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U. S. Department of Agriculture Year Books for:

U. S. Department of Agriculture, Division of Entomology Bulletins:

U. S. Department of Agriculture, Bureau of Plant Industry, Bulletins:

THE STRUCTURE OF WOOD.

When it is remembered that the suitability of wood for a particular purpose depends most of all upon its internal structure, it is plain that the woodworker should know the essential characteristics of that structure. While his main interest in wood is as lumber, dead material to be used in woodworking, he can properly understand its structure only by knowing something of it as a live, growing organism. To facilitate this, a knowledge of its position in the plant world is helpful.

All the useful woods are to be found in the highest sub-kingdom of the plant world, the flowering plants or Phanerogamia of the botanist. These flowering plants are to be classified as follows:

Under the division of naked-seeded plants , practically the only valuable timber-bearing plants are the needle-leaved trees or the conifers, including such trees as the pines, cedars, spruces, firs, etc. Their wood grows rapidly in concentric annual rings, like that of the broad-leaved trees; is easily worked, and is more widely used than the wood of any other class of trees.

Of fruit-bearing trees , there are two classes, those that have one seed-leaf as they germinate, and those that have two seed-leaves.

The most useful of the monocotyledons, or endogens, are the bamboos, which are giant members of the group of grasses, Fig. 1. They grow in dense forests, some varieties often 70 feet high and 6 inches in diameter, shooting up their entire height in a single season. Bamboo is very highly valued in the Orient, where it is used for masts, for house rafters, and other building purposes, for gutters and water-pipes and in countless other ways. It is twice as strong as any of our woods.

Under the fruit-bearing trees , timber trees are chiefly found among those that have two seed-leaves and include the great mass of broad-leaved or deciduous trees such as chestnut, oak, ash and maple. It is to these and to the conifers that our principal attention will be given, since they constitute the bulk of the wood in common use.

The timber-bearing trees, then, are the:

Conifers, the needle-leaved, naked-seeded trees, such as pine, cedar, etc. Fig. 45, p. 199.

Endogens, which have one seed-leaf, such as bamboos, Fig. 1.

Broad-leaved trees, having two seed-leaves, such as oak, beech, and elm. Fig. 48, p. 202.

The common classifications of trees are quite inaccurate. Many of the so-called deciduous trees are evergreen, such as holly, and, in the south, live oak, magnolia and cherry. So, too, some of the alleged "evergreens," like bald cypress and tamarack, shed their leaves annually.

Not all of the "conifers" bear cones. For example, the juniper bears a berry. The ginko, Fig. 2, tho classed among the "conifers," the "evergreens," and the "needle-leaf" trees, bears no cones, has broad leaves and is deciduous. It has an especial interest as being the sole survivor of many species which grew abundantly in the carboniferous age.

Also, the terms used by lumbermen, "hard woods" for broad-leaved trees and "soft woods" for conifers, are still less exact, for the wood of some broad-leaved trees, as bass and poplar, is much softer than that of some conifers, as Georgia pine and lignum vitae.

Another classification commonly made is that of "endogens" including bamboos, palms, etc., and exogens which would include both conifers and broad-leaved trees.

One reason why so many classifications have come into use is that none of them is quite accurate. A better one will be explained later. See p. 23.

As in the study of all woods three sections are made, it is well at the outset to understand clearly what these are.

The sections of a tree made for its study are :

Transverse, a plane at right angles to the organic axis.

Radial, a longitudinal plane, including the organic axis.

Tangential, a longitudinal plane not including the organic axis.

If a transverse section of the trunk of a conifer or of a broad-leaved tree is made, it is to be noted that it consists of several distinct parts. See Fig. 4. These, beginning at the outside, are:

Rind or bark Cortex Bast

Cambium

Wood Sap-wood Heart-wood

Pith.

The characteristics of the tree bark are due to the positions and kinds of tissue of these new layers of cork. Each tree has its own kind of bark, and the bark of some is so characteristic as to make the tree easily recognizable.

Bark may be classified according to formation and method of separation, as scale bark, which detaches from the tree in plates, as in the willows; membraneous bark, which comes off in ribbons and films, as in the birches; fibrous bark, which is in the form of stiff threads, as in the grape vine; and fissured bark, which breaks up in longitudinal fissures, showing ridges, grooves and broad, angular patches, as in oak, chestnut and locust. The last is the commonest form of bark.

