We endeavoured in our last article to describe the principal
circumstances of interest with respect to the ascending or unelaborated
sap. We have found that it is derived from the aliment which consists
of water and carbonic acid; that it is composed of a solution of sugar
and gum in water; that it ascends in the ordinary trees of this country
through the wood, which is situated between the bark and pith; that
the causes which elevate it are partly a vital attraction or suction
exercised by the buds, and partly an endosmose, by which, in consequence
of its superior density, it draws in its aliment through the spongy
extremities of the roots; that its use is not only to furnish materials
for the descending or elaborated sap, but by developing the fleshy part
of plants to cause the growth of stems in length and roots in thickness.
We shall now proceed to show the origin, the course, the composition, and
the uses of the descending or elaborated sap.
The elaborated sap is formed out of the ascending sap. The place where
this change takes place is in the leaves and green parts of vegetables;
it is generally in the spring season that the ascending sap pushes
out the buds into branches, and developes the little scales which
had surrounded these organs into leaves; but when these leaves are
formed, the sap continues to ascend into them, and there undergoes
those alterations from whence the elaborated sap results. Now, these
alterations consist in the getting rid of all superfluous water and
carbonic acid, which, originally absorbed as aliment, had not undergone
the conversion into gum and sugar during the ascent of the sap; secondly,
in the acquisition of additional nutriment from the atmosphere; and,
thirdly, in the conversion of these substances into a variety of new
compounds.
Let us examine each of those changes to which the ascending sap is
subjected, in succession; and, first, with respect to the disengagement
of superfluous water and carbonic acid, every one must have observed
drops of water collected on the leaves of cabbages and other vegetables,
when examined early in the morning. These are commonly supposed to
be dew-drops, but are truly in great part the result of a kind of
perspiration which is always taking place from the surface of plants.
That this is the case, can be proved by covering a cabbage-plant with a
bell-glass, and placing it in a room sufficiently heated to prevent the
deposition of dew, when drops of water will be found equally to collect
upon its leaves. These drops are not observed during the day, because the
temperature is then commonly so high as to evaporate them as fast as they
are transuded; but the fact is, that plants actually give off much more
water during the day than night. The escape of carbonic acid is not so
easily detected as that of water; it can, however, be proved, through the
resources of chemistry. Unlike water, which is liberated both night and
day, and indeed in greatest quantity during the latter period of time,
carbonic acid is found to be disengaged during the night only. As long as
plants are exposed to the light of the sun, their green parts liberate
none of this gas.
We have mentioned that when the ascending sap arrives into the leaves,
it not only throws off superfluous water and carbonic acid, but likewise
derives an additional quantity of nutriment from the atmosphere. The
presence of light is necessary for this latter circumstance to take
place. The nutriment which, under the influence of sunlight, it acquires
from this source, is a substance named “carbon;” this substance is a
constituent of carbonic acid, which is indeed composed of carbon and
oxygen; carbonic acid is contained in the atmosphere in the proportion of
one part in a thousand; the green parts of plants absorb it, and under
the influence of light decompose it; the carbon is retained, but the
oxygen is again liberated. We now may perceive the reason of the fact
mentioned in the preceding paragraph: plants give out no carbonic acid
during the day, because the superfluous carbonic acid of the ascending
sap becomes decomposed under the influence of light, in the same way as
that which has been absorbed from the atmosphere.
A great many compound products are obtained from the vegetable kingdom.
We need merely recall to the reader’s recollection starch, resin,
camphor, bland and aromatic oils, bitter principles, colouring matters,
the acids of the grape, the lemon, and the apple, &c. to assure him of
this truth. All these different substances form themselves out of the
sugar and gum of the ascending sap, together with the carbon absorbed
under the influence of light.
When the ascending sap has parted with its superfluous water and carbonic
acid, when under the influence of light it has absorbed carbon from the
atmosphere, and when its constituents arrange themselves anew, so as to
produce some or all of the substances above enumerated, its name as well
as its functions cease: it has now become the descending or elaborated
sap.
Let us now inquire the course which the descending sap pursues. We have
stated in our last article, that if a ligature be twisted tightly round
a branch of one of our common trees, the portion immediately above the
ligature will become swollen, while that beneath it will retain its
former thickness. If instead of a ligature we remove a circular ring of
bark, the same phenomenon will take place: the part above this annular
incision will swell out on every side. From this experiment we derive
several important inductions. We learn from hence that this kind of sap
descends, and moreover that the channel which conveys it is the _bark_.
Having ascertained the course which the elaborated sap pursues, let
us now turn our attention to its _composition_. This is found to vary
in different plants: thus in some, bitter principles are the chief
constituents; in others, aromatic substances; in others it is principally
resinous; but whatever may be the principal components, they may always
be divided into two groups--namely, those which are subservient to the
growth of the vegetable, and those which, becoming deposited in the
different organs, confer on them those properties which entitle them to
be employed as articles of medicine or aliment for animals, and by means
of which different plants are in this respect distinguished from each
other. The portion of the descending sap which serves for the growth of
the vegetable, exudes in ordinary trees between the bark and the wood,
forming a glutinous layer which separates these organs, and is the cause
of the facility with which in autumn the bark can be detached from the
stem: this portion is called cambium. In palms, and other trees of warm
climates, there is no bark, and in such vegetables the nutritive part of
the descending sap passes down through the centre of the stem.
The portion of elaborated sap which becomes deposited in the organs,
and which varies more or less in every plant, is called the proper
juice: proper vessels is the name given to the reservoirs which contain
the proper juices; and according to the nature of their contents, the
proper vessels are called milk-vessels, turpentine-vessels, vesicles of
essential oil, &c.
In the foregoing paragraphs we have somewhat anticipated the uses of
the descending sap: we have found that one portion of it is destined
for the nutrition of the vegetable. Now, the same means which revealed
to us the uses of the ascending sap, will also tell us how far the
elaborated sap is concerned in vegetable nutrition. In the dark no sap
is elaborated, and no vegetable fibre is developed. Are we not therefore
justified in supposing that vegetable fibre is formed out of this
elaborated sap? Again, let our readers call to their remembrance the
experiment of tying a ligature around a branch: in that experiment not
only does a considerable swelling take place above the ligature, but
from this swollen portion cereal roots frequently protrude. These facts
afford us a clue to the uses of the descending sap, for by developing
vegetable fibre, it increases the thickness of the stem and the length
of the roots, just as the ascending sap, by developing vegetable flesh,
lengthens the stem, and enlarges the root in diameter.