Evolution of Oxygen. 199 



during its photo-oxidation does not appear to be directly derived 

 from carbon dioxide. 



Kohl states that etiolated plants exposed to light in the absence 

 of oxygen, but in the presence of a little carbon dioxide, slowly 

 turn green, the oxygen set free by the agency of the carotin being 

 used in the formation of chlorophyll. It is doubtful, however, 

 whether a complete absence of oxygen was assured at the outset, 

 for Correns''^ has shown that the production of chlorophyll is closely 

 dependent upon the presence of oxygen, and that a partial pressure 

 of oxygen, at which growth and lieliotropic cuivatuie aie still 

 possible, does not suffice for the formation chlorophyll. 



The evolution of oxygen from green plants in light. 



It is a diflficult problem to determine how the oxygen produced 

 in close contact with pigments capable of rapid photo-oxidation is. 

 able to escape from the cell. No production of oxygen could be 

 obtained from extracted chlorophyll, carotin or xanthophyll in the 

 presence of oxidase or reductase enzymes in light or darkness and 

 in the presence or absence of carbon dioxide. It seems legitimate, 

 however, to assume that the large chlorophyll molecules may have a 

 definite physical arrangement in the protoplasm of the chloroplas- 

 tids. If their phytyl radicles were all turned outwards towards the 

 entering carbon dioxide, and carotin or xanthophyll were pro- 

 duced by additive combination with the latter, the surplus oxygen 

 could either escape or disintegrate the carotin or xanthophyll, back 

 to phytyl. For the latter a portion only of the oxygen is required. 

 The oxidation films formed from carotin, chlorophyll and xantho- 

 phyll are very impermeable to oxygen, so that the excess would 

 diffuse outwards. So long as the supply of carbon dioxide was 

 sufficient to rei^lace the xanthophyll or carotin film, as it was 

 oxidized, and as the phytyl returned to the chlorophyll molecule, 

 the chlorophyll would remain unoxidized, but if the intensity of 

 illumination increased greatly, oxygen would slowly penetrate the 

 chloroplastid, bleaching it. 



Phytol itself forms a series of compounds with varying propor- 

 tions of oxygen (CooHshOo.C.joHs^O,,), etc.,- and it also foinis an ozoitide 

 CaoHjoOj, which spontaneously separates into C.20H4O.O.. and free oxygen.* 



The possibility of a reductase enzyme, converting zanthophyll 

 into carotin, with a liberation of free oxygen also needs considera- 



1 Correns, Flora, 189:^, p. U. 



•2 Willstatter and llocheiler, Aimaleii Htr Cheinie, 1907, 353, p. 205. 



3 Willstatter, iUyer luul llnni, Amialcn, 1^)10, p. 73 



