Daniel I. Anion 321 



mentioned findings on the role of metals in the nitrogen nutrition of 

 barley (/) suggested this particular grouping. The third supplementary 

 solution, designated C13, supplied thirteen elements: aluminum, ar- 

 senic, cadmium, strontium, mercury, lead, lithium, rubidium, bromine, 

 iodine, fluorine, selenium, and beryllium. Sodium and chlorine, though 

 not singled out, were provided from several sources in these solutions. 

 In experiments with lettuce and asparagus a marked improvement in 

 growth was observed from supplying, in addition to A4, the B7 solu- 

 tion. The further addition of thirteen more elements supplied by the 

 C13 solution produced no measurable effect on either the lettuce or the 

 asparagus plants. The results obtained with lettuce are given in Table I. 



TABLE I 



Effect of Adding Different Groups of Micronutrients on the Growth of Lettuce 



Plants in Culture Solution (from Anion, 2) 



(Average fresh weights in grams) 



Micronutrients Added Shoots Roots 



71.4 14.5 



22.0 



None 



A4 (B. Zn, Cu, Mn) 105.7 



A 4 + B 7 (Mo, Ti, V, Cr, W, Co, Ni) 1068.3 188.6 



A 4 + B7+C1? 



(Al, As, Cd, Sr, Hg, Pb, Li, Rb, Br, I, F, Se, Be) 984.4 196.2 



The results indicated that one or more of the seven elements con- 

 tained in the B7 solution is capable of markedly benefiting plant growth. 

 The question arose what significance is to be attached to an increase in 

 growth in evaluating the essentiality of an element in plant nutrition. 

 Mention was already made of the frequent "beneficial" effects from 

 various soil treatments, and that under special conditions of nitrogen 

 nutrition, molybdenum, chromium, and nickel were associated with 

 favorable responses on the growth of barley. It was undertaken, there- 

 fore, to formulate definite criteria of essentiality by means of which the 

 status of each of the seven elements comprising the B7 group could be 

 tested. 



The following criteria were set up (9) : an element is not considered 

 essential unless (a) a deficiency of it makes it impossible for the plant 



