IX 



PREFACE 



Trace elements, micro-nutrients and minor elements are terms 

 applied to a number of chemical elements which are essential 

 for the lives of plants and animals, but which are required in 

 extremely small quantity. The development of our knowledge 

 of the part played by the trace elements in the life of plants and 

 animals is very recent, nearly all our present information on the 

 subject having been acquired during little more than the last 

 twenty years. Nevertheless, in the course of that time a great 

 many observations have been made in the field, and much 

 experimental work carried out in both field and laboratory, 

 while many hundreds of publications, some of slight value, 

 others of considerable scientific and economic importance, 

 dealing with trace elements in living plants and animals have 

 appeared in the scientific press. The time thus seems ripe for the 

 presentation of a digest of this information, so that the salient 

 facts may be available in a convenient form, and the present 

 position of our knowledge of trace elements in living organisms 

 made plain. It is with this intention that the present book has 

 been written. 



The role of the trace elements in organisms is, in the first 

 place, a matter for the plant or animal physiologist, but as 

 deficiency or excess in the supply of the various trace elements 

 may lead to a diseased condition of the plant or animal with 

 serious economic consequences, the trace elements are also of 

 interest to the plant pathologist, the veterinary surgeon, the 

 agriculturist and horticulturist. Indeed, as regards plants at 

 any rate, much more is definitely known of the plant patho- 

 logical aspect of the trace elements than of their physiological 

 functions. 



I would take this opportunity of expressing my thanks to 

 Mr W. Morley Da vies of Harper Adams Agricultural College, 

 who first introduced to me the effects of trace-element defi- 

 ciencies in the field, and who has generously and constantly put 

 at my disposal both his knowledge of the pathological effects of 

 trace-element deficiency, and the pathological material itself. 



