A dictionary of modern gardening - University Library

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have been expected, before Mr. Hales
demonstrated it by experiment.
" Now, we know that a much less
pressure than any of those above mentioned
would be capable of bursting the
delicate membranes of any of their exterior
descending sap vessels, and it is
in such outer ducts that the injury first
occurs. When one exterior vessel is
ruptured, that next beneath it, having
the supporting pressure removed, is
enabled to follow the same course at
the same locality; and in proportion to
the length of the time that the sap continues
in excess, is the depth to which
the mischief extends, and the quantity
of sap extra vasated.
" If the extravasation proceeds from
this cause, there is but one course of
treatment to be pursued ; sever one of
the main roots to afford the tree immediate
relief, and reduce the staple of
the soil, by removing some of it, and
admixing less fertile earthy components,
as sand or chalk. This must be done
gradually, for the fibrous roots that are
suited for the collection of food from a
fertile soil are not at once adapted for
the introsusception of that from a less
abundant pasturage. Care must be
taken not to apply the above remedies
before it is clearly ascertained that the
cause is not an unnatural contraction of
the sap vessels, because, in such case,
the treatment might be injurious rather
than beneficial. I have always found it
arising from an excessive production of
sap, if the tree, when afflicted by extravasation,
produces at the same time
super-luxuriant shoots.
" 3. Local contractions of the sap
vessels.—If the extravasation arises
from this cause, there is usually a swelling
of the bark immediately above the
place of discharge.
"I had a cherry tree in my garden,
in Essex, of which the stock grew very
much less freely than the graft, consequently,
just above the place of union,
a swelling resembling a wen extended
round the whole girth of the tree, from
which swelling gum was continually
exuding. In the stem below it, I never
observed a single extravasation. In a
case such as this, the cultivator's only
resource is to reduce cautiously the
amount of branches, if the bleeding
threatens to be injuriously extensive,
otherwise it is of but little consequence,
acting like temporary discharges of
blood from the human frame, as a relief
to the system.
" 4. The extravasation of the sap from
a wound is usually the most exhausting,
and as the wound, whether contused or
not, is liable to be a lodgment for wa-
ter and other foreign bodies opposed to
the healing of the injured part, the discharge
is often protracted. This is
especially the case if the wound be
made in the spring, before the leaves
are developed, as in performing the
winter pruning of the vine later than is
proper. In such case, the vine always
is weakened, and in some instances it
has been destroyed. The quantity of
sap which may be made to flow from
some trees is astonishing, especially in
tropical climates. Thus, from a cocoanut
palm, from three to five pints of sap
will flow during every day for four or
five successive weeks. The best mode
of checking such exudations, is by placing
a piece of sponge dipped in a solu-
tion of sulphate of iron upon the discharging
place, covering the sponge
with a piece of sheet-lead, and binding
it on firmly. The sulphate acts as a
styptic, promoting the contraction of
the mouths of the vessels; the sponge
encourages cicatrization, and the lead
excludes moisture.
" 5. Heat attended by dryness of the
soil, as during the drought of summer,
is very liable to produce an unnatural
exudation. This is especially noticeable
upon the leaves of some plants,
and is popularly known as honey-dew.
It is somewhat analogous to that outburst
of blood, which in such seasons is
apt to occur to man, and arises from
the increased action of the secretory
and circulatory system, to which it affords
relief There is this great and
essential difference, that, in the case of
the plants, the extravasation is upon the
surface of the leaves, and conse(juently
in proportion to the abundance of the
extruded sap are their respiration and
digestion impaired.
" Azaleas sometimes, but rarely, have
the impubcscence on their leaves, especially
on their lower surface, beaded as
it were with a resinous exudation. This
can scarcely be called a disease. It is
never found but upon plants that have
been kept in a temperature too high,
and in a soil too fertile. It is an effort
to relieve the surcharged vessels, and
occurs in various forms in other plants.