THE NATURE OF PHOTOSYNTHESIS 95 



2. Factors Which Influence the Rate of Photosynthesis 



a. The Principle of Limiting Factors. 



In endeavoring- to understand the photosynthetic process it is essen- 

 tial to bear in mind that we are dealing with a complex system. It is 

 complex because there are a number of factors involved, each one is 

 necessary for the successful and continuous operation of the process. It 

 is complex because each of these factors influences the process more or 

 less independently. Add to this that we are dealing with a series of 

 chemical reactions of different types including photochemical and catalytic 

 reactions, and it must be evident that to express these relationships in 

 quantitative terms becomes an exceedingly difficult task. And finally the 

 fact must never be lost sight of that photosynthesis is a function of the 

 living plant. Although a great deal of work has been done to determine 

 the influence of the various factors involved in photosynthesis it is evident 

 now that no one of these factors can be studied without at the same time 

 taking into consideration all of the others. 



The factors which are primarily concerned in determining the rate of 

 photosynthesis in any chlorophyllous organ are as follows : 



1 — The partial pressure of carbon dioxide in the air or water sur- 

 rounding the plant. 

 2 — The intensity and frequency of the light used. 

 3 — Temperature, more particularly that of the chloroplasts. 

 A — The amount and composition of the chlorophyll. 

 5 — The amount of water available. 

 6 — Certain internal factors. 



It has been customary for physiologists to endeavor to determine three 

 cardinal points in studying quantitatively the influence of various factors 

 or conditions on any physiological phenomenon. These cardinal points 

 are: the minimum, below which the phenomenon ceases, the optimum, 

 at which the phenomenon takes place to the highest degree, and the maxi- 

 mum, above which it ceases. Many attempts which were made to estab- 

 lish the cardinal points of the various factors operative in photosynthesis 

 led to contradictory results. It was F. F. Blackman ^* who in 1905 called 

 attention to the fact that in a phenomenon such as photosynthesis where 

 there are several factors at work simultaneously, the focusing of attention 

 on a single factor with disregard to the influence of the others leads to 

 erroneous results. A study of the "inter-relation of conditioning factors" 

 led Blackman to formulate his principle of limiting factors which he stated 

 in the following axiom : "When a process is conditioned as to its rapidity 

 by a number of separate factors, the rate of the process is limited by the 

 pace of tbe 'slowest' factor." Any of the factors mentioned above can 



''Blackman, Ann. of Bot., 19. 281 (1905). 



