Chapter IV: The Stem (2)

=127. Ringing fruit trees.=—The course of the sap explains why farmers sometimes hasten the ripening of fruit by the practice of _ringing_. As the food material cannot pass below the denuded ring, the parts above become gorged, and a process of forcing takes place. The practice, however, is not to be commended, except in rare cases, as it generally leads to the death of the ringed stem. The portion below the ring can receive no nourishment from above, and will gradually be so starved that it cannot even act as a carrier of crude sap to the leaves, and so the whole bough will perish.

=128. Sap movement not circulation.=—It must not be supposed that this flow of sap in plants is analogous to the circulation of the blood in animals, though frequently spoken of in popular language as the “circulation of the sap.” There is no central organ like the heart to regulate its flow, and the water taken up by the roots does not make a continual circuit of the plant body as the blood does of ours, but is dispersed by a process of general diffusion, partly into the air through the leaves and partly through the plant body as food, wherever it is needed. Figure 131 gives a good general idea of the movement of sap in trees, the arrows indicating the direction of the movement of the different substances.

=129. Unexplained phenomena.=—Though the forces named above undoubtedly exert a powerful influence over sap movement, their combined action has not been proved capable of lifting the current to a height of more than 200 feet, while in the giant redwoods of California and the towering blue gums of Australia, it is known to reach a height of more than 400 feet. The active force exerted by the cell protoplasm has been suggested as an efficient cause, but as the upward flow takes place through the cells of the xylem, which contain no protoplasm (116), this explanation is inadequate, and we must be content, in the present state of our knowledge, to accept the fact as one which science has yet to account for.

Practical Questions

1. Why will a leafy shoot heal more quickly than a bare one? (125,
126; Exp. 58.)

2. Why does a transverse cut heal more slowly than a vertical one?
(126, 127.)

3. Why does a ragged cut heal less rapidly than a smooth one?

4. Why does the formation of wood proceed more rapidly as the
amount of water given off by the leaves is increased? (126; Exp.
59.)

5. Why do nurserymen sometimes split the cortex of young trees in
summer to promote the formation of wood? (116, 118.)

6. What is the advantage of scraping the stems of trees?

7. Explain the frothy exudation that often appears at the cut ends
of firewood, and the singing noise that accompanies it. [120, 124
(2).]

8. Of what advantage is it to high climbing plants, like grape and
trumpet vine (_Tecoma_), to have such large ducts? (111, 116, 122.)

9. Why is the process of layering more apt to be successful if the
shoot is bent or twisted at the point where it is desired to make
it root? (127; Exps. 60, 61.)

10. Why do oranges become dry and spongy if allowed to hang on the
tree too long? (72, 126; Exps. 60, 61.)

11. Why will corn and fodder be richer in nourishment if, at
harvest, the whole stalk is cut down and both fodder and grain are
allowed to mature upon it? (126, 127; Exps. 60, 61.)

12. Is the injury done to plants by freezing due, as a general
thing, to mechanical, or to chemical action? (33.)

13. Why in pruning a branch is it best to make the cut just above a
bud? (Exps. 60, 61.)

14. Why is the rim of new bark, or callus, that forms on the upper
side of a horizontal wound, thicker than that on the lower side?
(126, 127; Exps. 60, 61.)

15. Why is it that the medicinal or other special properties of
plants are found mostly in the leaves and bark, or in the parts
immediately under the bark? (120, 126.)

16. Why does twisting the footstalk of a bunch of grapes, just
before ripening, make them sweeter? (127.)

17. Is it a mere superstition to drive nails into the stems of plum
and peach trees to make them bear larger or more abundant fruit?
(126, 127.)

18. Why is a living corn stalk heavier than a dry one? (124.)

19. Why is a stalk of sugar cane heavier than one of corn?
Suggestion: Which is the heavier, pure water, or water holding
solids in solution?

