Daniel I. Anion 327 



the nutrient medium or the seed. On the analytical side, the element 

 may enter into the composition of some organic intermediate of such 

 great structural instability or low concentration in the tissue, or both, 

 as to escape detection. 



It is interesting to note that an approach to essentiality through the 

 study of function was initiated over a half a century ago, in a manner 

 which proved of historic importance to general biochemistry. In 1897 

 Bertrand (75) in France, reported that manganese was consistently 

 associated with the activity of an oxidizing enzyme in plants, laccase. 

 He came to regard manganese as an essential constituent of the oxidase 

 system and hence essential to plant life. This announcement linked for 

 the first time a metal with an oxidizing enzyme in living cells. It is 

 true that recent work has shown that the effective metal in laccase is 

 copper rather than manganese (31,51), but the principle first proposed 

 by Bertrand has retained its force. 



The most fruitful development of the functional approach to essenti- 

 ality of micronutrient had to await the evolution of modern biochemi- 

 cal techniques. The small amounts in which these nutrients were re- 

 quired by plants pointed very early to their probable catalytic function 

 and, as first suggested by Bertrand (75), their association with enzymes. 

 But the experimental proof for this hypothesis came much later. For 

 boron, manganese, and molybdenum, we lack to this day precise formu- 

 lation of their biochemical function in the plant. Their indispensa- 

 bility was established by growth experiments. Even for the others to 

 which recent research has assigned some biochemical function, their 

 essential status was already known from growth experiments. Thus 

 the history of the micronutrients reveals that important physiological 

 advances and the agricultural application of our knowledge of the 

 indispensability of iron, boron, manganese, copper, zinc, and molybde- 

 num occurred either in advance, or in the absence, of any knowledge 

 of their functions within the plant. 



The biochemical approach, however, has led to important advances 

 in recent years. The study of respiration led Keilin (28) and Keilin 

 and Hartree (29) to the identification of four iron porphyrin com- 

 pounds which constitute the cytochrome system and are essential in 

 the respiration of aerobic organisms. Here then was biochemical evi- 



