92 BIOLOGICAL CHEMISTRY 



fall on hydrogen on concentration, and possibly in some such 

 way as this it will be possible to discover which is the important 

 factor for photosynthesis. Provisionally we can consider 

 that the photosynthesis depends upon the carbonic acid in 

 solution. 



The rate of reaction can be measured by the rate of dis- 

 appearance of carbon dioxide, or from the rate of appearance 

 of oxygen. In land plants we rely on gas analysis, but in 

 water plants the rate can be measured by the number of 

 bubbles of oxygen escaping from the cut end of the stem. 

 In the latter case great care must be taken to ensure accuracy 

 as the size of the bubbles may vary and the rate of solution 

 of oxygen in the surrounding water must be borne in mind. 



T 



/ 



M- 



r 



A% 3% *% *% 6% 



FIG. 19. Diagram to show effect of limiting factors. 



Carbon assimilation increases from A to B, but with increasing carbon dioxide 

 runs from B toC, showing no further increase in rate of assimilation. If some limiting factor 

 is raised the increase now goes to D and increase of some further limiting factor allows the 

 assimilation to rise to F. 



(From "Annals of Botany," F. F. Blackman.) 



The plant is always decomposing food material, using up 

 oxygen and producing carbon dioxide. This process of 

 respiration is the reverse of photosynthesis, and we really 

 measure the excess of one process over the other. The process 

 of respiration by itself can be measured in the dark and, 

 assuming that the rate of respiration is the same in the light, 

 the figures can be corrected to show the total photosynthesis. 

 Unfortunately this assumption may not be correct. 



At low concentrations of carbon dioxide the photosynthesis 

 is found to be proportional to the tension of carbon dioxide. 

 When the tension of carbon dioxide rises above a certain 

 value there is no further increase in the rate of synthesis. 

 This limit to the rate of synthesis shows what is termed a 