The bark of certain kinds of trees, as cherry and birch, has peculiar markings which consist of oblong raised spots or marks, especially on the young branches. These are called lenticels , and have two purposes: they admit air to the internal tissues, as it were for breathing, and they also emit water vapor. These lenticels are to be found on all trees, even where the bark is very thick, as old oaks and chestnuts, but in these the lenticels are in the bottoms of the deep cracks. There is a great difference in the inflammability of bark, some, like that of the big trees of California, Fig. 54, p. 209, which is often two feet thick, being practically incombustible, and hence serving to protect the tree; while some bark, as canoe birch, is laden with an oil which burns furiously. It therefore makes admirable kindling for camp fires, even in wet weather.

Inside the cork is the "phloem" or "bast," which, by the way, gives its name to the bass tree, the inner bark of which is very tough and fibrous and therefore used for mat and rope making. In a living tree, the bast fibers serve to conduct the nourishment which has been made in the leaves down thru the stem to the growing parts.

In young plant cells, the whole cavity of the chamber is filled with protoplasm, but as the cells grow older and larger, the protoplasm develops into different parts, one part forming the cell wall and in many cases leaving cavities within the cell, which become filled with sap. The substance of the cell wall is called cellulose . At first it has no definite structure, but as growth goes on, it may become thickened in layers, or gummy, or hardened into lignin , according to the function to be performed. Where there are a group of similar cells performing the same functions, the group is called a tissue or, if large enough, a tissue system.

When cells are changed into new forms, or "differentiated," as it is called, they become permanent tissues. These permanent tissues of the tree trunk constitute the various parts which we have noticed, viz., the rind, the pith and the wood.

The essentially living part of the tree, it should be remembered, is the protoplasm: where there is protoplasm, there is life and growth. In the stems of the conifers and broad-leaved trees--sometimes together called exogens--this protoplasm is to be found in the buds and in the cambium sheath, and these are the growing parts of the tree. If we followed up the sheath of cambium which envelopes a stem, into a terminal bud, we should find that it passed without break into the protoplasm of the bud.

In the cross-section of a young shoot, we might see around the central pith or medulla, a ring of wedge-shaped patches. These are really bundles of cells running longitudinally from the rudiments of leaves thru the stem to the roots. They are made of protoplasm and are called the "procambium strands," Fig. 6.

In the monocotyledons these procambium strands change completely into wood and bast, and so losing all their protoplasmic cambium, become incapable of further growth. This is why palms can grow only lengthwise, or else by forming new fibers more densely in the central mass. But in the conifers and broad-leaved trees, the inner part of each strand becomes wood and the outer part bast . Between these bundles, connecting the pith in the center with the cortex on the outside of the ring of bundles, are parts of the original pith tissue of the stem. They are the primary pith or medullary rays . The number of medullary rays depends upon the number of the bundles; and their form, on the width of the bundles, so that they are often large and conspicuous, as in oak, or small and indeed invisible, as in some of the conifers. But they are present in all exogenous woods, and can readily be seen with the microscope. Stretching across these pith rays from the cambium layer in one procambium strand to that in the others, the cambium formation extends, making a complete cylindrical sheath from the bud downward over the whole stem. This is the cambium sheath and is the living, growing part of the stem from which is formed the wood on the inside and the rind on the outside.

In the first year the wood and the bast are formed directly by the growth and change of the inner and outer cells respectively of the procambium strand, and all such material is called "primary;" but in subsequent years all wood, pith rays, and bast, originate in the cambium, and these growths are called "secondary."

In a living tree, sap-wood and heart-wood perform primarily quite different functions. The sap-wood carries the water from the roots to the leaves, stores away starch at least in winter, and in other ways assists the life of the tree. The proportional amount of sapwood varies greatly, often, as in long-leaf pine, constituting 40 per cent. of the stem.

As the sap-wood grows older, its cells become choked so that the sap can no longer flow thru them. It loses its protoplasm and starch and becomes heartwood, in which all cells are dead and serve only the mechanical function of holding up the great weight of the tree and in resisting wind pressures. This is the reason why a tree may become decayed and hollow and yet be alive and bear fruit. In a tree that is actually dead the sap-wood rots first.

