8 BURTON EDWARD LIVINGSTON 



From this conception of multi-conditional control has developed 

 the well-known law of the minimum and of Hmiting conditions, 

 which seems to have been useful in the interpretation of many 

 physiological relations, but this principle is still quite incomplete 

 logically, and its statement will assuredly become much more 

 complex as our science advances. With a score or more of 

 different kinds of conditions acting together to control the 

 processes of our plants, and with an infinite number of possible 

 combinations of the intensities of these various conditions, the 

 prol51em of physiological control assumes almost a hopeless com- 

 plexity. To illustrate, for a given plant, for a given total con- 

 centration of the series of Shive's- three-salt nutrient solutions 

 and for a given set of climatic conditions, there is a certain set 

 of salt proportions that gives the best growth. For another 

 total concentration, however, all other conditions remaining as 

 before, it is quite another set of salt proportions that are most 

 favorable to growth. Adequately to work out the apparently 

 simple problem thus suggested requires literally thousands of 

 cultures. 



I shall revert to this matter a little later, but I wish now 

 simply to emphasize the point that we can no longer speak of a 

 single condition as being the cause of an observed effect. The 

 next generation of physiologists will have to learn to handle 

 mo]"e than a single variable and to deal with complexes of con- 

 ditions. They will not consider it a scientific statement to say 

 that a certain concentration of poison, in the solution about the 

 roots or in the air about the leaves, results in death, for they 

 will realize that, with different nutrient solutions, with differ- 

 ent rates of transpiration, and so forth, this concentration 

 limit may have any magnitude within a wide range. 



External and internal conditions. The idea of the conditional 

 control of plant processes has resulted in the rather arbitrary, 

 but very convenient, classification of the effective conditions 

 in question, into two groups, external and internal conditions, 



^Shive, J. W., A three-salt nutrient solution for plants. Amer. Jour. Bot. 

 2: 157-160. 1915. Idem, A study of physiological balance in nutrient media. 

 Physiol. Res. 1: 327-397. 1915. 



