THE PRINCIPLE OF LIMITING FACTORS 339 



If the light intensity is sufficient to permit the leaf to utilize lO cc. of 

 carbon dioxide per hour, then the rate of photosynthesis will rise with in- 

 crease in the carbon dioxide concentration up to a value about twice as great 

 as that at which the maximum rate of photosynthesis was attained at the 

 lower light intensity. The results under these conditions can be indicated 

 graphically hy A D E (Fig. 83). 



Light and carbon dioxide are not the only factors which can be limiting 

 in the photosynthetic process. Theoretically, as examples given later in the 

 chapter will show, any of the factors which influence this process can, under 

 certain conditions, become limiting. 



Most subsequent workers (Harder, 1 92 1, James, 1928, and others) have 

 been unable to accept the principle of limiting factors in quite the simple 

 form in which it was first proposed by Blackman. Most investigators have 

 found that when the rate of increase of photosynthesis is plotted along the 

 ordinate with the quantitative variations in some one factor as the abscissa, 

 the resulting curve is not found to show an abrupt transition to the horizontal 

 (points B and Dj Fig. 83) as postulated by Blackman's formulation of this 

 principle, but shows instead a gradual transition to a position approximately 

 parallel to the abscissa. The general type of curve found by most investi- 

 gators for this relation is shown in Fig. 85. Within this transition region it 

 is evident that increase in either of the two factors involved will result in an 

 increase in the rate of photosynthesis. 



The explanation for the occurrence of this gradual transition in the direc- 

 tion of the curve, rather than the abrupt change postulated by the original 

 Blackman theory probably rests principally upon two circumstances. In the 

 first place the seat of the photosynthesis is in the chloroplasts of which there 

 are millions in even a small leaf. Patently it is impossible that each and 

 every chloroplast will be subjected to exactly the same conditions at exactly 

 the same time. All of the chloroplasts are not equally exposed to light, 

 neither are all of them equally well supplied with carbon dioxide. As a 

 factor approaches a limiting value it may check the rate of photosynthesis 

 in some chloroplasts sooner than in others. It is quite possible therefore for 

 light to be the limiting factor for some chloroplasts, while carbon dioxide is 

 simultaneously the limiting factor for other chloroplasts. Similar comments 

 apply to most of the other factors influencing photosynthesis. Hence the rate 

 of photosynthesis as measured in terms of entire organs will exhibit only a 

 gradual change when factors affecting the process are modified, and there 

 exist well defined regions in curves such as those shown in Fig. 85, in which 

 two or even more factors may be considered to act simultaneously as limiting 

 factors. 



