SPECIFIC ROLES OF MINERAL ELEMENTS IN PLANTS 425 



to plants except in very dilute concentrations. A number of observations 

 are on record indicating that copper salts may have a beneficial effect upon 

 plants under cultural conditions. The productivity of peat soils in particular 

 can usually be increased by applications of copper sulfate. According to 

 Sommer (1931), copper can be shown to be essential for flax, tomato, and 

 sunflower plants by a solution culture technique. Similarly Lipman and 

 MacKinney (1931) seem to have demonstrated its indispensability for barley 

 and flax plants. Although critical experimental results are still too scanty 

 to permit a general conclusion, indications are that copper will prove to be 

 one of the essential elements for plants. 



Zinc. — A number of earlier investigations indicated that this element has 

 a beneficial effect upon plants. More recently Sommer and Lipman (1926) 

 have apparently demonstrated the necessity of zinc for sunflower and barley 

 by a highly refined solution culture technique. This element also seems to 

 be necessary for buckwheat, Windsor beans, and red kidney beans (Sommer, 

 1928). The quantity of zinc required is, as would be expected, exceedingly 

 minute, and in any appreciable concentrations this element is highly toxic. 

 There is a strong presumption, therefore, that zinc, like copper, will prove 

 to be an essential element for many species of higher plants. 



The remainder of the elements to be discussed in this section are not gen- 

 erally considered to be essential although some of them exert marked effects 

 on the development or metabolism of plants. 



Sodium. — This element practically always occurs in the ash of plants, 

 but does not seem to be one of the essential elements. In some halophytes 

 it is present in considerable amounts, most of it being dissolved in the cell 

 sap in the form of sodium chloride. In part sodium can replace potassium as 

 one of the essential elements, but no plant will survive in the total absence 

 of potassium, even if sodium is available. 



Silicon. — This element comprises a very large proportion of the ash of 

 some species, particularly of the aerial portions of members of the grass and 

 Equisetum families. It is also relatively abundant in the bark of trees. Earlier 

 investigators believed, principally because of the large amounts present in the 

 ash of many species, that silicon is essential for plants. Many years ago, 

 however, it was shown that even those species in which silicon was most 

 abundant could be grown to maturity in culture solutions to which no silicon 

 was added. It is doubtful, however, if plants have ever been grown in the 

 complete absence of this element since even in cultures to which it is not sup- 

 plied traces are probably present in the form of impurities from various 

 sources. There is therefore a possibility that minute traces of this element 

 may be necessary. Formerly it was believed that silicon was important in 



