THAT the trace elements are required by plants in very small amounts suggests that their function may be that of catalysts in vital reactions. At the same time it does not follow that this is their only function; they may form part of substances essential to the life of the plant other than catalysts but which are necessary in only small quantity. It is, however, now clear that all the generally recognized trace elements with the possible exception of boron, either form an integral part of certain enzymes or activate enzyme systems, at any rate when these are isolated from the living plant. Under such conditions it is frequently found that more than one metal may be capable of activating the same enzyme action and when this is so it would appear that no one of these can be regarded as essential for that particular action. Also it has to be borne in mind that the functioning of an element in the plant where many reactions are linked and interlocked may not be precisely indicated by the part it plays in an isolated action outside the plant. Nevertheless examination of the action of various elements, in activating enzyme actions in plant extracts and other dead plant material, has given valuable information on the functions of trace elements and in some instances is the most definite we have.

As well as the part trace elements play in enzyme actions a variety of other functions have been ascribed to them. This is particularly so with manganese, zinc and boron but less so with copper and molybdenum. In this connexion it may possibly be significant that the first three trace elements are generally present in plants in a significantly higher concentration than the other two (cf. Table 18, p. 129).

A point worthy of note is that the effects of boron deficiency are generally of quite a different kind from those of the other trace elements. Warington (1923) thought that the function of boron in the broad bean was nutritive rather than catalytic.