NON-ESSENTIAL CONSTITUENTS 175 



is a waste product is not apparently true in every instance, for 

 there is evidence to believe that it is dissolved again sometimes 

 and utilised as a reserve of calcium. 



(viii) Magnesium is found in the ash of all parts of the plant, 

 but more especially in that of seeds. About 12 per cent, of the 

 ash of wheat grains consists of magnesia (MgO), while the ash of 

 the straw and vegetative parts contains ~less than 2 per cent. 

 Magnesium is taken from the soil, chiefly as carbonate and 

 sulphate, but its use to the plant is still very obscure. 



(ix) Iron. — The amount of iron in green plants is generally 

 very small, rarely exceeding 0-2 per cent, of the ash. It is, 

 nevertheless, absolutely necessary for their nutrition since with- 

 out it no chlorophyll is formed. Sufficient iron is present in 

 seeds for the production of a certain amount of chlorophyll, 

 and the first few leaves of seedlings grown in culture solutions 

 free from iron are green; the subsequent ones are, however, 

 pale and incapable of utilising the carbon. 



3. Non-essential elementary constituents of plants. — Some 

 of the elements are of such rare and abnormal occurrence in 

 plants that they need not be mentioned. Others, such as silicon, 

 sodium and chlorine, although found to be non essential to the 

 growth of green plants, are universally met with in the ash and 

 demand brief notice. 



Although healthy plants can be grown in the absence of several 

 elements, which are commonly met with in ordinary plant ash, 

 ■these so-called non-essential constituents may be, and probably 

 are, of use in stimulating or depressing the activity of various 

 functions carried on by plants. 



Silicon is specially abundant in the cell-walls of the external 

 portions of the stems and leaves of barley, wheat, oats and grasses 

 generally : more than one-half of the total ash of the cereals 

 consists of silica (SiOj). 



The accumulation of silicon in the cell-walls was formerly 

 supposed to be the cause of the rigidity and firmness of well- 

 grown straw and the ' lodging' of cereal crops was attributed to a 



