98 



THE FUNCTIONS OF 



the cycle of life'. From this point of view he divided the 

 elements into eight groups as shown in Table VIII. 



Table VIII. Thatcher's classification of elements in plants 



It is not within the scope of the present discussion to con- 

 sider the value of this scheme of classification, but we may note 

 that three of the well-established trace elements, manganese, 

 copper and zinc, are placed, along with iron, itself often classed 

 as a trace element, in one group, that of oxidation-reduction 

 regulators. The fourth, boron, is placed in another group, one 

 with unknown function, that of ampholytes of varying valency. 

 At the time of publication of Thatcher's paper molybdenum had 

 not been recognized as a plant trace element; had it been, it 

 might have been included in the same group as boron. Further, 

 it is not clear why the ampholytes of Thatcher's group VIII. 

 which included gallium, were separated from those of his 

 group V, since in both the functions are listed as unknown. 

 However this may be, Thatcher's classification divides the 

 trace elements into two groups, the oxidation -reduction regu- 

 lators including manganese, copper and zinc, and also iron, the 

 other including boron of unknown function. With regard to the 

 oxidation-reduction regulators Thatcher pointed out that if 

 nickel and cobalt are included the group contains six elements 



