184 HOW CROPS GROW. 
The ash of the common scouring rush, (Zquisetum 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 Graminee, 
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 pit 82.5|¢ 
TG 24.1 
a BR te cc 15.1, and in 
Bark ‘ *% 9290 3 a 30.8 
“cc rr 10) “ce “144 
“ we 6 BR OTT 
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 Béttinger found the seeds of the pine 
richer in silica than the wood. 
In numerous instances, silica is so deposited in or upon 
