THE ASH OF PLANTS. 199 



bark greatly influences the content of silica. He found 

 in ash of the 



And in 



Wood of a tree, 220 years old, 32.5% 

 170 " 24.1 



135 " 15.1 



Bark " 220 30.3 



" " 170 14.4 



" " 135 " 11.9 



In the ash of the straw of the oat, Arendt found the 

 percentage of silica to increase as the plant approached 

 maturity. So the leaves of forest trees, which in autumn 

 are rich in silica, are nearly destitute of this substance 

 in spring time. 



Silica accumulates then, in general, in the older and 

 less active parts of the plant, whether these be external 

 or internal, and is relatively deficient in the younger and 

 really growing portions. This rule is not without excep- 

 tions. Thus, the chaff of wheat, rye and oats is richer 

 in silica than any other part of these plants, and Bottin- 

 ger found the seeds of the pine richer in silica than the 

 wood. 



In numerous instances, silica is deposited in or upon 

 the cell-wall in such abundance that when the organic 

 matters are destroyed by burning, or removed by sol- 

 vents, the form of the cell is preserved in a silicious 

 skeleton. This has long been known in case of the 

 Equisetums and Deutzias. Here the peculiar rough- 

 nesses of the stems or leaves are fully incrusted or inter- 

 penetrated by silica, and the ashes of the cuticle present 

 the same appearance under the microscope as the cuticle 

 itself. 



The hairs of nettles, hemp, hops, and other rough- 

 leaved plants, are highly silicious. 



According to Wicke, the beech owes the smooth and 

 undecayed surface which its trunk presents, to the silica 

 of the bark. The best textile materials, which are bast- 



