It is a very commonly received idea that the sap of trees descends in
autumn, and when the leaves fall, returns to the roots whence it came in
the spring. It does not seem to have once occurred to those who accept
this view of the matter, that there would be any difficulty in cramming
a quart of water into a pint cup; perhaps, in order to accommodate the
sap, the roots are supposed to be as large and capacious as the trunk
and head of the tree; in short, that there is as much tree underground
as above. Whence the idea sprang into existence that the sap retires to
the roots on the approach of winter, it is not now possible to say, but
its very general popular acceptation is an evidence of the way in which
false views gain currency because some one ventures to make the
assertion, and the public mind stops not even to weigh the
probabilities, much less to investigate the ground upon which it rests.
Surely the time has come when we should cease to accept assertions upon
trust, and demand the facts on which they are based. Having these, we
can consider the theory, and if it does not suit us we can make one for
ourselves.
Now, in this matter of the sap, by which we mean all the fluids which
are contained in the interior of a tree, the facts are these: if the
trunk be cut in spring, the sap will run out; in summer, autumn, and
winter it will not, except under exceptional circumstances. But
nevertheless the sap is in motion in the summer and autumn, and winter
too; nay, save when extreme cold may for a time interfere with its flow,
it is always in motion; and the reason why it runs out of the trunk in
the spring is because it is then present in much greater abundance than
at any other season of the year. During the summer, when the tree is
covered with foliage, the leaves are evaporating large quantities of the
fluid parts of the tree into the air, while another portion is being
elaborated and converted into the tissues and structure of the tree,
producing what we call growth. When the autumn has come, what with the
evaporation and solidification that has taken place, the interior of the
tree has become comparatively dry, so that the quantity of sap has
become so greatly diminished that it no longer exudes when an incision
is made. Our readers are, at least many of them, aware that if a branch
be cut off from a grape vine in spring when the buds are starting, the
sap will run out quite freely, producing what is called bleeding; but if
the same branch were allowed to remain until the leaves on the vine have
became fully expanded, then if it be cut off no bleeding will take
place. The reason is, that the evaporation which is taking place in the
leaves has exhausted the supply of sap to such an extent that there is
no surplus in the vine to escape in that way.
The leaves being the principal organs of assimilation and perspiration,
it follows that when they have fallen off there is no longer much loss
of fluid to the tree from these causes. But the power of the roots to
absorb moisture from the earth is not diminished by the loss of the
leaves; they continue to draw fluid from the earth, and to send it up
into the tree. This action continues, except as modified by extreme
cold, all winter; the fluids are drawn from the soil by the roots and
sent into the tree, and by the time that spring has come the tree is
full of fluids and every vessel distended with sap. During the winter we
are not able to find sap by cutting the tree, because the process of
filling with fluid is gradual.
M. Biot, many years ago, made some very interesting experiments on the
flow of sap, and made a contrivance by which the rate of motion could be
measured at any season, and showed that there was considerable activity
even in winter. He found that the direction in which the sap moved was
very considerably affected by frost. When the weather was mild the sap
was always ascending; but when it was freezing weather the sap flowed
down. This he attributed to the contraction of the sap-vessels by the
cold, which forced the sap into the larger vessels which were unaffected
by the frost under ground. When, however, the frost was sufficiently
severe and continued to reach the roots, then the sap was forced back
into the trunk; but when it came on to thaw and the frost left the
ground, the sap returned to the roots. Thus we see that, as a rule, the
sap is always ascending, and that when it descends it is because it is
forced to do so by some temporary cause, and when that cause ceases to
act the sap immediately begins to ascend again.
In connection with the supposed ascent of the sap in spring, and growing
out of it, is the popular idea that this ascent of the sap is the cause
of the expansion of the buds and leaves. It would be nearer the truth to
say that the expansion of the buds and leaves was the cause of the
motion of the sap. Any of our readers can make the following experiment
for themselves, and see the true state of the case. If a tree be cut
into or tapped in some of the upright branches near the top very early
in the spring, and be again tapped just below the branches on the trunk,
and again just above the surface of the ground, it will be found that
the sap will flow from the wound that is nearest to the top first, from
the one just below the branches next, and last of all from the one near
the ground. The reason for this is, that the light and warmth tell first
upon the excitable buds at the extremities of the tree, and therefore
the sap is set into an accelerated motion that lies nearest to them. The
gentleman who first made this discovery came to the conclusion that in
the spring the sap of trees descended instead of ascending, but he
forgot that the moment the buds begin to expand they draw the sap from
the parts nearest to them; this causes the sap just beyond to push
upwards to supply the place of that taken up by the buds, and as the
buds increase and expand is this absorption increased, and the circle of
motion enlarged from the buds downwards.
Were it not true that the sap is constantly ascending, we should lose
all our evergreens during the very first winter, for the evaporation
that goes on in winter from their leaves would soon season the wood of
the tree were it not made good from the roots below; and hence it is
that some evergreens are killed by a severe winter while they are small,
which would survive without injury had they attained to a greater size;
for, being small, the roots have not penetrated to sufficient depth to
reach below the frozen ground, and consequently can not draw from the
frozen earth in sufficient quantity, nor with sufficient rapidity, to
supply the waste by evaporation; whereas when larger the roots will have
penetrated quite below the reach of frost, and will be able to draw from
the soil sufficient moisture to supply the loss.
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