60 
HISTOLOGY OF VEGETABLES. 
consider the vast amount of silica that must be removed 
from the soil with the straw of wheat, barley, oats, 
and other grasses, it must be evident that a supply of 
this substance ought to be restored to the soil in order 
to insure good crops ; hence, it follows, that the value 
of good guano as a manure may depend, not entirely 
upon its ammonia, lime, and potash, but in a certain 
degree also upon the silica it contains. 
