THE STEM--KINDS AND FORMS; PRUNING
=The Stem System.=--The stem of a plant is the part that _bears the
buds_, _leaves_, _flowers_, _and fruits_. Its office is _to hold these
parts up to the light and air_; and through its tissues the various
food-materials and the life-giving fluids _are distributed to the
growing and working parts_.
The entire mass or fabric of stems of any plant is called its =stem
system=. It comprises the trunk, branches, and twigs, but not the
stalks of leaves and flowers that die and fall away. The stem system
may be herbaceous or woody, annual, biennial, or perennial; and it may
assume many sizes and shapes.
=Stems are of Many Forms.=--The general way in which a plant grows is
called its =habit=. The habit is the _appearance or general form_.
Its habit may be open or loose, dense, straight, crooked, compact,
straggling, climbing, erect, weak, strong, and the like. The roots and
leaves are _the important functional or working parts_; the stem merely
connects them, and its form is exceedingly variable.
=Kinds of Stems.=--_The stem may be so short as to be scarcely
distinguishable._ In such cases the crown of the plant--that part
just at the surface of the ground--bears the leaves and flowers; but
this crown is really a very short stem. The dandelion, Fig. 33, is an
example. Such plants are often said to be =stemless=, however, in order
to distinguish them from plants that have long or conspicuous stems.
_These so-called stemless plants die to the ground every year._
[Illustration: FIG. 53.--STRICT SIMPLE STEM OF MULLEIN.]
[Illustration: FIG. 54.--STRICT UPRIGHT STEM OF NARROW-LEAVED DOCK.]
[Illustration: FIG. 55.--TRAILING STEM OF WILD MORNING GLORY
(_Convolvulus arvensis_).]
Stems are =erect= when they grow straight up (Figs. 53, 54). They
are =trailing= when they run along on the ground, as melon, wild
morning-glory (Fig. 55). They are =creeping= when they run on the
ground and take root at places, as the strawberry. They are =decumbent=
when they lop over to the ground. They are =ascending= when they lie
mostly or in part on the ground but stand more or less upright at their
ends; example, a tomato. They are =climbing= when they cling to other
objects for support (Figs. 36, 56).
[Illustration: FIG. 56.--A CLIMBING PLANT (a twiner).]
Trees in which the main trunk or the “leader” continues to grow from
its tip are said to be =excurrent= in growth. _The branches are borne
along the sides of the trunk_, as in common pines (Fig. 57) and
spruces. Excurrent means _running out_ or _running up_.
[Illustration: FIG. 57.--EXCURRENT TRUNK. A pine.]
Trees in which the main trunk does not continue are said to be
=deliquescent=. _The branches arise from one common point or from each
other._ The stem is lost in the branches. The apple tree, plum (Fig.
58), maple, elm, oak, China tree, are familiar examples. Deliquescent
means _dissolving or melting away_.
[Illustration: FIG. 58.--DELIQUESCENT TRUNK OF PLUM TREE.]
=Each kind of plant has its own peculiar habit or direction of
growth=; spruces always grow to a single stem or trunk, pear trees are
always deliquescent, morning-glories are always trailing or climbing,
strawberries are always creeping. We do not know why each plant has its
own habit, but the habit is in some way _associated with the plant’s
genealogy or with the way in which it has been obliged to live_.
The stem may be =simple= or =branched=. A simple stem usually grows
from the terminal bud, and side branches either do not start, or, if
they start, they soon perish. Mulleins (Fig. 53) are usually simple. So
are palms.
_Branched stems may be of very different habit and shape._ Some stem
systems are narrow and erect; these are said to be _strict_ (Fig. 54).
Others are _diffuse_, _open_, _branchy_, _twiggy_.
=Nodes and Internodes.=--The parts of the stem at which buds grow
are called =nodes= or =joints= and the spaces between the buds are
=internodes=. The stem at nodes is usually enlarged, and the pith is
usually interrupted. The distance between the nodes is influenced by
the vigor of the plant: how?
[Illustration: FIG. 59.--RHIZOME OR ROOTSTOCK.]
