204 STRUCTURAL BOTANY 



It will be noticed that no compound of silicon wag 

 added to the water, and this element certainly cannoi 

 be obtained from the air. Some trifling quantity 

 might possibly be derived from the glass of the bottle, 

 but the amount to be thus obtained is too small to 

 be of any importance. We infer, then, that silicon, 

 iu. spite of its general presence in plants, is not 

 essential to their nutrition. This does not prove that 

 it is of no use. To many plants, such as grasses, 

 silicon is of great importance in hardening the outer 

 surface of the epidermis, and thus rendering it a 

 better protection against parasites, but as food silicon 

 is not necessary to the plant. 



Of the salts offered to the plant, sodium chloride 

 can easily be proved not to be essential. If it is left 

 out altogether, the plant will still flourish and come 

 to maturity. The sodium, then, can certainly be 

 dispensed with. It is said that for some plants 

 chlorine in some form or other is needed, but certainly 

 this is not the case with most ; yet these two elements 

 are constantly present in plants, simply because they 

 are constantly present in the soil. 



If, however, we omit any of the other elements 

 from our food-solution, the plant suffers. Suppose, 

 for example, that no compound of iron be added to 

 the water ; for a time the seedling grows normally 

 the new leaves which are formed, however, are not 

 green, but white in fact, they contain no chlorophyll. 

 Hence no assimilation of carbon can take place, and 

 no further increase in the organic substance of the 

 plant can be produced. The reason why the first few 