V. WOOD STRUCTURE IN ITS RELATION TO INDUSTRIAL USES

MATERIAL.—Select from the billets of wood cut for the fire, sticks
of various kinds; hickory, ash, oak, chestnut, maple, walnut,
cherry, pine, cedar, tulip tree, all make good specimens. Red
oak shows the medullary rays well. Get sticks of green wood, if
possible, and have them planed smooth at the ends. Collect also,
where they can be obtained, waste bits of dressed lumber from a
carpenter or joiner. If nothing better is available, any pieces of
unpainted woodwork about the schoolroom will furnish subjects for
study.

=130. Detailed structure of a woody stem.=—Select a good-sized billet of hard wood, and count the rings of annual growth. How old was the tree or the bough from which it was taken? Was its growth uniform from year to year? How do you know? Are the rings broader, as a general thing, toward the center or the circumference? How do you account for this? Is each separate ring of uniform thickness all the way round? Mention some of the circumstances that might cause a tree to grow less on one side than on the other. Are the rings of the same thickness in all kinds of wood? Which are the more rapid growers, those with broad or with narrow rings? Do you notice any difference in the texture of the wood in rapid and in slow growing trees? Which makes the better timber as a general thing, and why?

=131. Heartwood and sapwood.=—Notice that in some of your older specimens (cedar, black walnut, barberry, black locust, chestnut, oak, Osage orange, show the difference distinctly) the central part is different in color and texture from the rest. This is because the sap gradually abandons the center (116, 123) to feed the outer layers, where growth in dicotyls takes place; hence, the outer part of the stem usually consists of sapwood, which is soft and worthless as timber, while the dead interior forms the durable heartwood so prized by lumbermen. The heartwood is useful to the plant principally in giving strength and firmness to the axis. It will now be seen why girdling a stem,—that is, chipping off a ring of the softer parts all round, will kill it, while vigorous and healthy trees are often seen with the center of the trunk entirely hollow.

=132. Different ways of cutting.=—In studying the vertical arrangement of stems, two sections are necessary, a radial and a tangential one. The former passes along the axis, splitting the stem into halves (Fig. 135); the latter cuts between the axis and the perimeter, splitting off a segment from one side (Fig. 136). The appearance of the wood used in carpentry and joiner’s work is due largely to the manner in which the planks are cut.

=133. The cross cut.=—The section seen at the end of a log (Figs. 132, 134) is called by carpenters a cross cut. It passes at right angles to the grain of the wood, and severs what important structures? (116, 119, 122.) Examine a cross cut at the end of a rough plank, or the top of a stump or an old fence post, and tell why this kind of cut is seldom used in carpentry.

=134. The tangent cut= is so called because it is made at right angles to the radius of a log. Repeat the geometrical principle upon which such a cut is described as “tangential.” It passes through the medullary rays and the annual rings diagonally (Fig. 136), and is the cheapest way of cutting timber, since the entire log is made into planks and there is no waste except the “slabs” and “edgings,” as shown in Fig. 138. The cut ends of the medullary rays appear on the surface as small lines or slits (Fig. 137), and give to this kind of plank its peculiar graining. The wavy or “watered” appearance of the annual rings (Figs. 133, 136, 140, 141), so often seen in cheap furniture and in the woodwork of cheaply constructed houses, is caused by the tangential cut, which strikes them at various angles.

=135. The radial, or quartered cut=, familiar to most of us in the “quartered oak” of commerce, passes through the center of the log and cuts the rings of annual growth perpendicularly, giving it the “striped” appearance (Fig. 135) seen in the best woodwork. It gets its name from the practice of dealers in first sawing a log into quarters and then cutting parallel to the radius passing through the middle of each quarter, as shown in Fig. 139. In this way each cut strikes the rings perpendicularly, but except in the case of very large logs, only narrow planks can be obtained in this manner. A better way of treating small logs is shown in Fig. 138, where the three central planks, _r_, _r_, _r_, on and near the diameter, will give the “quartered” effect, while the rest can be used for the cheaper tangential cuttings. Examine a piece of quartered board, or a log of wood that has been split down the center, and notice that the medullary rays appear as silvery bands or plates (Figs. 140, 141). This is because the cut runs parallel to them. It is the medullary rays chiefly that give to commercial woods their characteristic graining. Knots, buds, and other adventitious causes also influence it in various degrees.

