12 CHEMISTRY OF PLANT LIFE 



of plant growth. It appears to be needed in largest proportion 

 by plants which contain high percentages of nitrogen in their 

 foliage, such as the legumes. There is some evidence that sulfur 

 has a particular role in promoting the growth of bacteria, and it 

 may be that the high percentages of total sulfur which are found 

 in the tissues of legumes are due to the presence of the symbiotic 

 nitrogen-gathering bacteria in the nodules on the roots of these 

 plants. This point has not yet been investigated, however. 



Sodium is probably not essential to plant growth, although 

 it is present in small proportions in the ash from practically all 

 plants. In cases of insufficient supply of potassium, sodium can 

 apparently perform at least a part of the role of the former ele- 

 ment; but this seems not to be a normal relationship or use. 



Chlorine is found in small amounts in the sap and in the ash 

 of nearly all plants. However, it does not appear to be essential 

 to the growth of a plant, except possibly in the case of certain 

 species, such as asparagus, buckwheat, and, perhaps, turnips 

 and some other root crops. Whether the benefit which these crops 

 derive from the application of common salt to the soil in which 

 they are growing is due to the direct food value of either the 

 chlorine, or the sodium, or to some indirect effect, is not yet 

 known. The presence of chlorine in the sap of plants is undoubt- 

 edly due to the inevitable absorption of soluble chlorides from the 

 soil and apparently has no connection with the nutritional needs 

 of the plant. 



Silicon is always considered as a non-essential element, although 

 it occurs in such large proportions in some plants as to indicate 

 that it cannot be wholly useless. It accumulates in the stems 

 of plants, chiefly in the cell-wall, and has sometimes been sup- 

 posed to aid in giving stiffness to the stems. But large numbers of 

 analyses have failed to show any direct correlation between the 

 stiffness of straw of cereal plants and the percentage of silicon 

 which they contain. Further, plants will grow to full maturity 

 and with erect stems when no silicon is present in the mineral 

 nutrients which are furnished to them. On the other hand, cer- 

 tain experiments appear to indicate that silicon can perform some 

 of the functions of phosphorus, if soluble silicates are supplied to 

 phosphorus-starved plants. But under normal conditions of 

 plant nutrition, it seems to have no such function. 



