588 FACTORS AFFECTING GROWTH 



effects upon internal physiological conditions which are also conditioned, as 

 we have already seen, by the genetic makeup of the organism. Different 

 combinations of factors often have a similar effect upon the internal physio- 

 logical conditions in a plant. It is therefore entirely possible for approximately 

 the same end result in terms of plant development to be induced by dissimilar 

 combinations of environmental factors. The precept that the same internal 

 physiological conditions, and the same developmental reactions can be brought 

 about by different combinations of environmental factors is sometimes called 

 the principle of multiple causation. This principle is well illustrated by the 

 effects of various factors on the shoot-root ratio of plants (Chap. XXXIV). 

 In addition to the physical factors discussed above, plants are subject 

 to the influence of another entirely different group of factors — the other 

 living organisms in their environment. Among these are micro-organisms, 

 animals, and other plants. Man himself, from the standpoint of a plant, is 

 merely one of the factors in its environment. The influence of such biotic 

 factors is not generally considered to come within the scope of a discussion of 

 plant physiology, but their effects upon growth and development of plants is 

 often as pronounced as the effects of physical factors. Biotic factors often 

 operate as limiting factors in the survival or distribution of plants. The elimi- 

 nation of that once prominent tree species — the chestnut — from eastern North 

 America by the chestnut blight disease is an example of the profound effects 

 sometimes wrought by biotic factors. 



In order to interpret the effect of changes in the magnitude of any one of 

 the various factors influencing a process such as growth it is necessary to 

 formulate certain guiding principles. In 1843 Liebig proposed his well known 

 "law of the minimum," which was the first attempt at such a formulation. 

 Liebig was thinking primarily of the effect of fertilizers upon the yield of crop 

 plants when he suggested this "law," which states in essence that the yield is 

 limited by the factor which is present in relative minimum. Blackman's "prin- 

 ciple of limiting factors" as applied to photosynthesis (Chap. XXI) is essen- 

 tially an elaboration of Liebig's principle. 



Mitscherlich (1909) has proposed a somewhat different concept of the law 

 of the minimum. He also was thinking principally of factors which influence 

 the yield of crops. Like Liebig's principle, his statement affirms that factors 

 present in minimal amounts are the most important in governing growth. His 

 conception of the operation of the "limiting factor" may be stated as follows: 

 "the increase in any crop produced by a unit increment of a deficient factor 

 is proportional to the decrement of that factor from the maximum." 



Both of these interpretations of the effect of minimal factors can be 

 illustrated by a diagram (Fig. 131) in which it is supposed that five factors 



