stances of its entire absence are rare. In the cruciferous plants
(turnip, rape, etc.) it is found abundantly, and to them it appears
indispensable, but in most other plants it admits of replacement by
potash. It seems probable that where the soil is rich in the latter
substance, plants will select that alkali in preference to soda; but as
they must have a certain quantity of alkali, the latter may supply the
place of the former where it is deficient. Cultivation, probably by
enriching the soil in that element, increases the proportion of potash
found in the ash of plants, as is remarkably seen in the asparagus,
which gave the following quantities of alkalies and chlorine:--

Wild. Cultivated.
Potash 18.8 50.5
Soda 16.2 trace.
Chlorine 16.5 8.3

The soda having almost entirely disappeared in the cultivated plant,
while a corresponding increase had taken place in the quantity of
potash.

Potash is one of the most important elements of the ash of all plants,
rarely forming less than 20, and sometimes more than 50 per cent of its
weight. The latter proportion occurs chiefly in the roots and tubers,
but it is also abundant in all seeds and in the grasses. The straw, and
particularly the chaff of the cereals, and the leaves of most plants,
contain it in smaller quantity, although exceptions to this are not
unfrequent, one of the most curious being the case of poppy-seed, which
contains only about 12 per cent, while the leaves yield upwards of 37
per cent.

The proportion of lime varies within very wide limits, being sometimes
as low as 1, and in other plants reaching 40 per cent of their ash. The
former proportion occurs in the grains of the cerealia, and the latter
in the leaves of some plants, and more especially in the Jerusalem
artichoke. The turnip and some of the leguminous plants also contain it
abundantly.

Magnesia is generally found in small quantity. It is largest in the
grains, amounting in them to about 12 or 13 per cent of the ash, but in
other plants it varies from 2 to 4 per cent. Although small in quantity,
it is an important substance, and apparently cannot be dispensed with;
at least there is no instance known of its entire absence.

_Chlorine_ is by no means an invariable constituent of the ash, although
it is generally present, and sometimes in considerable quantity. It is
most abundant when the proportion of soda is large, and exists in t