426 UTILIZATION OF MINERAL SALTS 



contributing to the stiffness of the straw in cereal crops, but more recent 

 experiments do not support this view. Some investigators have also con- 

 sidered that the silicified cell walls of some species, such as the cereals, render 

 them more resistant to the attacks of fungous and insect parasites. 



Silicon appears, however, to exert an important influence upon the phos- 

 phate metabolism of plants. Application of sodium silicate results in an in- 

 crease in the yields of plants growing in plots inadequately supplied with 

 phosphates. Some workers believe this effect of silicon to be upon the metabo- 

 lism of the plant itself. They believe that the presence of this element in some 

 way increases the efficiency of the plant in the utilization of phosphorus. 

 Others believe that silicon increases the availability of the phosphates in the 

 soil. It is possible that both such effects may occur. 



Chlorine. — This element was earlier considered to be essential for plants, 

 although in more recent times this view has been abandoned. Chlorine seems, 

 however, to be of universal occurrence in plants but is apparently present 

 almost wholly in the form of inorganic chlorides. Chlorine is a constituent, 

 however, of the molecules of anthocyanins (Chap. XXII). The experi- 

 mental results which have been obtained upon supplying chlorides to plants 

 have been very variable. In some species a definite beneficial effect has been 

 noticed, in others applications of chlorides have resulted in a retardation of 

 plant growth, while in still others no apparent influence could be detected. 

 The apparent results of supplying chlorides to plants may be largely if not 

 entirely due to changed ionic relationships in the soil or solution culture, 

 induced by the introduction of the chloride ion, rather than to direct effects 

 of this element on the metabolism of the plant. There is no convincing 

 evidence that this element is essential for plants, although it is doubtful if 

 any plant has ever been grown in an environment — artificial or otherwise — 

 in which at least traces of chlorine were not present. 



Plants indigenous to salt marshes and saline soils can endure the presence 

 of relatively large quantities of chlorides, usually sodium chloride, in the soil. 

 Asparagus is an example of a crop plant which not only tolerates but actually 

 requires treatment with sodium chloride for its best development. Salting of 

 asparagus beds has long been a standard agricultural practice. 



Aluminum. — This element is one of the most abundant of those present 

 in the soil, although it occurs chiefly in insoluble forms. A larger proportion 

 of soluble aluminum is generally present in relatively acid soils (below pH 5.0) 

 than in soils of higher pH. 



Aluminum is probably universally present in plants although in terms 

 of percentage composition the amount in the ash of most species is very small. 

 While aluminum is not considered to be one of the essential elements it is 