=Stems vs. Roots.=--Roots sometimes grow above ground (Chap. VII); so,
also, _stems sometimes grow underground_, and they are then known as
=subterranean stems=, =rhizomes=, or =rootstocks= (Fig. 59).
=Stems normally bear leaves and buds, and thereby are they
distinguished from roots=; usually, also, they contain a pith. The
leaves, however, may be reduced to mere scales, and the buds beneath
them may be scarcely visible. Thus the “eyes” on a white potato
are cavities with a bud or buds at the bottom (Fig. 60). Sweet
potatoes have no evident “eyes” when first dug (but they may develop
adventitious buds before the next growing-season). The white potato is
a stem: the sweet potato is probably a root.
[Illustration: FIG. 60.--SPROUTS ARISING FROM THE BUDS, or eyes, of a
potato tuber.]
=How Stems elongate.=--_Roots elongate by growing near the tip. Stems
elongate by growing more or less throughout the young or soft part_ or
“between joints” (Figs. 48, 49). But any part of the stem soon reaches
a limit beyond which it cannot grow, or becomes “fixed”; and the new
parts beyond elongate until they, too, become rigid. When a part of the
stem once becomes fixed or hard, it never increases in length: that is,
_the trunk or woody parts never grow longer or higher; branches do not
become farther apart or higher from the ground_.
=Stems are modified in form= by the particular or incidental conditions
under which they grow. _The struggle for light_ is the chief factor
in determining the shape and direction of any limb (Chap. II). This
is well illustrated in any tree or bush that grows against a building
or on the margin of a forest (Fig. 4). In a very dense thicket the
innermost trees shoot up over the others or they perish. Examine any
stem and endeavor to determine why it took its particular form.
=The stem is cylindrical=, _the outer part being bark and the inner
part being wood or woody tissue_. In the dicotyledonous plants, the
bark is usually easily separated from the remainder of the cylinder at
some time of the year; in monocotyledonous plants the bark is not free.
Growth in thickness takes place inside the covering and not on the very
outside of the plant cylinder. It is evident, then, that the covering
of _bark must expand in order to allow of the expansion of the woody
cylinder within it_. The tissues, therefore, must be under constant
pressure or tension. It has been determined that the pressure within a
growing trunk is often as much as fifty pounds to the square inch. The
lower part of the limb in Fig. 61 shows that the outer layers of bark
(which are long since dead, and serve only as protective tissue) have
reached the limit of their expanding capacity and have begun to split.
The pupil will now be interested in the bark on the body of an old elm
tree (Fig. 62); and he should be able to suggest one reason why stems
remain cylindrical, and why the old bark becomes marked with furrows,
scales, and plates.
[Illustration: FIG. 61.--CRACKING OF THE BARK ON AN ELM BRANCH.]
[Illustration: FIG. 62.--PIECE OF BARK FROM AN OLD ELM TRUNK.]
Most woody plants increase in diameter by the addition of an _annual
layer or “ring”_ on the outside of the woody cylinder, underneath the
bark. The monocotyledonous plants comprise very few trees and shrubs in
temperate climates (the palms, yuccas, and other tree-like plants are
of this class), and they do not increase greatly in diameter and they
rarely branch to any extent. Consult the woodpile for information as to
the annual rings.
=Bark-bound Trees=.--If, for any reason, the bark should become so
dense and strong that the trunk cannot expand, the tree is said to be
“bark-bound.” Such condition is not rare in orchard trees that have
been neglected. When good tillage is given to such trees, they may
not be able to overcome the rigidity of the old bark, and, therefore,
do not respond to the treatment. Sometimes the thinner-barked parts
may outgrow in diameter the trunk or the old branches below them.
The remedy is to _release the tension_. This may be done either by
softening the bark (by washes of soap or lye), or by separating it. The
latter is done by slitting the bark-bound part (in spring), thrusting
the point of a knife through the bark to the wood and then drawing
the blade down the entire length of the bark-bound part. The slit is
scarcely discernible at first, but it opens with the growth of the
tree, filling up with new tissue beneath. Let the pupil consider the
ridges which he now and then finds on trees, and determine whether
they have any significance--whether the tree has ever been released or
injured by natural agencies.
