184 HOW CROPS GROW. 



The ash of the common scouring rush, (Equisetum hye- 

 male,) has been found to contain 97.5 | of silica. The 

 straw of the cereal grains, and the stems and leaves of 

 grasses, both belonging to the botanical family Graminece, 

 are specially characterized by a large content of silica, 

 ranging from 40 to 70 | . The sedge and rush families 

 likewise contain much of this substance. 



The position of silica in the plant would appear, from 

 the percentages above quoted, to be, in general, at the sur- 

 face. Although it is found in all parts of the plant, yet 

 the cuticle is usually richest, and this is especially true in 

 cases where the content of silica is large. Davy, in 1799, 

 drew attention to the deposition of silica in the cuticle, and 

 advanced the idea that it serves the plant an office of sup- 

 port similar to that enacted in animals by the bones. 



In the ash of the pine, (Pinus sylvestris^ Wittstein has 

 obtained results which indicate that the age of wood or 

 bark greatly influences the content of silica. He found in 

 Wood of a tree, 220 years old, 32.5| c 



Bark 



170 

 135 

 220 

 170 

 135 



24.1 



15.1, and in 



30.3 



144 



11.9 



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

 centage of silica to increase as the plant approached maturi- 

 ty. 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 exceptions. Thus, the chaff of 

 wheat, rye, and oats, is richer in silica than any other part 

 of these plants, and Bottinger found the seeds of the pine 

 richer-in silica than the wood. 



In numerous instances, silica is so deposited in or upon 



