THE STALK OF THE PLANT
“The stalk is the common support of the plant’s various parts. It is
called annual or herbaceous when it lives only one year, as in the
potato, spinach, parsley, and all forms of vegetation that from their
soft structure belong to the class of herbs. Ligneous is the name given
to the stalk when, designed to live for a greater or less number of
years, it is made of strong woody fibers, such as we find in the trunks
of trees.
“Let us make a clean cut through any tree-trunk, that of an oak for
example. We shall find it divided into three parts: in the center the
pith or marrow, very slightly developed; around the marrow the wood
proper; and, finally, on the outside, the bark. A closer examination
shows that the wood is formed of concentric layers which are indicated
in the cross-section by a series of circles having the marrow for a
common center. These layers are called ligneous zones or, since one is
formed every year, annual layers. During the summer there is a downward
flow, throughout the tree, of a peculiar liquid, the descending sap,
which constitutes the fluid nourishment of the tree. This liquid runs
between the wood and the bark and becomes, little by little in its
course, on one side a layer of wood which attaches itself to the outer
surface of the preceding year’s layer, and on the other side a thin
sheet of bark which is added to the inner surface of the bark already
formed.
“Thus each year both bark and wood form a new layer; but this added
layer is applied in opposite ways in the two instances,—outside on the
wood, inside on the bark. The wood thus encircled from year to year by
new layers increases in age toward the center and becomes younger and
younger toward the circumference, whereas the bark, lined every year
with a fresh sheet, shows its youth on the inside and its age on the
outside. The first buries inside the trunk its decrepit and dead
layers; the second thrusts its old layers outside, where they crack and
fall off in large scales. This aging process is simultaneous on the
outside and in the center of the tree-trunk; but between the wood and
the bark life is always at work, creating fresh accretions.
“Here are some experimental proofs of this annual formation of a
ligneous layer. A strip of bark is removed from the trunk of a tree,
and on the wood thus laid bare is fastened a thin sheet of metal. The
bark is then replaced and bound with ligatures so that the wound may
heal. We will suppose ten years have passed. The bark is raised again
at the same place. The metal sheet is no longer visible; to find it you
must bore deep into the wood. Now, if you count the ligneous layers
removed before reaching the metal sheet, you will find precisely ten,
just the number of years that have passed.
“A number of observations like the following are familiar: Some
foresters cut down a beech bearing on its trunk the date 1750. The same
inscription was found again in the inner substance of the wood, but to
reach it they had to cut through fifty-five layers on which no mark
whatever appeared. If now, we add 55 to 1750 we obtain precisely the
year when the tree was felled, or 1805. The inscription carved on the
trunk in the year 1750 had passed through the bark and reached the
layer of wood that was then outermost. Since that event fifty-five
years had passed and new layers, exactly the same in number, had grown
over the first.
“Thus a tree is composed of a succession of woody sheaths, the outer
ones enveloping the inner. The stem or trunk contains them all; the
branches, according to their age, contain more or fewer. Each one
represents a single year’s growth. The woody sheath of the present year
occupies the exterior of the trunk, immediately under the bark; those
of former years occupy the interior, and the nearer they are to the
center the older they are. The layers of future years will come one at
a time and take their places over preceding layers, so that what is now
the outermost layer will in its turn be found embedded in the body of
the trunk.
“Of all these ligneous zones of unequal age the most important to-day
is the outside one; its destruction would cause the death of the tree,
since through it the nutritive juices of the earth reach the buds,
leaves, and young branches. In their time the interior layers, one by
one, when they formed the surface, rendered the same service to the
buds of their day; but now that these buds have become branches the
inner layers have only a secondary office, or even none at all. Those
nearest the outside still have some aptness for work and help the layer
of the year to carry the juices from the earth to the branches. As to
the innermost ones, they have lost all activity; their wood is hard,
dried up, encrusted with inert matter. In their decrepitude these
interior layers are incapable of service in the work of vegetation; the
most they can do is by the support of their firm woody structure to
give solidity to the whole. Thus the tree’s activity decreases from the
outside toward the center. On the surface are youth, vigor, labor; in
the center old age, ruin, repose.”