s. The peculiar juices of plants differ much in their
nature, not only in different species of vegetables, but frequently in
different parts of the same individual plant: they are sometimes
saccharine, as in the sugar-cane, sometimes resinous, as in firs and
evergreens, sometimes of a milky appearance, as in the laurel.

EMILY.

I have often observed, that in breaking a young shoot, or in bruising a
leaf of laurel, a milky juice will ooze out in great abundance.

MRS. B.

And it is by making incisions in the bark that pitch, tar, and
turpentine are obtained from fir-trees. The durability of this species
of wood is chiefly owing to the resinous nature of its peculiar juices.
The volatile oils have, in a great measure, the same preservative
effects, as they defend the parts, with which they are connected, from
the attack of insects. This tribe seems to have as great an aversion to
perfumes, as the human species have delight in them. They scarcely ever
attack any odoriferous parts of plants, and it is not uncommon to see
every leaf of a tree destroyed by a blight, whilst the blossoms remain
untouched. Cedar, sandal, and all aromatic woods, are on this account of
great durability.

EMILY.

But the wood of the oak, which is so much esteemed for its durability,
has, I believe, no smell. Does it derive this quality from its hardness
alone?

MRS. B.

Not entirely; for the chesnut, though considerably harder and firmer
than the oak, is not so lasting. The durability of the oak is,
I believe, in a great measure owing to its having very little
heart-wood, the alburnum preserving its vital functions longer than in
other trees.

CAROLINE.

If incisions are made into the alburnum and cortical layers, may not the
ascending and descending sap be procured in the same manner as the
peculiar juice is from the vessels of the parenchyma?

MRS. B.

Yes; but in order to obtain specimens of these fluids, in any quantity,
the experiment must be made in the spring, when the sap circulates with
the greatest energy. For this purpose a small bent glass tube should be
introduced into the incision, through which the sap may flow without
mixing with any of the other juices of the tree. From the bark the sap
will flow much more plentifully than from the wood, as the ascending sap
is much more liquid, more abundant, and more rapid in its motion than
that which descends; for the latter having been deprived by the
operation of the leaves