188 PLANT PHYSIOLOGY 



display high sensitivity to the reagents mentioned. In 

 its character, Blackman's reaction approaches the catalytic 

 decomposition of peroxides, especially of hydrogen peroxide, 

 occurring under the influence of catalase and in which likewise 

 ions of iron play a role. 



Warburg represents his theory by the following equations: 



H2C03->CH20< I 



(photochemical reaction) 



CH20< I -> CH2O + O2 



(Blackman's reaction) 



As according to Willstatter's theory, the last phase of photo- 

 synthesis also consists in the splitting off of oxygen from the 

 peroxide compound, both Willstatter's and Warburg's theories 

 may be united into one general scheme, the last phase of 

 Willstatter's scheme representing Blackman's reaction. 



Both theories, Willstatter's as well as Warburg's, are to a 

 considerable degree hypothetical and as yet are not quite com- 

 plete. Their advantage when compared with Baeyer's and 

 other purely chemical schemes lies in the fact that they explain 

 the necessity of the whole complex for photosynthesis: chloro- 

 phyll + living stroma + protoplasm with all its enzymes and 

 specific structure, and the impossibility of reproducing photo- 

 synthesis in solutions of chlorophyll. 



That is the reason why repeated attempts to explain the 

 course of photosynthesis by means of a study of chemical reac- 

 tions occurring in water solutions of carbon dioxide under the 

 influence of ultraviolet rays, electric discharges, and the like, 

 must be recognized as grossly mechanistic and having no direct 

 relation to what proceeds during photosynthesis in the green 

 plastids of the plant. 



44. Structure of the Leaf as an Organ of Photosynthesis. 

 Penetration of Carbon Dioxide into the Leaf. — The process of 

 photosynthesis takes place in the chloroplasts. Hence, in order 

 that the carbon dioxide may serve as material for the synthesis 

 of carbohydrates, it must be absorbed by the chlorophyll-bearing 

 cells. These cells, forming the mesophyll of the leaf, are not in 

 direct contact with the outer atmosphere, as, on either side, the 