Chemical substances infiltrate into the cell walls of heart-wood and hence it has a darker color than the sap-wood. Persimmon turns black, walnut purplish brown, sumac yellow, oak light brown, tulip and poplar yellowish, redwood and cedar brownish red. Many woods, as mahogany and oak, darken under exposure, which shows that the substances producing the color are oxidizable and unstable. Wood dyes are obtained by boiling and distilling such woods as sumach, logwood, red sanders, and fustic. Many woods also acquire distinct odors, as camphor, sandalwood, cedar, cypress, pine and mahogany, indicating the presence of oil.

As a rule heart-wood is more valuable for timber, being harder, heavier, and drier than sap-wood. In woods like hickory and ash, however, which are used for purposes that require pliability, as in baskets, or elasticity as in handles of rakes and hoes, sap-wood is more valuable than heart-wood.

In a radial section of a log, Fig. 8, these "rings" appear as a series of parallel lines and if one could examine a long enough log these lines would converge, as would the cut edges in a nest of cones, if they were cut up thru the center, as in Fig. 9.

In a tangential section, the lines appear as broad bands, and since almost no tree grows perfectly straight, these lines are wavy, and give the characteristic pleasing "grain" of wood. Fig. 27, p. 35. The annual rings can sometimes be discerned in the bark as well as in the wood, as in corks, which are made of the outer bark of the cork oak, a product of southern Europe and northern Africa. Fig. 10.

The growth of the wood of exogenous trees takes place thru the ability, already noted, of protoplasmic cells to divide. The cambium cells, which have very thin walls, are rectangular in shape, broader tangentially than radially, and tapering above and below to a chisel edge, Fig. 11. After they have grown somewhat radially, partition walls form across them in the longitudinal, tangential direction, so that in place of one initial cell, there are two daughter cells radially disposed. Each of these small cells grows and re-divides, as in Fig. 12. Finally the innermost cell ceases to divide, and uses its protoplasm to become thick and hard wood. In like manner the outermost cambium cell becomes bast, while the cells between them continue to grow and divide, and so the process goes on. In nearly all stems, there is much more abundant formation of wood than of bast cells. In other words, more cambium cells turn to wood than to bast.

In the spring when there is comparatively little light and heat, when the roots and leaves are inactive and feeble, and when the bark, split by winter, does not bind very tightly, the inner cambium cells produce radially wide wood cells with relatively thin walls. These constitute the spring wood. But in summer the jacket of bark binds tightly, there is plenty of heat and light, and the leaves and roots are very active, so that the cambium cells produce thicker walled cells, called summer wood. During the winter the trees rest, and no development takes place until spring, when the large thin-walled cells are formed again, making a sharp contrast with those formed at the end of the previous season.

It is only at the tips of the branches that the cambium cells grow much in length; so that if a nail were driven into a tree twenty years old at, say, four feet from the ground, it would still be four feet from the ground one hundred years later.

Looking once more at the cross-section, say, of spruce, the inner portion of each ring is lighter in color and softer in texture than the outer portion. On a radial or tangential section, one's finger nail can easily indent the inner portion of the ring, tho the outer dark part of the ring may be very hard. The inner, light, soft portion of the ring is the part that grows in the spring and early summer, and is called the "spring wood" while the part that grows later in the season is called "summer wood." As the summer wood is hard and heavy, it largely determines the strength and weight of the wood, so that as a rule, the greater the proportion of the summer growth, the better the wood. This can be controlled to some extent by proper forestry methods, as is done in European larch forests, by "underplanting" them with beech.

In a normal tree, the summer growth forms a greater proportion of the wood formed during the period of thriftiest growth, so that in neither youth nor old age, is there so great a proportion of summer wood as in middle age.

It will help to make clear the general structure of wood if one imagines the trunk of a tree to consist of a bundle of rubber tubes crushed together, so that they assume angular shapes and have no spaces between them. If the tubes are laid in concentric layers, first a layer which has thin walls, then successive layers having thicker and thicker walls, then suddenly a layer of thin-walled tubes and increasing again to thick-walled ones and so on, such an arrangement would represent the successive annual "rings" of conifers.

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