One more word before we part about the last of the eaters, about
Vegetables. They will furnish you with a new and very clearly marked
proof of the uniformity of the fundamental conditions to which the
Author of life has subjected all organised beings.
Let us look once more at this oak, of whose manner of growth I was
obliged to give you a sketch beforehand, in order to show you the ties
which unite it with its immediate neighbors in the animal kingdom. How
does it feed? I need not tell you this. It feeds by its roots, which
suck up in the bosom of the earth the water charged with the juices
which form its nourishment. Are you aware that every large branch had
its subterranean fellow or representative, and that the annual shoot
at the top of the tree is reproduced at the base by fresh fibres, which
extend themselves in the soil of the earth, in proportion as their
sisters above make their way in the air? And thus, by means of organs
ever young, the life and progress of the great association is kept up,
while those members whose day of work is over still remain there as
the supports of the edifice. It is the same with human societies. They
are sustained by what is old, but they live and progress only by what
is young. The sap, then, which is the name given to the moisture or
water sucked in by the young roots, having once got into the cells of
which the tissue of the fibres is composed, passes from one to another,
and travels thus to the top of the tree, where it is wanted by the
leaves.
There is no obvious machinery here, however, to impel it forward. It
journeys on of itself, as it were, under the action of laws which have
never been satisfactorily explained, but all of which are dependent
on the vital force or life-power of the tree, inasmuch as without it
there is no circulation. One agent, but by no means the principal, or
it would act as well in a dead tree as a living one, is _capillary
attraction_; and, if you wish to know what that is, you have only
to think of what happens to a towel, if you hang it upon a peg, and
leave the end of it soaking in water. Does not the "wet" seem to climb
up it thread by thread, till it is damp from one end to the other? A
little in this way--but these similes are very imperfect, and will not
bear close application--the sap rises in a tree, stealing up branch
by branch; and it is then called _ascending sap_. [Footnote: M. Macé
speaks of this sap as the _blood of the tree_, and of the leaves only as
_lungs_. These statements have been modified so as to meet the fact that
_ascending sap_ consists of, and conveys the raw elements of _food_ to,
the leaves; that in the leaves this food is _digested_, as well as
brought in contact with the air, and that it is thus converted into that
nourishing fluid, the _descending sap_, which certainly plays the part
of steward to the tree as our blood does to us, and therefore may now be
called the blood of the tree. It must be remembered, however, that each
tree has its own sort of steward, as the case of the _Euphorbia_ (quoted
afterwards) plainly shows. The analogy with the more general substance
of blood is therefore not very complete.-TR.]
It arrives at last at the leaves, which it enters as our food enters
our stomachs, and for the same purpose; for in them takes place, as
in all true stomachs, that process of digestion by which the elements
of the crude sap-food are decomposed from their first condition, and
converted into a nourishing chyle; in each tree of a sort "after its
kind."
But more than this. Like the outer coat of the earthworm, the coat of
the leaf affords a passage to air and moisture through its surface;
and here, therefore, takes place that mysterious exchange which is
everywhere the essential condition of life. Here is the charcoal-market
as before, only the bargainers have changed parts. The air, which in
the other case received the _carbon,_ delivers it up, now, and
receives oxygen in exchange; exactly the reverse of its traffic with
animals. In other words, the tree inhales through its leaves the
carbonic acid gas thrown into the atmosphere by our lungs. On its own
responsibility it breaks through the alliance between the carbon and
oxygen contracted in our organs; keeps the carbon for its own use, to
restore it to us another day under the form of wood, or, by the aid
of the charcoal-burner, in the pure and simple state of charcoal; and
sets at liberty the oxygen, which once more goes off in search of new
lungs and a fresh alliance. Thus a constant equilibrium is maintained
in the atmosphere; and thus, by a system of perpetual rotation or
everlasting merry-go-round, the same substances serve, indefinitely,
to support life of every opposite description.
Now there are two things to be remembered in this inverted respiration
of vegetables. In the first place, it occurs only in the parts which
are _green_. Flowers, fruit, the root, and every part of any other
color, do as we do when we breathe; _i.e._ deprive the air of its
oxygen, charging it with carbonic acid instead. For which reason,
by-the-by, we ought not to keep flowers in a bedroom at night. Charming
as they are, they are _poisoners_, and a headache is what we may
fairly expect after sleeping shut in with them in the same room. It
is almost as bad to allow green boughs to remain there either, for,
in the dark, even the green parts cease to purify the air, and begin
like the others to manufacture carbonic acid, at the expense of course
of their carbon, which thus by degrees is used up. Now, as it is the
carbon which constitutes the solid fibres of plants and produces their
green color, they soon become yellow and limp when deprived of light.
