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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.

The function of the pith rays is twofold. They transfer formative material from one part of a stem to another, communicating with both wood and bark by means of the simple and bordered pits in them, and they bind the trunk together from pith to bark. On the other hand their presence makes it easier for the wood to split radially.

The substance of which they are composed is "parenchyma" , which also constitutes the pith, the rays forming a sort of connecting link between the first and last growth of the tree, as the cambium cells form new wood each year.

If a cambium cell is opposite to a pith ray, it divides crosswise into eight or ten cells one above another, which stretch out radially, retaining their protoplasm, and so continue the pith ray. As the tree grows larger, new, or secondary medullary rays start from the cambium then active, so that every year new rays are formed both thinner and shorter than the primary rays, Fig. 14.

In order to fully understand the structure of wood, it is necessary to examine it still more closely thru the microscope, and since the three classes of wood, non-porous, ring-porous and diffuse-porous, differ considerably in their minute structure, it is well to consider them separately, taking the simplest first.

The most noticeable characteristic is the regularity of arrangement of the cells. They are roughly rectangular and arranged in ranks and files.

Another noticeable feature is that they are arranged in belts, the thickness of their walls gradually increasing as the size of the cells diminishes. Then the large thin-walled cells suddenly begin again, and so on. The width of one of these belts is the amount of a single year's growth, the thin-walled cells being those that formed in spring, and the thick-walled ones those that formed in summer, the darker color of the summer wood as well as its greater strength being caused by there being more material in the same volume.

Running radially directly thru the annual belts or rings are to be seen what looks like fibers. These are the pith or medullary rays. They serve to transfer formative material from one part of the stem to another and to bind the tree together from pith to bark.

Looking now at the radial section, Fig. 18:

The first thing to notice is the straightness of the long cells and their overlapping where they meet endwise, like the ends of two chisels laid together, Fig. 11.

On the walls of the cells can be seen round spots called "pits." These are due to the fact that as the cell grows, the cell walls thicken, except in these small spots, where the walls remain thin and delicate. The pit in a cell wall always coincides with the pit in an adjoining cell, there being only a thin membrane between, so that there is practically free communication of fluids between the two cells. In a cross-section the pit appears as a canal, the length of which depends upon the thickness of the walls. In some cells, the thickening around the pits becomes elevated, forming a border, perforated in the center. Such pits are called bordered pits. These pits, both simple and bordered, are waterways between the different cells. They are helps in carrying the sap up the tree.

The pith rays are also to be seen running across and interwoven in the other cells. It is to be noticed that they consist of several cells, one above another.

In the tangential section, Fig. 18:

The straightness and overlapping of the cells is to be seen again, and

The numerous ends of the pith rays appear.

In a word, the structure of coniferous wood is very regular and simple, consisting mainly of cells of one sort, the pith rays being comparatively unnoticeable. This uniformity is what makes the wood of conifers technically valuable.

But in other woods, the ends of some cells which meet endwise are absorbed, thus forming a continuous series of elements which constitute an open tube. Such tubes are known as pores, or vessels, or "tracheae," and sometimes extend thru the whole stem. Besides this marked difference between the porous and non-porous woods, the porous woods are also distinguished by the fact that instead of being made up, like the conifers of cells of practically only one kind, namely tracheids, they are composed of several varieties of cells. Besides the tracheae and tracheids already noted are such cells as "wood fiber," "fibrous cells," and "parenchyma." Fig. 19. Wood fiber proper has much thickened lignified walls and no pits, and its main function is mechanical support. Fibrous cells are like the wood fibers except that they retain their protoplasm. Parenchyma is composed of vertical groups of short cells, the end ones of each group tapering to a point, and each group originates from the transverse division of one cambium cell. They are commonly grouped around the vessels . Parenchyma constitutes the pith rays and other similar fibers, retains its protoplasm, and becomes filled with starch in autumn.

