DESCENDING SAP
“Ascending sap, a liquid composed of a large quantity of water and a
very small proportion of dissolved nutritive substances, is absorbed in
the ground by the roots and carried to the leaves through the sap-wood.
It is not yet a nutritive fluid for the plant; it becomes so in the
foliage by a double process. First, on being distributed to the leaves,
which furnish a vast surface for evaporation, it exhales its
superabundant water in the form of vapor and thus concentrates its
usable ingredients. Then, under the influence of the sun’s rays and
through the medium of the green matter contained in the leaves, it
undergoes modifications that work a fundamental change in its
character. Among the processes here taking place, one of the best known
is the decomposition of the carbonic acid gas taken from the air by the
leaves and from the soil by the roots.
“We have seen that this gas, the plant’s chief source of nourishment,
is composed of carbon combined with the breathable part of the air, or
oxygen. Under the action of the sun’s light the leaves decompose this
gas, liberating the oxygen in a condition henceforth fit for the
respiration of animals and for combustion, while the carbon remains in
the plant, mixes with the substances brought by the ascending sap, and
with them becomes the nourishing liquid, the descending or elaborated
sap, from which all future parts of the plant are to be formed. This
liquid cannot be called wood, bark, leaf, flower, or fruit; it is not
at all like any of these, and yet it is essentially a little of them
all. An animal’s blood is neither flesh, bone, nor fleece; but bone,
flesh, and fleece are of its substance. Likewise the elaborated sap is
a liquid designed for the sustenance of all parts of the plant; it
contains matter for fruit and wood, leaves and flowers, bark and buds.
It is the plant’s blood; everything in the plant gets from it its
nourishment, its wherewithal to develop. What a wonderful, what an
incomprehensible process its production appears to us! In the crowded
ranks of the leaf-cells, where one would suppose everything to be at
rest, what activity, what transformations beyond the reach of human
science! Liquids swell the cells, ooze from one to another, transpire,
infiltrate, circulate, exchange their dissolved substances; vapors are
exhaled, gases come, others go; the sun’s light separates what was
united, unites what was separated, and the raw materials of the
ascending sap combine henceforth with the materials of life.
“The elaborated sap descends from the leaves to the twigs, from the
twigs to the branches, from the branches to the stalk or trunk, and
from the latter to the root, distributing itself here and there on its
way. It circulates between the wood and the bark. It is this sap that,
in the spring, when it is in great abundance, forms between the wood
and the bark a thin layer of slightly viscous moisture and makes the
bark easy to peel from its branch. Which of you in the month of May has
not taken advantage of this peculiarity to peel off all in one piece a
tube of bark from a very smooth twig of willow or lilac in order to
make a whistle, trumpet, or other noisy plaything, the delight of boys
of your age?
“Nothing is easier than to prove the passage of sap from above
downward. If you remove from a tree-trunk an annular band of bark, the
nourishing liquid oozes and accumulates at the upper edge of the wound,
but nothing of the sort takes place at the lower edge. Arrested thus by
a break in its path, the sap accumulates above the uncovered ring and
causes there an abundant growth of wood and bark, which piles up in the
form of a thick circular swelling, while below the ring the trunk
preserves its former size.
“A tight ligature, by compressing and obstructing the passages through
which the nutritive fluid has to pass, causes the formation of a
similar swelling above the line of stoppage. You may have seen a
sapling, bound too tightly to the stake intended for its support,
strangled by its own growth if the gardener has forgotten to loose the
band in time. Little by little the trunk swells above this band, which
is finally overgrown by the bark and even hidden within its substance.
Indeed, it is not rare to find a tree with its trunk caught fast in a
narrow passage, as for example in the crevice of a rock, and swollen
above the obstacle into an unsightly excrescence. The stoppage of the
sap in its downward course explains this phenomenon.
“If the tree-trunk is not completely encircled by the stricture, if
somewhere there is a strip of bark left free to serve as a passage, the
nourishing juice takes this way to get around the obstacle, and so
pursues its course to the roots. Then the tree continues to live. But
if the barrier is absolutely insuperable, as in the case of an
unyielding ligature or when the tree has been girdled, the sap cannot
descend to the roots to nourish them; and with the death of these the
end of the tree is not far distant.
“An important lesson remains to be drawn from these details concerning
the circulation of this nutritive liquid in plants. Henceforth, when we
fasten a plant to its prop or supporting stake, we shall be careful not
to tie the string too tight or else to loosen it at the proper time,
since otherwise we should run the risk of strangling the plant and so
causing its death.”