340 FACTORS AFFECTING PHOTOSYNTHESIS 



A second possible explanation of the fact that the rate of photosynthesis 

 shows a gradual rather than an abrupt change with modification in the in- 

 tensity of factors influencing the process is based upon more theoretical 

 grounds. Certain external factors may not affect the process directly, but 

 influence instead some internal condition which, in turn, affects photosyn- 

 thesis directly. Within certain ranges of values which show up as "transi- 

 tion zones" on curves plotted as in Fig. 85 increase in either of a pair of 

 external factors may result in an increase in the magnitude of this internal 

 factor, and hence in the rate of photosynthesis. Beyond one limit of the 

 transition range the effect of one of the factors is so predominant that it acts 

 essentially as a limiting factor, while beyond the other limit the other factor 

 acts in a similar manner. Such relationships may actually be more compli- 

 cated than has been indicated, as it is possible that a single internal factor 

 may often be influenced by more than two external factors. 



It should be clearly understood that in speaking of "limiting factors" 

 as applied to physiological processes, that it is not their absolute magnitude 

 which is significant, but their relative magnitude in proportion to the amounts 

 actually required in the process. The quantitatively smallest factor does not 

 necessarily condition the rate of the process, since it may be necessary only 

 in traces, while larger amounts of some other material, or a greater intensity 

 of some other factor may be necessary. To illustrate, suppose that we assume 

 ten units of a, two units of hj and one unit of c are necessary for the forma- 

 tion of one unit of d. If we suppose that only five units of a are available, none 

 of d can be formed regardless of the quantities of b and c available. Al- 

 though c is an absolute minimum, a is in relative minimum, and thus acts as 

 the limiting factor. For this reason many authorities consider it to be more 

 satisfactory to speak of the "relatively limiting factor," "factor in relative 

 minimum," or "most significant factor," rather than of the "limiting factor." 



The modifications which have been imposed upon the original concept 

 of limiting factors do not invalidate this principle as a good approximation 

 to the facts, nor destroy its value as a point of view from which to interpret 

 the influence of various factors upon the rate of photosynthesis. Whether 

 the effect of an external factor is direct or indirect, whether the curves break 

 sharply or show a gradual change of direction, the significant fact is that the 

 rate of the process, except in relatively narrow transition regions, is usually 

 largely determined by the least favorable factor, which may for convenience 

 be spoken of either as the limiting factor, or as the factor in relative minimum. 



The principle of limiting factors is applicable to all physiological processes 

 and will receive further evaluation in relation to growth phenomena in Chap. 

 XXXIII. 



