ASSIMILATION OF CARBON BY PLANTS 187 



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

 middle one represents a photochemical process taking place with 

 the absorption of light energy. The other two phases are purely 

 chemical processes. Such a complexity of the photosynthetic 

 process explains the fact long ago established that photosynthesis 

 follows van't Hoff's temperature rule, that with every 10°C. 

 increase in temperature the rate of the reaction doubles. Now 

 a characteristic feature of purely photochemical reactions is 

 their almost entire independence of temperature. The effect 

 of temperature on photosynthesis will be comprehensible if it 

 is assumed that it is built up of three phases, of which two 

 represent ''dark" reactions, i.e., chemical reactions that proceed 

 without the participation of light. 



Warburg's concept of photosynthesis approaches in some 

 degree the scheme of Willstatter. It is based chiefly on his study 

 of photosynthesis of the unicellular alga Chlorella. The sim- 

 plicity of the structure of this alga makes it possible to detect 

 more easily many of the important relations during the process 

 of photosynthesis. 



According to Warburg, photosynthesis should be regarded 

 first of all as a reaction proceeding on the surface of the chloro- 

 plasts. This is indicated by the fact that the process is strongly 

 hindered by the influence of surface-tension reducing agents. 

 According to this view, chlorophyll is distributed on the surface 

 of the chloroplasts in a very thin layer 1 molecule in thickness, 

 a monomolecular film, and it is in this layer that the carbon 

 dioxide is decomposed. The decomposition proceeds in two 

 phases, of which the first represents a photochemical reaction. 

 It is independent of temperature and proceeds with extreme 

 rapidity, according to Emerson's measurement in 0.00001 sec. 

 The second phase is designated by Warburg ''Blackman's reac- 

 tion," (named in honor of the English plant physiologist F. F. 

 Blackman, who has contributed much to the understanding of 

 photosynthesis). This is of a purely chemical nature and pro- 

 ceeds without the participation of light. It requires at 25°C. 

 about 0.04 sec; this chemical reaction is considerably slower 

 than the physical reaction of the first phase. 



Blackman's reaction is impeded by hydrocyanic acid and by 

 hydrogen sulphide. According to Warburg, this indicates that 

 here is involved the catalytic influence of iron ions, which 



