45
Barley, grain, 0.053
straw 0.191
Oats, grain 0.103
straw 0.289
Rye, grain 0.051
Beans 0.056
Peas 0.127
Lentils 0.110
Hops 1.063
Gold of Pleasure 0.253
Black Mustard 1.170
White Mustard 1.050

_Phosphoric acid_, which may be looked upon as the most important
mineral constituent of plants, is found to be present in very variable
proportions. The straws, stems, and leaves contain it in comparatively
small quantity, but in the seeds of all plants it is very abundant. In
these of the cereals it constitutes nearly half of their whole mineral
components, and it rarely falls below 30 per cent.

_Carbonic acid_ occurs in very variable quantities in the ash. It is of
comparatively little importance in itself, and is really produced by the
oxidation of part of the carbonaceous matters of the plant; but it has a
special interest, in so far as it shows that part of the bases contained
in the plant must in its natural state have been in union with organic
acids, or combined in some way with the organic constituents of the
plant.

_Silica_ is an invariable constituent of the ash, but in most plants
occurs but in small quantity. The cereals and grasses form an exception
to this rule, for in them it is an abundant and important element. It is
not, however, uniformly distributed through them, but is accumulated to
a large extent in the stem, to the strength and rigidity of which it
greatly contributes. The hard shining layer which coats the exterior of
straw, and which is still more remarkably seen on the surface of the
bamboo, consists chiefly of silica; and in the latter plant this element
is sometimes so largely accumulated, that concretions resembling opal,
and composed entirely of it, are found loose within its joints. The
necessity for a large supply of silica in the stems of other plants does
not exist, and in them it rarely exceeds 5 or 6 per cent, but in some
leaves it is more abundant.

A knowledge of the composition of the ash of plants is of considerable
importance in a practical point of view, and enables us in many
instances to explain why some plants will not grow upon particular soils
on which others flourish. Thus, for instance, a plant which contains a
large quantity of lime, such as the bean or turnip, wi