20 HISTORICAL INTRODUCTION 



Avena sativa (oat) Piper, 1940 

 Hordeum distichum (barley) Arnon, 1937 

 Lemna minor (duckweed) Steinberg, 1941 



Penicillium Javanicum Lockwood, 1933 

 Aspergillus niger Steinberg, 1936 



Tungsten 



Penicillium Javanicum Lockwood, 1933 



Gallium 



Lemna minor (duckweed) Steinberg, 1941 



Aspergillus niger Steinberg, 1938 



In addition to the elements included in the above list, claims 

 have been made that others, for example, tin and uranium, 

 'stimulate' plant growth. Until further information accumu- 

 lates it would be wiser to defer judgement on the significance of 

 these claims. It should also be noted that some of the elements 

 in the list are scarcely generally accepted at present as micro - 

 nutrients. In this category are columbium and tungsten. The 

 elements most definitely accepted as micro -nutrients are man- 

 ganese, zinc, boron, copper and molybdenum, but the evidence 

 for aluminium, silicon, chlorine and gallium for the plants cited 

 in the list is very strong. 



