TRACE ELEMENTS IN PLANTS 99 



with consecutive atomic numbers, namely, 25-30. It is inter- 

 esting to note that gallium, only recently shown to be essential 

 for any plant, is the next element in the series with an atomic 

 number of 31. This means that each successive member of the 

 series differs as regards atomic structure from the element before 

 it only by the addition of one electron to the proton nucleus. 

 Whether this close connexion between these elements really has 

 any significance in respect of their physiological function it 

 would be premature to say, but it is a fact that several of them 

 have rather similar chemical properties. Thatcher expressed the 

 opinion that there was sufficient evidence to justify the opinion 

 that manganese and iron on the one hand, and copper and zinc 

 on the other, were pairs of ' mutually co-ordinating catalysts for 

 oxidation-reduction reactions', the former pair for reactions in 

 which the addition or removal of oxygen is involved, the latter 

 for reactions which concern the transference of hydrogen. As we 

 shall see in the sequel there is a certain amount of experimental 

 evidence in favour of at least one of these hypotheses. 



Frey-Wyssling (1935) has also attempted to find a relation 

 between essentiality and position of elements in the periodic 

 table. He uses a table in which group O appears both on the 

 extreme left and the extreme right, but with each element of the 

 group one period higher in the right than in the left column. If 

 a line is then drawn through the table from argon on the left 

 to carbon and then on to argon on the right, this line passes 

 through or near the positions in the table of all the essential 

 elements with the exception of hydrogen. This line Frey- 

 Wyssling calls the nutrient -line. This relationship expresses with 

 greater precision the fact pointed out by Thatcher, that all the 

 essential elements occur in the first four periods of the periodic 

 table. The further the position of an element is from the nutrient- 

 line the more toxic it is in general. It will be observed that 

 essential elements occur in all groups I-VIII. 



A very interesting discussion on the relation between the 

 biological essentiality of the elements and their atomic structure 

 has been contributed by Steinberg (1938c). When considered 

 from the point of view of their position in the periodic table 

 Steinberg concludes that three and no more than three essential 

 elements are to be found in each group. Where less than three 



