12 HISTORICAL INTRODUCTION 



Aluminium, cadmium, copper, fluorine, lanthanum, lead, mer- 

 cury, tin and zinc gave an increased growth when supplied at 

 the rate of 100 p. p.m., while with 10 p.p.m. arsenic, beryllium, 

 chromium, iodine, lithium, selenium, thorium, titanium, vana- 

 dium and zirconium were beneficial. At a concentration of 

 0-1 p.p.m., boron, nickel and thallium brought about an increase 

 in growth. Of the thirty-five elements, the effects of which were 

 tested, only cobalt appeared to be slightly toxic at this lowest 

 concentration employed, while silver at this concentration 

 appeared to have no effect. At higher concentrations than those 

 stated for the respective elements the action of these was 

 depressing on growth. 



Experiments with cultures of two green algae, species of 

 Chlorella and Crucigina respectively, led Young to conclude that 

 on the whole any element will stimulate the growth of algae at 

 a definite concentration which depends on the element and the 

 species. 



We certainly cannot conclude that an element which stimu- 

 lates growth is necessarily essential for growth, although if 

 increased growth is observed to result from the presence of a 

 particular element there is always the possibility that that 

 element may be an essential one. Whether it is so or not can 

 only be proved by growing the plant in carefully controlled 

 cultures, in which every care is taken rigidly to exclude from the 

 culture medium the element under examination. 



To conclude this introduction to our subject here is appended 

 a list of those elements which, it has been claimed, constitute 

 micro-nutrients of plants. Under the name of each of these 

 elements are listed those species for which it has been claimed 

 that the element either is essential or induces an increase in rate 

 of growth or in yield. In the latter circumstance it does not 

 necessarily follow that the element is essential although the 

 evidence of increased growth indicates that this may be so. 



In the following list the name of the first worker to call 

 definite attention to the favourable effect of each particular 

 element on the growth of the species concerned is given, whether 

 that worker regarded the element as essential for the species 

 or not. Thus Nakamura in 1903 reported increased growth of 

 peas and spinach as a result of adding boron to the soil, but it 



