860 



EXTERNAL AND INTERNAL FA f 'TORS 



CHAr. 20 



of being determined uniquely by the momentary values of the variables 

 Fi, F2, . . ., becomes time-dependent, thus revealing a progressive (and often 

 irreversible) destruction of the biochemical apparatus. 



Compared with the hypothesis of the three cardinal points, Blackman's 

 principle of limiting factors represented substantial progress in the inter- 

 pretation of the kinetics of photosynthesis. Since its first enunciation in 

 1905, it has been accepted and widely used by students of photosynthesis. 

 Unfortunately, Blackman was not satisfied with the improvement of the 

 general qualitative picture: In the belief that the efficiency of biological 

 processes must be subject to simple quantitative ndes, he insisted on treat- 

 ing the concept of "limiting factors" as an exact law of nature. He postu- 

 lated that the fimctious P = /(F,) must have precisely the shape shown in 



Increase 

 of F2 



Fig. 26.2. Kinetic curves of the first type (Jlackman t3T)e). 



figure 26.1, i. e., consist of a linear ascending part, terminated by a sharp 

 break and followed by a horizontal plateau. In order that the same rule 

 l)e true also for the dependence of photosynthesis on all the other factors, 

 F2, Fi, . . . , he had to assume that for different values of a parameter, F2, 

 the yield P as a function of Fi is represented by a set of broken fines — such 

 as ABC, ABDE, ABDFG. . .in figure 26.2— which coincide at the low Fi 

 values (part AJ5 in the figure) and are distinguished only by the position 

 of the break, where the ascending part goes over into the horizontal plateau. 

 This means that the rate of photosynthesis was assumed to be proportional 

 to the one factor that is limiting under the given conditions, and entirely 

 independent of all the other factors. 



Only in the case of temperature as the "limiting factor" did Blackman 

 admit the possibility of a different shape of the ascending part of the curve, 

 an exponential (instead of a linear) rise, in agreement with the general ex- 

 perience in the field of temperature dependence of chemical reactions. 



In the belief that the law of limiting factors must be strictly obeyed, 



