206 PLANT PHYSIOLOGY 



The fact established by Blackman and his coworkers that 

 photosynthesis follows van't Hoff's rule, though only within 

 comparatively narrow limits, contradicted the view prevailing 

 at that time that photosynthesis represents a purely photo- 

 chemical reaction, for as such it is almost independent of tem- 

 perature. As has been seen, this contradiction led Warburg 

 and other investigators to the analysis of the process of photo- 

 synthesis into at least two phases, one of which is photochemical 

 and does not depend upon temperature and the other purely chemi- 

 cal (Blackman's reaction) and thus subject to van't Hoff's rule. 



The influence of temperature would show itself most clearly 

 only when the experiments were conducted with sufficiently 

 intense light and with sufficiently high concentrations of carbon 

 dioxide. Under natural conditions, these two factors, especially 

 a sufficient amount of carbon dioxide, are seldom secured. This 

 circumstance complicates the experiment, and the primary 

 influence of temperature on photosynthesis may be entirely 

 disguised. In studying the simultaneous effect of several 

 environmental factors on photosynthesis, Blackman found that 

 some of them, being in the minimum, limit the influence of the 

 other factors. This dependence of one factor on another is 

 called ''the law of limiting factors." 



This law operates distinctly when at least two external factors 

 are acting simultaneously, e.g., light and the carbon dioxide 

 content of the atmosphere. It has been pointed out that the 

 dependence of photosynthesis on each of these two factors 

 separately may be expressed by a logarithmic curve. Suppose, 

 now, that the intensity of the light increases, while the content 

 of carbon dioxide remains the same, a condition that may be 

 observed, for instance, during the day in a greenhouse. From 

 the moment when the intensity of light attains a level at which 

 the carbon dioxide absorbed by the chloroplasts during a certain 

 period is completely used up, further increase of assimilation 

 becomes impossible, and the curve, showing the dependence of 

 photosynthesis on the intensity of light, proceeds in a horizontal 

 direction. If the CO2 content of the atmosphere is increased, 

 then the bending of the curve takes place at a higher light 

 intensity than before, and only with a certain excess of carbon 

 dioxide in the air may the actual dependence of photosynthesis 

 on the intensity of light energy be clearly ascertained. 