=136. The swelling and shrinking of timber.=—The capacity possessed by certain substances of bringing about an increase of volume by the absorption of liquids is termed _imbibition_. Care must be taken not to confound imbibition with capillarity. (Exp. 53.) When liquids are carried into a body by capillary attraction, they merely fill up vacant spaces already existing between small particles of the substance, and therefore do not cause any swelling or increase in size. When imbibition takes place, the _molecules_, or chemical units of the liquid, force their way between those of the imbibing substance, and thus, in making room for themselves, bring about an increase in volume of the imbibing body. To this cause is due the alternate swelling and shrinking of timber in wet and dry weather.

=137. Knots.=—Look for a billet with a knot in it. Notice how the rings of growth are disturbed and displaced in its neighborhood. If the knot is a large one, it will itself have rings of growth. Count them, and tell what its age was when it ceased to grow. Notice where it originates. Count the rings from its point of origin to the center of the stem. How old was the tree when the knot began to form? Count the rings from the origin of the knot to the circumference of the stem; how many years has the tree lived since the knot was formed? Does this agree with the age of the knot as deduced from its own rings? As the tree may continue to live and grow indefinitely after the bough which formed the knot died or was cut away, there will probably be no correspondence between the two sets of rings, especially in the case of old knots that have been covered up and embedded in the wood. The longer a dead branch remains on a tree the more rings of growth will form around it before covering it up, and the greater will be the disturbance caused by it. Hence, timber trees should be pruned while very young, and the parts removed should be cut as close as possible to the main branch or trunk. Sometimes knots injure lumber very much by falling out and leaving the holes that are often seen in pine boards. In other cases, however, when the knots are very small, the irregular markings caused by them add greatly to the beauty of the wood. The peculiar marking of bird’s-eye maple is caused by abortive buds buried in the wood.

Practical Questions

1. Is the swelling of wood a physical or a physiological process?

2. Does wood swell equally with the grain and across it?
(Suggestion: test by keeping a block under water for 10 to 20 days,
measuring its dimensions before and after immersion.)

3. In building a fence, what is the use of “capping” the posts?
(133.)

4. In laying shingles, why are they made to touch, if the work is
done in wet weather, and placed somewhat apart, if in dry weather?
(136.)

5. What is the difference between timber and lumber? Between a
plank and a board? Between a log, stick, block, and billet?

6. Why does sapwood decay more quickly than heartwood? (131.)

7. Explain the difference between osmosis, diffusion, capillarity,
and imbibition. (9, 56, 57, 136; Exp. 53.)

VI. FORESTRY

=138. Practical bearings.=—This part of our subject is closely related to lumbering and forestry. The business of the lumberman is to manufacture growing trees into merchantable timber, and to do this successfully he must understand enough about the structure of wood to cut his boards to the best advantage, both for economy and for bringing out the grain so as to produce the most desirable effects for ornamental purposes.

=139. Forestry has for its object=: (1) the preservation and cultivation of existing forests; (2) the planting of new ones, or the reforestation of tracts from which the timber has been destroyed. Forests may be either _pure_, that is, composed mainly of one kind of tree, as a pine or a fir wood; or _mixed_, being made up of a variety of different growths, as are most of our common hardwood forests.

=140. Enemies of the forest.=-The first step in the preservation of our forests is to know the dangers to be guarded against. The chief of these are: (1) fires; (2) the ignorance or recklessness of man in cutting for commercial purposes; (3) fungi; (4) injurious insects; (5) sheep, hogs, and other animals that eat the seeds and the young, tender growth.

=141. How to protect the forests.=—The annual destruction of forests by fires probably exceeds that from all other causes combined. The only effectual safeguard against this danger is watchfulness on the part of _everybody_. We can each one of us help in this work by at least being careful ourselves never to kindle a fire in the woods without taking every precaution against its spreading. A single match, or the glowing stump of a cigar, carelessly thrown among dry leaves or grass, may start a conflagration that will destroy millions of dollars’ worth of standing timber.

