PLANT GROWTH. 9 



concentrated in the upper parts of the plant, the water 

 being rapidly evaporated by the leaves, as already men- 

 tioned. The essential ash constituents are employed in 

 the formation of new tissues. The non-essential ash 

 constituents which have been taken up by the roots are 

 partly disposed of in a sohd form, as a permanent incrus- 

 tation of the older tissues. The soluble salts which are 

 not thus disposed of, at first accumulate in the sap, and 

 are probably more or less removed from the surface of the 

 leaves and stem by the washing effect of rain. 



The deposition of silica upon the external tissues of 

 wheat, barley, and other graminaceous plants is a familiar 

 example of the excretion of a non-essential ash constituent. 

 Silica is also abundant in the old leaves, and in the outer 

 bark of many trees, and is commonly found as an incrust- 

 ing constituent of old tissues. Insoluble calcium salts, 

 frequently the oxalate, are also deposited as incrusting 

 matters in old tissues. These incrustations are indirectly 

 of service to the plant, as they tend to harden the tissues 

 and thus protect them from injury. 



Soluble non-essential ash constituents, as chloride of 

 sodium, are found abundantly in the succulent parts of 

 plants when such ash constituents have been present in 

 the soil. They generally diminish in quantity as the 

 plant matures, and are never stored up in the seed. 



The amount and composition of the ash of succulent 

 plants, as meadow grass, clover, and mangel, is greatly 

 influenced by the character of the soil, and the manure 

 applied. The ash of a seed, on the other hand, is very 

 constant in composition, resulting from the selective 

 powers of the plant. 



Of the particular action of the ash constituents within 



