Daniel I. Anion 319 



now known as micronutrients. Despite these obstacles the concept of a 

 micronutrient, an element which though used by the organism in 

 minute amounts is nevertheless essential for life, was formulated with 

 remarkable insight as early as 1869 by Raulin (40), who discovered 

 the indispensability of zinc for Aspergillus niger, and considered this 

 element essential for higher plants as well. He explained growth with- 

 out the addition of zinc as being due to the presence of zinc as an 

 impurity in the medium. This point of view was further elaborated 

 at the turn of the present century by Javillier (26), who found that zinc 

 was present as a contaminant not only in the purest chemicals then 

 available on the market but was also derived from the glass used for 

 containers. Strong support for the essential nature of nutrients required 

 in minute quantity by higher plants was also provided during this early 

 period by the work of Maze (^7), who used corn as the test plant. 



Important and fundamental as these concepts of the French workers 

 proved to be, they were not generally accepted even in the country of 

 their origin, where other investigators claimed that the effects were 

 not specific (34). Only with the evolution of careful procedures for the 

 removal of accidental impurities was decisive evidence obtained for the 

 essentiality of micronutrients for plant growth. The careful work of 

 Steinberg (47) provided confirmation for the early thesis of Raulin 

 that zinc is essential for Aspergillus niger. An extension of the purifi- 

 cation techniques to other elements combined in some cases with a 

 felicitous choice of test plants resulted in the demonstration that zinc 

 and three other elements — boron, copper, manganese — are essential for 

 higher plants as well (^5, 53, 45, 44). The biological significance of these 

 findings was reflected in the fact that species differing so widely 

 taxonomically and physiologically as Aspergillus niger and the tomato 

 were found to share the requirements for zinc, manganese, and copper. 

 The practical significance of these discoveries was soon attested by a 

 vast and ever-growing list of hitherto obscure diseases of crops grown 

 in the field which were identified as micronutrient deficiencies (48). The 

 latter development has incidentally provided once more a demonstra- 

 tion of the fitness of the nutrient solution technique in elucidating prob- 

 lems of plant nutrition in soils. 



In the early part of the thirties of the present century, the evidence 



