is seldom below
forty in winter.

It is from the sap, after it has been elaborated by the leaves, that
vegetables derive their nourishment; in its progress through the plant
from the leaves to the roots, it deposits in the several sets of vessels
with which it communicates, the materials on which the growth and
nourishment of each plant depends. It is thus that the various peculiar
juices, saccharine, oily, mucous, acid, and colouring, are formed; as
also the more solid parts, fecula, woody fibre, tannin, resins, concrete
salts; in a word, all the immediate materials of vegetables, as well as
the organised parts of plants, which latter, besides the power of
secreting these from the sap for the general purpose of the plant, have
also that of applying them to their own particular nourishment.

EMILY.

But why should the process of vegetation take place only at one season
of the year, whilst a total inaction prevails during the other?

MRS. B.

Heat is such an important chemical agent, that its effect, as such,
might perhaps alone account for the impulse which the spring gives to
vegetation. But, in order to explain the mechanism of that operation, it
has been supposed that the warmth of the spring dilates the vessels of
plants, and produces a kind of vacuum, into which the sap (which had
remained in a state of inaction in the trunk during the winter) rises:
this is followed by the ascent of the sap contained in the roots, and
room is thus made for fresh sap, which the roots, in their turn, pump up
from the soil. This process goes on till the plant blossoms and bears
fruit, which terminates its summer career: but when the cold weather
sets in, the fibres and vessels contract, the leaves wither, and are no
longer able to perform their office of transpiration; and, as this
secretion stops, the roots cease to absorb sap from the soil. If the
plant be an annual, its life then terminates; if not, it remains in a
state of torpid inaction during the winter; or the only internal motion
that takes place is that of a small quantity of resinous juice, which
slowly rises from the stem into the branches, and enlarges their buds
during the winter.

CAROLINE.

Yet, in evergreens, vegetation must continue throughout the year.

MRS. B.

Yes; but in winter it goes on in a very imperfect manner, compared to
the vegetation of spring and summer.

We have dwelt much longer on the history of vegetable chemistry than I
had intended