THE "cardinal POINTS" AND THE "LIMITING FACTORS" 



859 



yields are determined by the quantity of the one nutrient element (such as 

 potassium, phosphorus or nitrogen) present in the lowest concentration 

 (relative to its optimum quantity). This postulate became known as the 

 "law of the minimum." 



Sixty years later, Blackman (1905) suggested that a generalized form 

 of this law can be applied to photos\Tithesis, and can explain the obsen-a- 

 tions in this field better than the concept of the three cardinal points. He 

 took from Liebig the idea that the rate of a biological process (in this case, 

 photosynthesis) is determined, under given conditions, by a single "limit- 

 ing" factor; but, in addition to the supply of material ingredients (the 

 only kind of factors with which Liebig was concerned), Blackman con- 

 sidered also tem-pcraUtre and light intensity as potential "limiting factors." 



Fig. 26.1. Photosynthesis according to the concepts of "cardinal points' 

 {MOM') and "limiting factors" {ABCD). 



He suggested that the rate of photosynthesis increases with the increase in 

 value of any one of these factors {Fi) as long as this factor is "slowest" 

 and ceases to be dependent on Fi when one of the other factors (F2, F3, . . . ) 

 becomes limiting. In other words, the plot of the yield of photosynthesis, 

 P, versus a variable Fi (at constant values of all the other kinetic factors) 

 was postulated by Blackman to have the shape ABC (fig. 26.1), instead of 

 the shape MOM' required by the optimum theory. Blackman could not 

 deny that extreme conditions generally inhibit biological processes — in 

 other words, that the horizontal plateau BC in figin-e 26.1 cannot extend 

 indefinitely, but that sooner or later, P must begin to decline (as indicated 

 by the broken line in the figure). However, he attributed this decline to 

 destnictive phenomena {e. g., freezing of cell water, or denaturation of pro- 

 teins) not inherent in the kinetic mechanism of photosynthesis itself. Ac- 

 cording to Blackman, these inhibitions merely limit the range in which the 

 "law of limiting factors" can be verified; their superimposed character is 

 indicated by the fact that, whenever they come into play, the rate, instead 



