30 TEXTBOOK OF PLANT PHYSIOLOGY 



ous. Moreover, this theory is confirmed by the fact that the dry 

 matter of the green leaf readily absorbs carbon dioxide and is in 

 this respect not inferior to venous blood or even alkaline solutions. 

 Regarding the decomposition of the oxidized product with the 

 liberation of free oxygen, this is evidently called forth by a special 

 enzyme belonging to the group of catalases — enzymes splitting 

 the peroxide with the formation of free oxygen. 



Thus, the process of photosynthesis shows at least three 

 phases : the union of carbon dioxide with chlorophyll, the internal 

 rearrangement of the atoms in this compound, transforming it 

 into an oxidized form, and the decomposition of the oxide obtained, 

 with the liberation of free oxygen. Of these three phases, only the 

 middle one is a photochemical process, being produced at the 

 expense of the absorbed light energy. The two remaining phases, 

 the first and the last, are purely chemical reactions. This com- 

 plexity of the photosynthetic mechanism explains the well-known 

 fact that its course may not be explained by the usual mechanism 

 of photochemical reactions. This explains likewise the fact 

 already mentioned, that solutions of chlorophyll are not able 

 to decompose carbon dioxide, and that the assimilation process 

 requires the presence of the whole complex: 



chlorophyll + stroma + yellow pigments 



The idea of Willstatter is hypothetical to a considerable de- 

 gree, and is not yet commonly accepted. It must be recognized, 

 however, as a big step forward in the study of the internal 

 chemistry of this most important physiological process in green 

 plants. 



That the participation of chlorophyll in photosynthesis is not 

 only of an optical but also of a chemical character is shown by the 

 ease with which it undergoes decomposition when exposed to light. 

 Alcohol and benzine solutions of chlorophyll, when brought into 

 contact with air, are readily discolored by sunlight, and even by 

 diffused light. This may be due to the acceleration of the oxida- 

 tion processes under the influence of light. Chlorophyll is much 

 more stable in the living plastids of the plant, where the photo- 

 synthetic work is going on under the conditions of strong illumina- 

 tion. The causes of this stability have not been discovered as 

 yet. Some authors are of the opinion that as the chlorophyll 



