Daniel I. Anion 325 



improvement was produced by supplying 0.0005 m S- eacn °f l ea d> 

 cadmium, and mercury. These results, while confirming the indispen- 

 sability of zinc and copper in amounts greater than those found in the 

 nutrient medium, were interpreted as permitting no final conclusion 

 as to the role of cadmium, lead, and mercury. The possibility that these 

 elements, or others studied by a similar technique, may be required in 

 amounts smaller than the incidental impurities which could not be re- 

 moved from culture solution by the present technique cannot be a 

 priori excluded. 



If these views are accepted there can be no objection to regarding 

 almost every element in the periodic table, and particularly those most 

 frequently encountered in plant tissues, as susceptible of being shown 

 at some time to be essential for plants. What can be asserted definitely 

 is that, if an element now regarded as dispensable for a given plant 

 should at some future time be found essential, it will be shown to be 

 required in exceedingly small amounts — within the limits of con- 

 tamination still encompassed by the refined methods now used for 

 purifying the nutrient medium. This quantitative approach to the 

 problem of essentiality of micronutrients is regarded not as a mere 

 theoretical generalization but as a point of view conducive to a search 

 for more refined analytical methods and procedures for growing plants 

 which would make it possible to investigate the status of a number of 

 new elements in plant nutrition. 



The discussion thus far has dealt with those advances in the explora- 

 tion of essentiality which were made through experimental modification 

 of the external medium. Failure of the plant to grow was taken as the 

 physiological yardstick by which nutrient requirements were measured. 

 Except as a general surmise, experiments of this kind do not help to 

 determine the function which the essential element plays within the 

 plant. Regardless of how many different functions an element may 

 perform within the plant, it is obvious that if the insufficiency of a 

 nutrient resulted in blocking only one crucial reaction, growth would 

 be arrested. 



These considerations suggest an alternative approach to the problem 

 of essentiality of inorganic nutrients. Rather than measuring the effect 

 of the removal of an element from the external nutrient medium on 



