CHAPTER VI 



CONCLUDING REMARKS 



I t will be clear from the preceding review of the present position 

 of our knowledge of trace elements in plants and animals that 

 these elements present the biologist with two sets of problems, 

 the one pathological, the other physiological. The pathologist is 

 concerned with the abnormal conditions resulting in plants and 

 animals from deficiency, and in a few cases from excess, of the 

 various trace elements, and of the means by which the deficiency 

 or excess can be removed. The problems of the physiologist are 

 more subtle, for it is his business to discover the functions in 

 the life of the organism of these various elements which are 

 present in only minute amounts. The two sets of problems are 

 of course interdependent, for the work of the pathologist in 

 discovering the effects of deficiency greatly aids the physiologist, 

 while knowledge of the part played by the trace elements in the 

 life of the organism must necessarily help the pathologist in the 

 diagnosis and treatment of deficiency diseases. But on the 

 whole, knowledge of the pathology of trace elements, particu- 

 larly in plants, is much more advanced than our knowledge of 

 their physiological functions. A considerable number of well- 

 recognized and defined plant diseases are now correctly attri- 

 buted to various trace-element deficiencies, and means of con- 

 trolling these diseases have been determined with considerable 

 precision. Future work on the pathological side will no doubt 

 extend the number of such known deficiency diseases. In 

 addition to such work it would appear that two lines of research 

 in the field of pathology would well repay attention. The first of 

 these is the development of rapid means of early diagnosis. The 

 usual method of observing deficiency symptoms, employed with 

 outstanding success by Wallace, is naturally only applicable 

 after visible external symptoms have developed. The injection 

 methods elaborated by Roach appear to be particularly useful 

 for trees, but although certainly usable for annual crops, and 

 affording a means of early diagnosis, require some degree of 

 technical skill for their employment with small herbaceous plants. 



