Botanists describe two kinds of vegetable sap; the one is called the
ascending or unelaborated sap, the other the descending or elaborated
sap. If a young branch be cut across in the spring season, the newly
exposed surfaces will be found rapidly to cover themselves with a dew,
especially that portion which is continuous with the trunk--this moisture
is the ascending sap: while if during the summer or autumn a piece of
twine be tightly drawn and knotted round a young branch of lilac, the
part above this ligature will shortly become swollen, and will bulge out
on every side, in consequence of an impediment having been thus presented
to the downward flow of the descending sap, which will be therefore
forced to accumulate in the situation described. The reader may perceive
that the origin from whence these two kinds of sap are derived, their
chemical composition, the part of the vegetable through which they pass,
the causes which produce the ascent of one and the descent of the other,
together with the uses of both in the vegetable economy, are questions of
great interest, as well to the farmer as the horticulturist.
The source from whence the ascending sap is derived is the aliment
absorbed by the roots from the soil. This aliment consists essentially of
two substances; one of these being sufficiently familiar, namely, water;
and the other commonly existing in the atmosphere under the form of gas
or air, but likewise capable of solution in water, namely, carbonic acid;
this substance is known to every one as the cause, by its escape, of
the boiling appearance seen in freshly uncorked soda water. Those two
substances constitute the necessary aliment of vegetables: at the same
time it is notorious that various matters, such as manures, earths, &c.,
greatly facilitate the growth of plants; but these matters produce this
effect either by supplying a greater quantity of carbonic acid, or by
acting in a manner similar to condiments; for in the same way as spices
taken into the stomach along with food invigorate the digestive power, so
do many minerals, when absorbed by the roots, operate in promoting the
nutrition of vegetables.
The chemical composition of the ascending sap is chiefly a solution of
sugar and gum in water. In the northern states of America, sugar in
large quantities is obtained from some species of maple, principally the
sugar maple and swamp maple of Canada, by boring the stem, collecting
the ascending sap which flows from the wound, and evaporating away its
watery portions. It is an interesting question, from whence proceed the
sugar and gum contained in this ascending sap? The only satisfactory
reply to this question is, that these substances become formed out
of the water and carbonic acid absorbed from the soil; but this is a
transformation which cannot be effected by the most expert chemist, so
that we find in this, as in many other instances, a living body is a
laboratory in which Nature executes changes far transcending the loftiest
efforts of man’s ingenuity.
The part of the vegetable through which the sap ascends can be easily
shown in any of the ordinary trees of this country. If a branch from a
currant shrub be placed with its inferior and newly cut surface immersed
at first in a solution of green vitriol and afterwards in an infusion of
nutgalls, the course through which these fluids ascend may be traced by
the black colour produced by their mixture; for every one knows that a
mixture of green vitriol and nutgalls produces ink, and in the experiment
just described, the solutions of these substances following each other
in their ascent, inscribe in a manner on the interior of the branch the
path which they successively pursued. This course will be found to exist
between the bark and the pith, these parts being quite unchanged, while
the intermediate portion of wood will be deeply coloured.
The causes which produce the ascent of the sap are of a very powerful
nature. The celebrated Hales ascertained that a vine branch, in a few
days, sucked up water with a force equal to the weight of sixteen pounds
on the square inch: this was a power greater than atmospheric pressure;
and when it is recollected that the pressure of the atmosphere is capable
of lifting thirty-three or thirty-four feet of water in a common pump,
some estimate may be formed of the force with which the sap ascends. This
ascent appears to be produced by the influence of two causes: the one,
a quality peculiar to living beings, by which the buds in common with
all growing organs are capable of attracting or sucking towards them the
juices necessary for their nutrition; and in agreement with this, the
sap is found to ascend in the first instance near the buds: the other, a
general property of all matter which has been but lately discovered. This
latter property, which has been called endosmose, is found to operate
when two fluids of different densities are separated by a membrane.
Under these circumstances, and in obedience to an attraction for each
other, both fluids pass through the membrane, and mix together; but the
denser and thicker fluid finding a greater difficulty to penetrate the
membrane than the lighter and thinner, consequently passes through in
less quantity. To illustrate this, let us suppose a bladder containing
a little syrup, and placed in a vessel of water, and we will have the
conditions necessary for endosmose: the syrup and water will both pass
through the bladder in opposite directions, but a greater quantity of
water will pass into the syrup, than of the latter into the water. It
will be evident to the reader that this excess of thin liquid passing
into the denser will constitute a force or power which will require
an equal force to neutralise it; and it has been ascertained that the
tendency of water to penetrate a membrane for the purpose of mixing with
a syrup of once and a third its own specific weight, required a force
equal to sixty-three pounds on the square inch to overcome it. Now, a
plant growing in the ground is similarly circumstanced to the bladder
in this experiment: its roots furnished with extremities of spongy
membrane are interposed between thin water and carbonic acid externally,
and a syrupy solution of sugar and gum internally. Now, under those
circumstances we need not be surprised if an endosmose should operate,
abundantly sufficient to elevate the sap with a force even greater than
that determined by Hales.
The use of the ascending sap in the vegetable economy is the last subject
which we shall consider in this article. On a future occasion we shall
endeavour to show that it is out of the ascending sap that the descending
or elaborated sap is chiefly formed; but besides this utility of the
ascending sap, as the source of the descending sap, the former has
special functions of its own to perform. If we inquire what period of
the year is the ascending sap in greatest quantity, we shall find it to
be during the spring season. Now, this is the time when the buds become
pushed out into branches, and the young leaves peep forth: the roots also
during this season increase in thickness. Another means which we possess
of ascertaining the uses of this sap, is by protecting plants from the
influence of light: in total darkness no elaborated sap is ever formed;
therefore, whatever vegetation may then take place, must be solely at the
expense of the ascending sap. Under such circumstances the plant becomes
very succulent, its stems grow to a great length, no vegetable fibre can
be detected in its substance, its colour is blanched, it possesses no
bitter or aromatic properties, and it does not develope flowers. Potatoes
growing in a dark cellar, or celery protected from the light, by earth
heaped around its foot-stalks, will afford familiar examples. These
considerations lead us to the belief that out of the ascending sap is
formed the fleshy part of vegetables, which, by its production, increases
the length of the stem, and the thickness of the roots. In our next
article we will describe the most remarkable properties of the ascending
sap.