is equal in volume to the carbon dioxide 

 absorbed, is exhaled, and the surplus formaldehyde is polymerised to hexoses, 

 either by contact with the chlorophyll or with dilute alkali in the protoplasm 

 around the chloroplastid. 



The one point which cannot so far be repeated outside the living chloro- 

 plastid is the evolution of oxygen. Possibly with extracted chlorophyll, 

 any oxygen set free in its substance at once combines with it. It is 

 possible that in the living plastid some special separation is effected, or 

 the magnesium of the chlorophyll may reduce some of the xanthophyll to 

 the very readily oxidisable carotin, which might protect the chlorophyll 

 from oxidation. Further, carotin and chlorophyll, being insoluble in water, 

 oxidise much less rapidly when suspended in water than does a solution 

 of xanthophyll. This gives a further possibility for the separation of 

 oxygen at the surface of the chloroplastid. The separation of oxygen can 

 hardly be produced by the action of a reductase reducing xanthophyll to 

 carotin, for this would only give one of oxygen for every 19 of C0 2 , and in 

 addition, when magnesium (or zinc) reduces xanthophyll to carotin, no 

 • oxygen is set free. 



The exact means by which the oxygen formed escapes without wholly 

 * For simplicity the rest of the chlorophyll molecule is omitted from the equation. 



