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 the 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 the leaves are _the important functional or working parts_; the
stem merely connects them, and its form is exceedingly variable.
[Illustration: FIG. 53.—STRICT SIMPLE STEM OF MULLEIN.]
[Illustration: FIG. 54.—STRICT UPRIGHT STEM OF NARROW-LEAVED DOCK.]
=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 the 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. 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 flop 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_.
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. 57.—EXCURRENT TRUNK. A pine.]
[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 vigour 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.
=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_.
[Illustration: FIG. 60.—SPROUTS ARISING FROM THE BUDS, or eyes, of a
potato tuber.]
=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 the 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 endeavour to determine why it took its
particular form.
[Illustration: FIG. 61.—CRACKING OF THE BARK ON AN ELM BRANCH.]
=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. 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.
=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 parts with thinner
bark 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 a 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=.
[Illustration: FIG. 64.—ERRONEOUS PRUNING.]
=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. 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.= Examine 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 fibres. Are there similar fibres 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.]