862 



EXTERNAL AND INTERNAL FACTORS 



CHAP. 26 



The conditions that bring about the three types of curves will be discussed 

 later in this chapter. 



While the limitations of Blackman's law of limiting factors were de- 

 bated, Liebig's notion of the "absolute minimum" of one nutritive factor, 

 from which Blackman's concept was derived, also was found to be too 

 rigid, and attempts were made to change it so as to admit the possibility of 

 simultaneous sensitivity of a crop to several nutrient factors, each of which 

 was said to be in a "relative minimum." New analytical formulations 

 of the minimum law, suggested, e. g., by Mitscherhch (1909, 1916, 1919, 

 1921) and Baule (1918, 1920), led to yield curves that approached the 

 maximum asymptotically, without a sudden break. 



Similar compromise solutions were suggested for photosynthesis. 

 Harder (1921), Lundegardh (1921, 1924) and Singh and Lai (1935) con- 



Fig. 26.3. Kinetic curves of the 

 second type (Bose type). 



Fig. 26.4. Kinetic curves of tlie third t3Tie. 

 Arrow indicates increase of F-2. 



eluded from their measurements that the rate of photosynthesis may de- 

 pend on several factors at the same time; when one "factor" gradually 

 ceases to be "limiting," the influence of another one increases. This con- 

 cept stands midway between the two postulates of Blackman and Bose; 

 attempts have been made to use it for a general formulation of the kinetic 

 relationships in photosynthesis. 



However, because of the diversity of factors and conditions encoun- 

 tered in photosynthesis, it is unlikely that any analytical approximation 

 will prove equally satisfactory for all the cases studied. Liebig's law of 

 the minimum applied to one kind of factor only — nutrient elements; it 

 was not unreasonable to suggest that it represents an equally good first 

 approximation for all such factors, and that an improved mathematical 

 formulation may provide a generally useful second approximation. Such 

 hopes are much less justified in the case of photosynthesis, where Blackman 

 included under the heading of "limiting factors" such heterogeneous mag- 



