ance--the stalk, the leaf, and the grain, show in
their ashes, important difference of composition. The stalk or straw
contains three or four times as large a proportion of ash as the grain,
and a no less remarkable difference of composition may be noticed in
the ashes of the two parts. In that of the straw, we find a large
proportion of silica and scarcely any phosphoric acid, while in that of
the grain there is scarcely a trace of silica, although phosphoric acid
constitutes more than one half of the entire weight. The leaves contain
a considerable quantity of lime.

[What is the reason for this difference?

In what part of the grain does phosphoric acid exist most largely?]

This may at first seem an unimportant matter, but on examination we
shall see the use of it. The straw is intended to support the grain and
leaves, and to convey the sap from the roots to the upper portions of
the plant. To perform these offices, _strength_ is required, and this is
given by the _silica_, and the woody fibre which forms so large a
proportion of the stalk. The silica is combined with an alkali, and
constitutes the glassy coating of the straw. While the plant is young,
this coating is hardly apparent, but as it grows older, as the grain
becomes heavier, (verging towards ripeness), the silicious coating of
the stalk assumes a more prominent character, and gives to the straw
sufficient strength to support the golden head. The straw is not the
most important part of the plant as _food_, and therefore requires but
little phosphoric acid.

[Why is Graham flour more wholesome than fine flour?

Are the ashes of all plants the same in their composition?]

The grain, on the contrary, is especially intended as food, and
therefore must contain a large proportion of phosphoric acid--this
being, as we have already learned, necessary to the formation of
bone--while, as it has no necessity for strength, and as silica is not
needed by animals, this ingredient exists in the grain only in a very
small proportion. It may be well to observe that the phosphoric acid of
grain exists most largely in the hard portions near the shell, or bran.
This is one of the reasons why Graham flour is more wholesome than fine
flour. It contains all of the nutritive materials which render the grain
valuable as food, while flour which is very finely bolted[L] contains
only a small part of the outer portions of the grain (where the
phosphoric acid, protein and fatty