154 CONCLUDING REMARKS 



assume that they play the part of catalysts, although there is 

 quite a definite suggestion on the part of some workers that the 

 function of boron is of a different kind from those of man- 

 ganese, zinc and copper. 



Two lines of attack on the role of the trace elements which 

 have developed during recent years appear to give promise of 

 fruitful results. The first of these is that displayed in particular 

 by the work of Keilin and Mann in which compounds involving 

 the trace elements in their molecules have actually been isolated 

 and the chemical properties of these compounds determined. It 

 has been mentioned earlier that several copper compounds and 

 one zinc compound have been isolated in this way. They all 

 appear to be protein compounds of the metal concerned, and 

 two of them, the catechol oxidase of plants and carbonic 

 anhydrase of animals, are, indeed, enzymes which had been 

 previously known, and the properties of which were well 

 established. As far as these results go they thus confirm the view 

 of the catalytic nature of the trace elements in organisms, and 

 suggest the possibility that manganese and molybdenum also 

 will turn out to be essential constituents of enzymes. The further 

 view that the trace elements are concerned in vital oxidations 

 and reductions is also supported by our knowledge of the nature 

 of at least two of these compounds, for catechol oxidase is 

 definitely an oxidizing enzyme, while carbonic anhydrase is con- 

 cerned in the release of an end-product of oxidation, carbon 

 dioxide. The condition of boron is more problematic. 



The second promising line of attack on the physiological 

 aspects of trace elements lies in growing plants under experi- 

 mental conditions in which the supply of various mineral 

 nutrients is controlled and determining the resultant effects on 

 growth and, as far as possible, the fate in the plant of the mineral 

 elements concerned . The recent work of Shive and his associates 

 along these lines, which has been dealt with in an earlier chapter 

 of this book, has already produced very promising results with 

 regard to the functions of manganese and boron in plants, and 

 it would seem probable that a development of work along these 

 lines will serve greatly to increase our understanding of the role 

 of the trace elements in plants. 



The realization of the importance of trace elements in plants 