To prevent the spread of fungi, dead trees should be removed, and broken or decayed branches trimmed off and the cut surfaces painted. Birds which destroy insects should be protected; sheep and hogs should be kept out, and dead leaves left on the ground to cover the roots and fertilize the soil with the humus created by their decay. Finally, none but mature trees should be cut for industrial purposes, and the cutting ought to be done in such a way that the young surrounding growth will not be injured by the falling trunks.

=142. The usefulness of forests.=—Aside from the value of their products, forests are useful in many other ways. They influence climate beneficially by acting as windbreaks, by giving off moisture (Exp. 58), by shading the soil, and thus preventing too rapid evaporation. Their roots also help to retain the water in the soil, and by this means tend to prevent the washing of the land by heavy rains and to restrain the violence of freshets.

=143. Forests and water supply.=—It is especially important that the watershed of any region should be well protected by forests, to prevent contamination of the streams and to insure an unfailing supply of water by checking the escape of the rainfall from the soil.

Practical Questions

1. Explain the difference between a forest, grove, copse, wood,
woodland.

2. In pruning a tree why ought the branch to be cut as close to the
stock as possible? (137.)

3. Name the principal timber trees of your neighborhood. What gives
to each its special value?

4. Name six trees that produce timber valuable for ornament; for
toughness and strength.

5. Which is the better for timber, a tree grown in the open, or one
grown in a forest, and why? (Plate 7.)

6. What are the objects to be attained in pruning timber trees?
Orchard and ornamental trees?

7. Is the outer bark of any use to a tree, and if so, what?

8. Why should pruning not be done in wet weather? [140 (3), 141.]

9. Why should vertical shoots be cut off obliquely? [133, 140 (3),
141.]

Field Work

(1) Make a study of the various climbing plants of your
neighborhood with reference to their modes of ascent, and the
effect, injurious, or other, upon the plants to which they attach
themselves. Note the origin and position of tendrils, and try to
make out what modification has taken place in each case. Consider
the twining habit in reference to parasitism, especially in the
case of soft-stemmed twiners when brought into contact with
soft-stemmed annuals. Observe the various habits of stem growth:
prostrate, declined, ascending, etc., and decide what adaptation to
circumstances may have influenced each case.

(2) Notice the shape of the different stems met with, and learn
to recognize the forms peculiar to certain of the great families.
Observe the various appliances for defense and protection with
which they are provided, and try to find out the meaning of the
numerous grooves, ridges, hairs, prickles, and secretions that are
found on stems. Always be on the alert for modifications, and learn
to recognize a stem under any disguise, whether thorn, tendril,
foliage, water holder, rootstock, or tuber.

(3) Note the color and texture of the bark of the different trees
you see and learn to distinguish the most important kinds:

(_a_) scaly—peeling off annually in large plates, as sycamore,
shagbark-hickory;

(_b_) fibrous—detached in stiff threads and fibers, as grape;

(_c_) fissured—split into large, irregular cracks by the growth
of the stem in thickness, as oak, chestnut, and most of our large
forest trees;

(_d_) membranous—separating in dry films and ribbons, as common
birch (_Betula alba_).

Observe the difference in texture and appearance of the bark on
old and young boughs of the same species. Try to account for the
varying thickness of the bark on different trees and on different
parts of the same tree. Notice the difference in the timber of
the same species when grown in different soils, at different ages
of the tree, and in healthy and weakly specimens. Find examples
of self-pruning trees (Plate 7), and explain how the pruning was
brought about.

(4) Select a small plot, about a fourth of an acre, of any wooded
tract in your neighborhood, and make a study of all the trees and
shrubs it contains. Make a list of the different kinds, with the
number of each. Take note of those that show themselves, by vigor
and abundance of growth, best adapted to the situation. These are
the “climax” or dominant vegetation of the plot. Find out, if you
can, to what cause their superiority is due.