HISTORICAL INTRODUCTION 9 



plants, and Roberg himself says that these elements are to be 

 considered as acting as stimulants. However, having regard to 

 work with plants of other groups, it may well be that these 

 elements v/ill prove to be actually essential for the growth of 

 these and other algae. 



From what has so far been discovered regarding the essenti- 

 ality of trace elements, two questions arise which only further 

 research can answer. The first is how far the necessity for these 

 elements is general throughout the plant kingdom, and the 

 second is whether we now know the complete list of these 

 elements. As regards the first question, it would seem probable 

 rather than merely possible that when plants differing as widely, 

 both taxonomically and anatomically, as well as physiologically, 

 as angiosperms and fungi, both exhibit these requirements, we 

 are dealing with something very fundamental in plant nutrition, 

 and we are justified in concluding that the best established of the 

 trace elements, manganese, boron, zinc and copper, are likely 

 to be found essential for the nutrition of plants in general. In 

 regard to the second question, Steinberg (1938 c) has contributed 

 an interesting discussion on the relations between essentiality 

 of elements and their atomic structure, and he draws the con- 

 clusion from such considerations that scandium may be an 

 essential element for plant nutrition. In this connexion some 

 experiments carried out by Arnon are of interest. In 1937 this 

 worker described the results of water-culture experiments in 

 which the growth of barley plants was improved by the addition 

 of small quantities of molybdenum, chromium and nickel. In 

 the following year further experiments were described in which 

 plants of asparagus and lettuce were grown in four different 

 culture solutions. The first of these contained the ordinary 

 nutrient elements. The second contained these together with the 

 four well-established micro-nutrients, manganese, boron, zinc 

 and copper (designated A 4) and some chlorine. The third con- 

 tained all the elements present in the second solution together 

 with another seven (designated B7); these were molybdenum, 

 titanium, vanadium, chromium, tungsten, cobalt and nickel. 

 The fourth solution contained all the elements present in the 

 third solution together with thirteen other elements, namely, 

 aluminium, arsenic, cadmium, strontium, mercury, lead, lithium, 



