37 Arnon: Criteria of Essentiality 



deficient cultures was accompanied by the other elements in the 

 B7 and CI 3 solutions. 



To test the results by the last criterion, that of the direct effect 

 of an element on the plant as distinguished from its possible in- 

 fluence on the root environment, molybdenum-deficient plants 

 were sprayed with a dilute solution of molybdic acid (1 p.p.m. 

 Mo) so as to bring about absorption only through the aerial parts 

 of the plant. Recovery and resumption of normal growth with 

 the disappearance of the molybdenum-deficiency symptoms took 

 place. This provided the last link in the chain of evidence for 

 the indispensability of molybdenum for the tomato plant. 

 • The results with tomato were subsequently repeated with 

 mustard and lettuce. The essentiality of molybdenum was soon 

 confirmed by Piper (9) for oats and by Hoagland (19) for My- 

 robalan plum. Molybdenum was recognized as an essential ele- 

 ment and included with the A4 solution which was redesignated 

 A5, whereas the B7 was changed to B6. The addition of A5 to 

 a basic nutrient solution gave for the species tested the same re- 

 sults as a further addition of B6 and C13. 



Does the inclusion of molybdenum among the essential ele- 

 ments complete their list? An unequivocal answer to this ques- 

 tion cannot be given, notwithstanding the fact that a number of 

 different species of plants have been grown successfully in rigidly 

 purified nutrient solutions which supplied only boron, manganese, 

 copper, zinc and molybdenum. It is certain that, despite all cau- 

 tion, minute impurities of other elements persisted in the nutrient 

 medium as well as in the seed. It would seem best to attempt to 

 answer this question in a quantitative rather than a qualitative 

 manner: to determine analytically whenever possible the upper 

 limit of impurity for a given element that may be contained in 

 the nutrient medium and to measure a growth response with and 

 without a further addition of the element in question. This can 

 be illustrated as follows. It was found with the dithizone test 

 that when the combined zinc, copper, lead, cadmium and mer- 

 cury content of a nutrient solution was less than 0.0001 mg per 

 plant, severe deficiency symptoms occurred in the tomato. Re- 

 covery was brought about by adding 0.002 mg Cu and 0.002 mg 

 Zn, but no further improvement was produced by supplying 

 0.0005 mg each of lead, cadmium and mercury. These results, 

 while confirming the indispensability of zinc and copper in 

 amounts greater than those found in the nutrient medium, were 



