HISTORICAL INTRODUCTION 11 



Using Arnon's technique, Twyman (1943) obtained a similar 

 result with oats. Four weeks after germination of the grains 

 the average dry weights of plants supplied with the A 4, B7 

 and A4 + B7 groups of trace elements were respectively 0-101, 

 0-130 and 0-238 g. This again shows the necessity of one or more 

 of the elements of Arnon's group B7. 



The question arises why for so many years the necessity for 

 the trace elements in plant nutrition was not recognized. The 

 answer given to this question by Maze was no doubt the correct 

 one. Maze considered that the importance of the micro- 

 nutrients had been overlooked because in water-culture experi- 

 ments a sufficient quantity of these was introduced into the 

 cultures from (1) the seeds used, (2) impurities in the salts used 

 in preparing the culture solutions, and (3) solution from the 

 vessels containing the culture solutions. Indeed, knowledge of 

 the existence of the various trace elements has only been 

 obtained through the purification of the water and nutrient 

 salts used, and the choice of suitable culture vessels. 



The securing of an adequate degree of purity of the materials 

 used is of the utmost importance in experimental work designed 

 to examine the indispensability or otherwise of particular 

 substances. The methods that have been developed to this end, 

 along with other experimental methods of value in work on 

 micro -nutrients, form the subject of the next chapter. 



Although addition of a compound of a particular element to 

 the nutrient medium in which plants are growing may bring 

 about increase in rate of growth of the plants, it does not follow 

 that the element is essential for the growth of plants. Indeed, 

 increases in growth rate as a result of the addition of com- 

 pounds of a number of different elements have been recorded 

 from time to time. Among recent observations of this kind 

 particular mention may be made of those of R. S. Young (1935), 

 who examined the effect On the growth of timothy (Phleum 

 pratense) of thirty -five of the rarer elements when added in 

 five different concentrations (2000, 500, 100, 10 and 0-1 p. p.m.) 

 to a sandy loam. Beneficial effects were observed with molyb- 

 denum, supplied in 2000 p.p.m., and with antimony, barium, 

 bismuth, bromine, cerium, manganese, strontium, tungsten, 

 uranium and yttrium, supplied at the rate of 500 p.p.m. 



