ASSIMILATION OF CARBON BY PLANTS 207 



Thus it is seen that a very low concentration of carbon dioxide 

 limits the influence of gradually increasing light. On the other 

 hand, there are conditions where too feeble light is the factor 

 limiting the influence of the other external conditions. Hence, 

 the law of limiting factors may be formulated as follows: The 

 rate or intensity of any physiological process is determined, in 

 the first place by the magnitude of that factor, of all the external 

 factors, which is at a minimum in proportion to demands. In 

 other words, this factor, being in the minimum, limits the influ- 

 ence of all other external factors. 



The law of limiting factors in its original form is of a pro- 

 nounced mechanistic character, for it assumes that each factor 

 is completely isolated and acts independently of all others. It 

 does not take into consideration the complicated interrelations 

 of all the factors, both with each other and with the organism 

 upon which they act. According to this law, if one of the factors 

 is limiting, an increase of any of the other factors will not be 

 able to intensify a certain physiological process. Later investi- 

 gations in respect to other factors and other physiological proc- 

 esses (Benecke, Lundegardh, and Harder) have likewise 

 established that the limiting action of the separate factors only 

 weakens but does not entirely stop the influence of other factors, 

 the controlling influence being not an absolute but only a relative 

 one. This relationship is represented graphically in Fig. 64, 

 showing the dependence of assimilation on the varying intensities 

 of two factors, light and carbon dioxide content, in the aquatic 

 moss Fontinalis. 



The law of limiting factors clearly shows that the process of 

 photosynthesis should be regarded as a chain of separate proc- 

 esses, each of which is regulated chiefly by some one of the 

 factors of the surrounding medium.^ The quantity of light is 

 reflected in the velocity of the photochemical reaction; tempera- 

 ture acts upon Blackman's reaction; the carbon dioxide content 

 determines first of all the rate of its diffusion into the leaf; etc. 

 In studying the influence of external conditions upon photo- 

 synthesis, there must always be held in view the complexity of 

 the process as well as the possibility of direct interrelations 

 between the separate factors. For instance, the rate of the 

 diffusion of carbon dioxide depends not only on the concentration 

 of CO2, but also on temperature, etc. The water content of the 



