IN the preparation of the new edition of this book considerable alterations have been made to the original text. In the first place, as the title indicates, the subject-matter has been limited to a discussion of trace elements in plants, the effects of trace-element deficiency or excess in animals being considered only where such effects arise directly from shortage or excess of these elements in the plants eaten by grazing animals. To have included a full discussion of trace elements in animals would have meant unduly increasing the size of this book; moreover the subject would be dealt with much more satisfactorily by a zoologist than a botanist.

A very great deal of fresh information about trace elements in plants has accumulated during the last sixteen years, to deal adequately with which has involved many additions to, and much rearrangement of, the text. In this new edition some consideration has been given to the results of excess absorption as well as to those of deficiency; the consequences of excess of a trace element may be as serious as those of its shortage. Some reference has also been made to soil conditions as they relate to availability of the trace elements.

The increase in size necessitated by additions to our knowledge has been lessened to some extent by the deletion of matter relating to trace elements in animals and by the removal of the lists of species for which various trace elements have been proved essential. Since there is now every reason to believe that the five universally recognized trace elements are essential for all higher plants, there is little point in listing those, now very numerous, in which essentiality of one or other of the trace elements has actually been demonstrated.

It is a pleasure to me to acknowledge with gratitude the very great help I have received from Dr E. J. Hewitt in providing a number of excellent photographs obtained at Long Ashton Research Station where the investigation of trace element problems has been in progress for many years.