ESSENTIAL AND NON-ESSENTIAL ELEMENTS 419 



substances makes it impossible to accept the results of their investigations as 

 the final word on the mineral salt requirements of plants. 



Recognition of these sources of contamination in solution culture technique 

 has led, in recent years, to refinements in such methods, which eliminate or 

 at least enormously reduce the possibility of introducing unknown solutes into 

 solutions used in plant culture work. By repeated crystallizations it is pos- 

 sible to obtain chemicals of a much higher degree of purity than those used 

 by the earlier workers. The water used may be distilled from special stills 

 and re-distilled a number of times to remove even the smallest traces of most 

 solutes. Containers can be used which are inert when in contact with the 

 culture solutions employed. By removal of the cotyledons at a very early 

 stage in germination, or by other procedures, the supply of elements obtained 

 by the plant from the seed can be largely eliminated. 



Various more or less critical observations have led to claims that many 

 other elements are essential for normal plant development in addition to the 

 ten which have long been accepted as essential to plants. Among these are 

 arsenic, aluminum, barium, boron, bromine, caesium, chromium, chlorine, co- 

 balt, copper, iodine, lithium, manganese, nickel, selenium, silicon, strontium, 

 tin, titanium, vanadium and zinc. Most of these elements are considered to 

 be necessary for plants only in traces. With only a few exceptions all of them, 

 in fact, are toxic in any appreciable concentration. 



In the face of this rather overwhelming array of possibly essential ele- 

 ments it appears desirable to adopt a criterion by which the indispensability 

 of an element can be judged. While it is undoubtedly true that any one of 

 these elements, at least when supplied to certain species of plants, under cer- 

 tain cultural conditions, will result in beneficial effects upon growth, this is 

 far from indubitable evidence of its indispensability. Necessity of an element 

 for normal plant metabolism is demonstrated only if lack of it can be shown 

 to result in injury, abnormal development, or death of plants when grown in 

 sand or solution cultures by a technique including the refinements of method 

 described above. Complete proof of the necessity of an element also requires 

 demonstration that no other element of similar properties can be substituted 

 for it. Furthermore, before it can be considered to be proved that a given 

 element is essential for green plants generally its indispensability must have 

 been demonstrated for a wide enough variety of species that a cross section of 

 the plant kingdom is represented. 



At the present time practically all botanists agree that at least two of the 

 elements in the list just cited — boron and manganese — must be added to the 

 group of elements essential in plant metabolism. In addition evidence for the 

 indispensability of the two others — copper and zinc — is believed by many in- 



