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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:

Phanerogamia, (Flowering plants)

I. Gymnosperms. (Naked seeds.) 1. Cycadaceae. (Palms, ferns, etc.) 2. Gnetaceae. (Joint firs.) 3. Conifers. Pines, firs, etc. II. Angiosperms. (Fruits.) 1. Monocotyledons. (One seed-leaf.) (Palms, bamboos, grasses, etc.) 2. Dicotyledons. (Two seed-leaves.) a. Herbs. b. Broad-leaved trees.

Under the division of naked-seeded plants (gymnosperms), 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 (angiosperms), there are two classes, those that have one seed-leaf as they germinate, and those that have two seed-leaves.

The one seed-leaf plants (monocotyledons) include the grasses, lilies, bananas, palms, etc. Of these there are only a few that reach the dimensions of trees. They are strikingly distinguished by the structure of their stems. They have no cambium layer and no distinct bark and pith; they have unbranched stems, which as a rule do not increase in diameter after the first stages of growth, but grow only terminally. Instead of having concentric annual rings and thus growing larger year by year, the woody tissue grows here and there thru the stem, but mostly crowded together toward the outer surfaces. Even where there is radial growth, as in yucca, the structure is not in annual rings, but irregular. These one seed-leaf trees (monocotyledons) are not of much economic value as lumber, being used chiefly "in the round," and to some extent for veneers and inlays; e. g., cocoanut-palm and porcupine wood are so used.

The most useful of the monocotyledons, or endogens, ("inside growers," as they are sometimes called,) 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 (angiosperms), timber trees are chiefly found among those that have two seed-leaves (the dicotyledons) 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:

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

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

(3) Broad-leaved trees, having two seed-leaves, such as oak, beech, and elm. Fig. 48, p. 203.

The common classifications of trees are quite inaccurate. Many of the so-called deciduous (Latin, deciduus, falling off) 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.

Fig. 1. A Bamboo Grove, Kioto, Japan.

Fig. 2. Ginko Leaf.

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" (inside growers) including bamboos, palms, etc., and exogens (outside growers) 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 (Fig. 3):

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

(2) Radial, a longitudinal plane, including the organic axis.

Fig. 3.

A.	B.
A, B, C, D, Transverse Section. B, D, E, F, Radial Section. G, H, I, J, Tangential Section.	A, B, C, Transverse Section. A, B, D, E, Radial Section. B, C, E, F, Tangential Section.

(3) 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:

 (1) Rind or bark(a) Cortex(b) Bast

 (2) Cambium

 (3) Wood(a) Sap-wood(b) Heart-wood

 (4) Pith.

Fig. 4. Diagram of Cross-section of Three Year Old Stem of Basswood.

(1) The rind or bark is made up of two layers, the outer of which, the "cortex," is corky and usually scales or pulls off easily; while the inner one is a fibrous coat called "bast" or "phloem." Together they form a cone, widest, thickest, and roughest at the base and becoming narrower toward the top of the tree. The cortex or outer bark serves to protect the stem of the tree from extremes of heat and cold, from atmospheric changes, and from the browsing of animals. It is made up of a tough water-proof layer of cork which has taken the place of the tender skin or "epidermis" of the twig. Because it is water-proof the outside tissue is cut off from the water supply of the tree, and so dries up and peels off, a mass of dead matter. The cork and the dead stuff together are called the bark. As we shall see later, the cork grows from the inside, being formed in the inner layers of the cortex, the outer layers of dry bark being thus successively cut off.

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 (Latin lenticula, freckle), 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. 208, 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.

(2) The cambium. Inside of the rind and between it and the wood, there is, on living trees, a slimy coat called cambium (Med. Latin, exchange). This is the living, growing part of the stem, familiar to all who have peeled it as the sticky, slimy coat between the bark and the wood of a twig. This is what constitutes the fragrant, mucilaginous inner part of the bark of slippery elm. Cambium is a tissue of young and growing cells, in which the new cells are formed, the inner ones forming the wood and the outer ones the bark.

Fig. 5. Young Stem, Magnified 18½ Diameters, Showing Primary and Secondary Bundles. By Courtesy of Mrs. Katharine Golden Bitting.

E, epidermis, the single outside layer of cells.

C, cortex, the region outside of the bundles.

HB, hard bast, the black, irregular ring protecting the soft bast.

SB, soft bast, the light, crescent-shaped parts.

Ca, cambium, the line between the soft bast and the wood.

W, wood, segments showing pores.

MR, medullary rays, lines between the bundles connecting the pith and the cortex.

MS, medullary sheath, the dark, irregular ring just inside the bundles.

P, pith, the central mass of cells.

In order to understand the cambium and its function, consider its appearance in a bud, Fig. 5. A cross-section of the bud of a growing stem examined under the microscope, looks like a delicate mesh of thin membrane, filled in with a viscid semi-fluid substance which is called "protoplasm" (Greek, protos, first; plasma, form). These meshes were first called "cells" by Robert Hooke, in 1667, because of their resemblance to the chambers of a honeycomb. The walls of these "cells" are their most prominent feature and, when first studied, were supposed to be the essential part; but later the slimy, colorless substance which filled the cells was found to be the essential part. This slimy substance, called protoplasm, constitutes the primal stuff of all living things. The cell walls themselves are formed from it. These young cells, at the apex of a stem, are all alike, very small, filled with protoplasm, and as yet, unaltered. They form embryonic tissue, i.e. one which will change. One change to which an cell filled with protoplasm is liable is division into two, a new partition wall forming within it. This is the way plant cells increase.

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 (cotton and flax fibers consist of almost pure 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 (wood), 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.

Fig. 6. Three Stages in the Development of an Exogenous Stem. P, pith; PB, primary bast; SB, secondary bast; C, cambium; PR, pith ray; PW, primary wood; SW, secondary wood; PS, procambium strands. After Boulger.

In the monocotyledons (endogens) 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 (bark). 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 (Latin, medulla, pith). 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 (bark) 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."