CHAPTER IX

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.]