SALTS 



429 



state from that in which it occurs as a poison — thus, soluble 

 barium, BaCU, is highly toxic; but insoluble barium, BaS04, is 

 not — or the element may be bound to other substances (proteins) 

 in such a way that its harmful influences are suppressed. What- 

 ever the answer to this question may be, it is now known that 

 the heavy elements found in tissues, though poisonous in high 

 concentrations, become not only harmless but in many cases 

 beneficial and necessary in the concentrations in which they occur 

 in plants and animals. 



The following list includes those elements so far known to 

 occur in plants and animals (listed approximately in the order of 

 their abundance) : 



That the first 10 are necessary is graphically shown in the case 

 of plants by the technique known as water culture, with which 

 much has been accomphshed by the plant physiologist B. E. 

 Livingston, his students, and others. J. E. McMurtrey has 

 obtained a series of excellent water cultures, showing the rate of 

 growth of tobacco when one of nine elements is omitted (Fig. 174). 

 Carbon, oxygen, and hydrogen are not considered, for they 

 cannot be excluded, as the plant gets them from water and air. 



That all of the 37 elements found in plants and animals are 

 absolutely necessary cannot be said, but certainly more than 

 10 or 12 are needed. Why is it, then, that a plant will do so well 

 in a water culture, such as the control in McMurtrey's experi- 

 ment (6, Fig. 174), which presumably contains but 10 elements — 

 carbon, hydrogen, and oxygen being otherwise available? 

 The answer may be one of two. Most chemicals contain minute 

 amounts of other elements, in sufficient quantity for the needs of 

 a plant. Only highly refined salts are free of foreign matter. 

 The "trace elements" necessary for plant life may be present 

 as impurities. Or it may be that a plant gets on quite well 

 in the laboratory without this or that element but that when 