You may, perhaps, have wondered why the gardener amused himself with
smothering his poor lettuces by tying them up at top like a knot of
"back hair," instead of letting them grow freely in the air and
sunshine. It is, my dear, to make them more tender and delicate for
you to eat; and those beautiful, crisp, yellow leaves, so delicious
to the tooth, would have been green and tough, had they not slowly and
quietly let out a great portion of their store of carbon in darkness
during the last few days, before being gathered. Even without playing
the gardener, you may assure yourself of this fact in a still more
simple manner. Put a flat board upon the lawn and leave it there for
three days; then take it up again, and you will find just where the
board has prevented the light from reaching the grass, a yellow mark
so distinctly traced as to be seen from the other end of the garden.
But to return to the sap, which we left undergoing a change from air
and solar influences in the leaves. The ascending sap was to all
appearance only clear water. When it returns from the leaves, charged
with carbon, it is a thick juice having almost the consistency, and
sometimes even the color of milk, and is possessed of properties
altogether new. The most striking example that I can give you of
thedifference of the two states of sap is the Euphorbia of the Canary
Islands, whose digestive or descending sap is a violent poison. When
the natives of the country are accidentally pressed by thirst, they
carefully remove the bark in which the fatal juice circulates, and are
then able to refresh themselves safely by sucking the stem, which
yields only the watery sap sucked from the ground, and as yet unaltered
and harmless.
Each of these two saps, in fact, has its path distinctly traced for
it: the first rises through the wood, the second descends through the
bark, whence it is called descending sap. If you wish to satisfy
yourself of this, fasten a rather tight knot of pack-thread round a
young branch, and after a time you will see it pine below the knot and
become swollen above it, an unanswerable proof that the nutritive
juices flowed downward through the bark; for the wood inside the branch
will have been uninjured by the strangling pressure. Remember this,
my dear, when you are playing in the garden, and do not injure the
bark of the young trees your father likes so much to see flourishing.
It is by the bark that they are nourished, and you might even kill
them by treating it too roughly.
And now I must show you how the nutrition is carried on, or, if you
like better, how the tree grows by means of this descending sap. See:
here is a fir tree, which has just been cut down to the ground. Now,
if you like, I will tell you in a moment how old it is. I will even
tell you the age of every branch, little and big ones both, without
making a mistake in a single year; and you know as well as I do that
I am no conjuror. You see these small circles so delicately drawn, as
it were, upon the face of the sawn trunk, each wider than the last,
as if they were composed of a set of tubes, of unequal sizes, fitting
exactly into each other. Now count them; and you will perhaps find
twenty-five; and as each of these circles represents the work of one
year, you will know that the tree is twenty-five years old. In spring,
when the sap begins to move more briskly, it deposits everywhere between
the wood and the bark, from the trunk to the farthest boughs of the
tree, a uniform layer of a thick liquid, which moulds itself exactly
upon the wood already formed. This layer stiffens during the year; it
gets filled with the carbon left in it atom after atom, by each drop
of the descending sap as it goes by, and thus insensibly becoming
organised and hardened. When winter arrives to interrupt the work, it
will have formed two _ligneous, i.e._ woody layers, as they are
called. Of these, one belongs to the wood, and will never move again
so long as the tree lasts, for it will be covered over, and as it were
buried, by the successive layers yet to come; while, on the contrary,
the other (layer) belongs to the bark, and is doomed to find itself
perpetually forced outwards by the fresh layers, which will after a
while insinuate themselves between it and the wood.
It is for this reason that the bark of old trunks of trees is so deeply
furrowed, and that the dry scales may be picked off the surface without
the slightest injury to the tree. It is part of the original bark,
dead long ago. The old wood also is dead inside, and even when it is
altogether gone, the glad youthful branches growing green in the
sunshine will scarcely find it out! This accounts for those oaks which
time has hollowed without destroying, as those of Allonville in
Normandy, in which mass is said, and which is moreover the greenest
tree in the country. But without going so far, who has not seen those
hollow old willows, sometimes pierced with holes letting in daylight,
yet proudly crowned above by a forest of young boughs, as green and
full of vigor as if the trunk were still in its prime? What was dead
has departed, but all that has life in it remains, and that is enough
for the tree.
Need I add that the descending sap, this steward of the vegetable, has
also his workmen to supply with materials, as in our case, and that
he is always falling in on his road with organs, all of which want
different things from him? That here a flower has to be formed, there
a fruit, there a leaf, or a bit of wood, and so on: and that a
mysterious intelligence--the same that we have found everywhere
else--presides over all these varied constructions, the materials for
which are mixed together pell-mell, in the imperceptible thread of sap
which oozes from the leaf to the bark? I recollect just as I am about
to conclude, my dear child, that I once told you, you were a small
temple in which God perpetually attests His presence, by a permanent
miracle. You may now henceforth look upon a tree as something more
than a bit of wood, yielding a pleasant shade. God is in it also.