In order to understand this subject we must first ascertain the
conditions under which sap is first produced, what it is, and how it
circulates.
To do this we must first know something of the structure of those parts
of the tree which serve as channels, or ducts, and those other parts
which gather the sap and dispose of the waste after it has completed
its mission.
To begin with, the tree is composed of small structures, too small for
the naked eye to distinguish. Each structure is, at least for a time,
a whole in itself, containing solid, semi-solid, and fluid parts which
differ in their chemical nature. These structures are the cells, and
when a large number of them are united in close contact they form a
cellular tissue through which the sap passes from the roots to the
leaves, and from the leaves to the growing parts of the young tree, or
shoot.
This cellular tissue is superseded by another tissue which is much
stronger and which takes up the work of the cellular tissue, when the
tree becomes too large to be supported by the weaker form. It is more
solidly formed and is composed of elongated cells which are joined
together in a series with their ends overlapping. This is known as
woody fiber. The cellular tissue now exists in the tree stem only in
the pith, and in the medullary rays which we see in the grain of any
hard wood, radiating from the pith.
With the statement, then, that these tissues form the timber, and that
the bark and roots only present a modification of the same structures,
we will pass to the tree as we see it with the naked eye.
If we saw the trunk of a tree, of any considerable size, squarely
in two, we find three forms which differ in solidity, rigidity, and
appearance; namely, the heart-wood, sap-wood, and bark. The heart-wood
is the firm, solid wood surrounding the center of the tree, the
sap-wood is the softer wood outside the heart-wood, while the bark
forms the skin or outer covering for the whole.
Trees grow from the center outward, hence the present sap-wood will in
time become heart-wood and be covered by a new layer of sap-wood, and
the present heart-wood is simply sap-wood which has become solidified
by the deposit within its tissues of resinous and other matter secreted
by the tree. It is now useless for sap-carrying purposes and seems to
exercise only the function of supporting the tree in its position. It
is through the outer, younger layer or sap-wood that the sap ascends.
Now, if we examine the end of our stick more closely we see a series of
rings, clearly marked, circling from the center of the tree and ranging
in size from the tiny one which encloses the pith, to the large one
which forms the outer surface next to the bark. They are caused by a
constant annual deposit and outward growth, by which a layer is added
to the outer surface of the sap-wood each season. Hence, by counting
these we may determine the age of the tree. Less distinct rings may
appear but they will not deceive us as we know that they are caused by
a cessation of growth, which may have been caused by drouth.
As a general rule these rings are more distinct in trees inhabiting a
climate where vegetation is entirely suspended by the cold after each
layer is formed. In warmer regions they are not so distinct. This is
especially interesting when we study fossils of trees which in many
cases show a great difference in climatic conditions in the early ages
from those we have at the present time.
The layers of bark are much thinner than those of the wood and are not
so readily distinguished. They are formed from the interior so that the
oldest are on the outside. The older ones fall off, however, so that we
cannot trace as many rings in the bark as we can in the wood, although
one is formed in each for every season that the tree lives.
The roots of the tree spread out underground and are the agents through
which the tree derives most of the moisture so necessary to its growth.
They absorb moisture only at their extremities and usually spread to
just such an extent that the water which falls off the outer branches
of a tree during a rain, falls exactly where the tender rootlets can
gather it up at once and hurry it back up the trunk of the tree. In
ground that is springy, or naturally moist, the roots do not depend so
much on the rainfall but reach out after moisture wherever it exists in
the soil.
Spring seems to give a new impulse to life, especially to vegetable
life, which always responds promptly to the genial rays of the sun.
During the winter, in our climate, the cells which form our trees are
contracted by the cold and when the warm days cause them to resume
their natural size, a small vacuum is formed in each cell, which the
first warm days proceed to enlarge by thawing only the trunk and
branches of the tree, leaving the roots below embedded in frozen
soil from which but little moisture can be drawn, while evaporation
draws moisture from the trunk and branches with irresistible force. A
warm rain now comes, thaws out the soil, and sets the juices therein
contained in motion. An immediate rush of sap up the trunk of the
tree is the result. It clears out the pores or channels, as a spring
freshet clears out the water courses, it rushes into the branches, and
the branches rejoice and put on their livery of green; it rushes out
through the porous surface of the limbs and rises in the air in the
form of vapor, while that which does not escape becomes charged with
life and returns down a devious pathway and lays the foundation for
another season's growth.
But why should the sap ascend the tree?
This is only one of many questions that the tree will not answer and no
one else ever has answered. If we take a strip of blotting-paper and
insert one end of it in an ink-well, the ink immediately begins to
climb up the blotting-paper by means of the force known as capillary
attraction. Here, says the seeker for truth, is the reason for the
ascent of sap, and many profound authors have agreed that he is right.
Others claim, however, that he is wrong, while still others think he
is only partly wrong and that this force has something to do with
it. If we cut the roots from a tree and insert the stem in water we
will soon find that this force is not the sole cause for the ascent
of sap. Another student has made experiments with the force called
diffusion, and claims that this explains the rise of sap to such
remarkable heights; but diffusion does not work fast enough and hence
must be thrown aside. Another finds that water is imbibed through fine
porous substances with great force and that air can thus be compressed
to several atmospheres, and this force is affirmed to be the one at
work in our trees. But the fact that the amputation of the leaves and
branches checks the ascent is brought forward and this theory falls
to the ground. The fact that liquid films have a tendency to expand
rapidly on wetable surfaces was next advanced, but the objection to the
first theory met it at once.
Another interesting theory is now brought forward and has the advantage
of practical demonstration, that is, an artificial model was made
through which water ascended. It is based on the principle that water
will pass through moist films that air will not penetrate, on the fact
that evaporation takes place under right conditions with force enough
to cause something of a vacuum, and also on the elasticity of the cells.
The model was constructed of glass tubes, closed at one end with
a piece of bladder, and joined together in series by means of
thick-walled caoutchouc tubing; the top which represented a leaf was a
funnel closed by a bladder. This artificial cell chain was filled with
water, mixed with carbolic acid to keep the pores from clogging, and
was set up with its base immersed. The fluid evaporated through the
membrane at the top of the funnel, which drew up more from the cells
below, the space so caused being continually filled from the base. This
is an interesting experiment and is said to solve the question, but it
is open to the same objection, that a tree will not absorb fluid and
carry it for any length of time after the roots are cut off. I regard
it, however as a long stride in the right direction.
To what source, then, must we look for an explanation of this process?
I think it is a fact that the small, new root-fibers imbibe fluid
with considerable force, but it is undoubtedly a fact that they soon
lose this force when deprived of the leaves; that the leaves with the
aid of evaporation, exert a great force, which the above experiment
plainly indicates; and I cannot consistently dismiss the idea that
capillary attraction has something to do with it. If we also add to
this the theory that the swaying of the stems and branches by the wind
is continually changing the shape and size of the cells and is thus
driving the juices wherever an opening will allow them to travel, thus
bringing the elasticity of the tree to our aid, we have again advanced.
But the principle of life is not discovered. Whenever it is we may find
it to be a force much greater than any we have so far examined, and
which may even cause the overthrow of all theories heretofore advanced.