[Illustration: FIG. 63.--PROPER CUTTING OF A BRANCH. The wound will
soon be “healed.”]
=The Tissue covers the Wounds and “heals” them.=--This is seen in Fig.
63, in which a ring of tissue rolls out over the wound. This ring of
healing tissue forms most rapidly and uniformly when the wound is
smooth and regular. Observe the healing on broken and splintered limbs;
also the difference in rapidity of healing between wounds on strong and
weak limbs. There is difference in the rapidity of the healing process
in different kinds of trees. Compare the apple tree and the peach.
This tissue may in turn become bark-bound, and the healing may stop.
On large wounds it progresses more rapidly the first few years than it
does later. This roll or ring of tissue is called a =callus=.
=The callus grows from the living tissue of the stem= just about the
wound. It cannot cover long dead stubs or very rough broken branches
(Fig. 64). Therefore, in pruning _the branches should be cut close to
the trunk_ and made even and smooth; _all long stubs must be avoided_.
The seat of the wound should be close to the living part of the trunk,
for the stub of the limb that is severed has no further power in itself
of making healing tissue. The end of the remaining stub is merely
covered over by the callus, and usually remains a dead piece of wood
sealed inside the trunk (Fig. 65). If wounds do not heal over speedily,
germs and fungi obtain foothold in the dying wood and _rot sets in_.
Hollow trees are those in which the decay-fungi have progressed into
the inner wood of the trunk; _they have been infected_ (Fig. 66).
[Illustration: FIG. 64.--ERRONEOUS PRUNING.]
[Illustration: FIG. 65.--KNOT IN A HEMLOCK LOG.]
=Large wounds should be protected= with a covering of paint, melted
wax, or other adhesive and lasting material, to keep out the germs and
fungi. A covering of sheet iron or tin may keep out the rain, but it
will not exclude the germs of decay; in fact, it may provide the very
moist conditions that such germs need for their growth. Deep holes in
trees should be treated by having all the decayed parts removed down to
the clean wood, the surfaces painted or otherwise sterilized, and the
hole filled with wax or cement.
[Illustration: FIG. 66.--A KNOT HOLE, and the beginning of a hollow
trunk.]
=Stems and roots are living=, and they should not be wounded or
mutilated unnecessarily. Horses should never be hitched to trees.
Supervision should be exercised over persons who run telephone,
telegraph, and electric light wires, to see that they do not mutilate
trees. Electric light wires and trolley wires, when carelessly strung
or improperly insulated, may kill trees (Fig. 67).
[Illustration: FIG. 67.--ELM TREE KILLED BY A DIRECT CURRENT FROM AN
ELECTRIC RAILROAD SYSTEM.]
SUGGESTIONS.--_Forms of stems._ =43.= Are the trunks of trees ever
perfectly cylindrical? If not, what may cause the irregularities?
Do trunks often grow more on one side than the other? =44.= Slit a
rapidly growing limb, in spring, with a knife blade, and watch the
result during the season. =45.= Consult the woodpile, and observe the
variations in thickness of the annual rings, and especially of the
same ring at different places in the circumference. Cross-sections of
horizontal branches are interesting in this connection. =46.= Note
the enlargement at the base of a branch, and determine whether this
enlargement or bulge is larger on long, horizontal limbs than on
upright ones. Why does this bulge develop? Does it serve as a brace
to the limb, and is it developed as the result of constant strain?
=47.= _Strength of stems._ The pupil should observe the fact that
a stem has wonderful strength. Compare the proportionate height,
diameter, and weight of a grass stem with those of the slenderest
tower or steeple. Which has the greater strength? Which the greater
height? Which will withstand the most wind? Note that the grass stem
will regain its position even if its top is bent to the ground. Note
how plants are weighted down after a heavy rain and how they recover
themselves. =48.= Split a cornstalk and observe how the joints are
tied together and braced with fibers. Are there similar fibers in
stems of pigweed, cotton, sunflower, hollyhock?
[Illustration: FIG. 68.--POTATO. What are roots, and what stems? Has
the plant more than one kind of stem? more than two kinds? Explain.]