A QUARTER-CENTURY OF PLANT PHYSIOLOGY 7 



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centration of solutions, and so forth) might be established by 

 maintaining all other conditions constant and causing the 

 intensity of the condition under investigation to vary from 

 experiment to experiment. No attention was apparently to 

 be given to the intensities of the other conditions; they were 

 merely to be constant, or were to vary in the same manner for 

 all comparable experiments. With more knowledge and with 

 deeper thinking we have become aware that the constellation 

 or complex of other conditions frequently exerts a pronounced 

 influence upon the effect produced in the organism by a given 

 difference in the intensity of the one condition studied. For 

 example, if you wished to determine how concentrated must be 

 a solution of copper sulphate in order to kill a wheat seedling, 

 with its roots in solution and its leaves in air, you would naturallj^ 

 make the nutrient solution precisely the same in all of your 

 cultures and you would add different amounts of the poison salt 

 to the respective jars. Also, you would see to it that all jars 

 were alike and that all cultures were subjected to the same tem- 

 perature and light conditions, whether these were to be main- 

 tained constant or allowed to vary with time. From such a 

 series of tests you would arrive at a fairly definite lethal con- 

 centration of the copper salt, and, until recently, you might 

 have published this result with the idea that a generalization, 

 at least for wheat seedhngs of a given age, had been attained. 

 But if you were to take another nutrient solution as foundation 

 for all the cultures, if you were to shade them all or subject them 

 all to another temperature or to a different series of temperature 

 changes, thus repeating the series of experiments, otherwise as 

 before, you would in all probability then estabhsh quite another 

 critical concentration of the poison salt. In short, how much 

 of the poison is required to produce death depends quite clearly 

 upon the other environmental conditions. Failure to realize 

 this, failure to measure and describe all the other effective con- 

 ditions — which were not thought of as being elements of the 

 problem in hand — has frequently given rise to more or less 

 serious polemics between different experimenters, and so to waste 

 of time, energy and accumulated wealth. 



