THE NATURE OF PHOTOSYNTHESIS 181 



not affect the mechanism of photochemical reaction, as is indicated by 

 the maintaining of the compensation point in plants treated with HNC, 

 but that this substance does affect the photochemical reactivity of the 

 carbon dioxide which the plant normally takes up from the surrounding 

 medium. 



An example of the effect of low concentration of cyanide on photo- 

 synthesis, as found by Warburg, is given below : 



In these plants the comi>ensation point was about 500 Lux. It appears, 

 therefore, that the light intensity of the compensation point does not 

 determine the point where the inhibiting action of HNC commences; the 

 critical light intensity is higher. In 0.0001 normal solution of HNC, 

 carbon dioxide is absorbed and reduced, but the rate is considerably 

 decreased as compared with plants not treated with cyanide. It seems 

 highly probable that an interpretation of Warburg's results will finally 

 be found in the effect of HNC on the absorptive capacity of the material 

 in the plant which first takes up the carbon dioxide from the surrounding 

 medium. 



