WATER CONTENT AND ASH OF PLANTS iyi 



is left. It was formerly supposed that silica gave the 

 stems of grains and grasses their stiffness. Perfect wheat, 

 however, with normal strength of straw has been grown 

 in the absence of silica, except for the small amount 

 originally present in the seed. Lawes and Gilbert have 

 shown that the lack of silica is not the cause of grain 

 lodging. Some authorities claim that silica takes a part 

 in plant economy and is necessary in seed formation. 

 Whatever its function, it is not an important element as 

 plant food, and there is always an abundance in the soil 

 for crop purposes. 



In the living plant, the mineral elements are not pres- 

 ent in the same form or combination as in the plant ash. 

 During growth, many of the ash elements are combined 

 with the organic compounds, for example phosphorus, 

 which forms phosphorized proteids and fats. The ash forms 

 a part of the plant tissue. When the plant is burned, the 

 organic compounds are volatilized, while the ash elements, 

 which are non-volatile, are left. The essential ash ele- 

 ments are absolutely necessary, as food, for the growth 

 and development of all crops, and plant growth is fre- 

 quently arrested because of the lack of a sufficient supply 

 for purposes of nutrition. The food requirements of in- 

 dividual farm crops are discussed in the ' ' Chemistry of 

 Soils and Fertilizers. ' ' 



