ater, tends to preserve the plant from the
destructive effects of violent rains, severe climates, or inclement
seasons, to which this species of vegetables is peculiarly exposed.

EMILY.

Resin must preserve wood just like a varnish, as it is the essential
ingredient of varnishes?

MRS. B.

Yes; and by this means it prevents likewise all unnecessary expenditure
of moisture.

The parenchyma is immediately beneath the epidermis; it is that green
rind which appears when you strip a branch of any tree or shrub of its
external coat of bark. The parenchyma is not confined to the stem or
branches, but extends over every part of the plant. It forms the green
matter of the leaves, and is composed of tubes filled with a peculiar
juice.

The cortical layers are immediately in contact with the wood; they
abound with tannin and gallic acid, and consist of small vessels through
which the sap descends after being elaborated in the leaves. The
cortical layers are annually renewed, the old bark being converted into
wood.

EMILY.

But through what vessels does the sap ascend?

MRS. B.

That function is performed by the tubes of the alburnum, or wood, which
is immediately beneath the cortical layers. The wood is composed of
woody fibre, mucilage, and resin. The fibres are disposed in two ways;
some of them longitudinally, and these form what is called the silver
grain of the wood. The others, which are concentric, are called the
spurious grain. These last are disposed in layers, from the number of
which the age of the tree may be computed, a new one being produced
annually by the conversion of the bark into wood. The oldest, and
consequently most internal part of the alburnum, is called heart-wood;
it appears to be dead, at least no vital functions are discernible in
it. It is through the tubes of the living alburnum that the sap rises.
These, therefore, spread into the leaves, and there communicate with the
extremities of the vessels of the cortical layers, into which they pour
their contents.

CAROLINE.

Of what use, then, are the tubes of the parenchyma, since neither the
ascending nor descending sap passes through them?

MRS. B.

They are supposed to perform the important function of secreting from
the sap the peculiar juices from which the plant more immediately
derives its nourishment. These juices are very conspicuous, as the
vessels which contain them are much larger than those through which the
sap circulate