100 THE FUNCTIONS OF 



essential elements are known to exist in a group there is a pre- 

 sumption that the missing ones are yet to be found. However, 

 Steinberg considers that correlations between essentiality and 

 atomic structure can best be shown by tabulating the elements 

 on the basis of their transition subshell, those in which the 

 electron numbers have undergone a regular change in the 

 formation of the elements and which largely determine the 

 chemical properties of the latter. From such considerations 

 Steinberg makes certain deductions regarding the relationship 

 between essentiality and atomic structure and of the possible 

 essential elements not yet recognized as such. Thus the non- 

 essentiality of silicon is indicated, while the possibility of 

 columbium as an essential element is suggested. Steinberg's 

 conclusions only claim to be very tentative. 



That the trace elements are required by plants in such small 

 quantities strongly suggests that they all function as catalysts 

 or are at least closely linked up with some catalytic process. 

 Miss Warington (1923) thought that the function of boron in 

 the broad bean was nutritive rather than catalytic, and it is 

 certainly true that the results of boron deficiency are generally 

 of a distinctly different kind from those resulting from de- 

 ficiency of manganese, zinc and copper. With boron deficiency, 

 as we have seen, the most characteristic effect is a breakdown 

 of thin- walled tissues, especially those of the meristematic 

 regions, followed by degeneration of the vascular tissues, whereas 

 with the other three well-established trace elements early 

 external symptoms are generally localized chloroses, although 

 these may precede more serious disturbances in growth such as 

 those which lead, in the case of zinc and copper, to die-back of 

 the terminal buds of the shoots. Nevertheless, chlorosis may 

 also be a symptom of boron deficiency, as, for example, in 

 tobacco (Van Schreven, 1934) and in sugar cane (Martin, 1934), 

 while plants do not appear to require more boron than man- 

 ganese. It is not very clear why a catalyst essential for the 

 maintenance of normal metabolism and growth should not be 

 regarded as fulfilling a nutritive function. 



Manganese, zinc and copper are generally regarded, like iron, 

 as playing a part in vital oxidations and reductions, and some 

 writers consider boron to play a similar role. It must be 