The most common type of structure among the broad-leaved trees contains tracheae, trachaeids, woody fiber, fibrous cells and parenchyma. Examples are poplars, birch, walnut, linden and locust. In some, as ash, the tracheids are wanting; apple and maple have no woody fiber, and oak and plum no fibrous cells.

This recital is enough to show that the wood of the broad-leaved trees is much more complex in structure than that of the conifers. It is by means of the number and distribution of these elements that particular woods are identified microscopically. See p. 289.

The summer wood is also distinguishable by the fact that, as with the conifers, its cells are smaller and its cell walls thicker than those of the spring wood. The summer wood appears only as a narrow, dark line along the largest pores in each ring.

The irregular arrangement and

Complex structure are evident, and these are due to the fact that the wood substance consists of a number of different elements and not one as in the conifers.

Looking at the radial section, Fig. 20:

If the piece is oak, the great size of the medullary rays is most noticeable. Fig. 32, p. 38. They are often an inch or more wide; that is, high, as they grow in the tree. In ash they are plain, seen thru the microscope, but are not prominent.

The interweaving of the different fibers and the variety of their forms show the structure as being very complex.

In the tangential section, Fig. 20:

The pattern of the grain is seen to be marked not so much by the denseness of the summer wood as by the presence of the vessels .

The ends of the pith rays are also clear.

The irregularity with which the pores are scattered,

The fine line of dense cells which mark the end of the year's growth,

The radiating pith rays,

The irregular arrangement and,

The complex structure.

In the radial section, Fig. 21:

The interweaving of the fibers is to be noted and also their variety.

In the tangential section, Fig. 21:

The grain is to be traced only dimly, but the fibers are seen to run in waves around the pith rays.

The pith rays, the ends of which are plainly visible.

THE GRAIN OF WOOD.

The term "grain" is used in a variety of meanings which is likely to cause confusion. This confusion may be avoided, at least in part, by distinguishing between grain and texture, using the word grain to refer to the arrangement or direction of the wood elements, and the word texture to refer to their size or quality, so far as these affect the structural character of the wood. Hence such qualifying adjectives as coarse and fine, even and uneven, straight and cross, including spiral, twisted, wavy, curly, mottled, bird's-eye, gnarly, etc., may all be applied to grain to give it definite meaning, while to texture the proper modifying adjectives are coarse and fine, even and uneven.

Usually the word grain means the pattern or "figure" formed by the distinction between the spring wood and the summer wood. If the annual rings are wide, the wood is, in common usage, called "coarse grained," if narrow, "fine grained," so that of two trees of the same species, one may be coarse grained and the other fine grained, depending solely on the accident of fast or slow growth.

The terms coarse grain and fine grain are also frequently used to distinguish such ring-porous woods as have large prominent pores, like chestnut and ash, from those having small or no pores, as cherry and lignum vitae. A better expression in this case would be coarse and fine textured. When such coarse textured woods are stained, the large pores in the spring wood absorb more stain than the smaller elements in the summer wood, and hence the former part appears darker. In the "fine grained" woods the pores are absent or are small and scattered, and the wood is hard, so that they are capable of taking a high polish. This indicates the meaning of the words coarse and fine in the mind of the cabinet-maker, the reference being primarily to texture.

If the elements of which a wood are composed are of approximately uniform size, it would be said to have a uniform texture, as in white pine, while uniform grain would mean, that the elements, tho of varying sizes, were evenly distributed, as in the diffuse-porous woods.

The term "grain" also refers to the regularity of the wood structure. An ideal tree would be composed of a succession of regular cones, but few trees are truly circular in cross-section and even in those that are circular, the pith is rarely in the center, showing that one side of the tree, usually the south side, is better nourished than the other, Fig. 14, p. 23.

The normal direction of the fibers of wood is parallel to the axis of the stem in which they grow. Such wood is called "straight-grained," Fig. 22, but there are many deviations from this rule. Whenever the grain of the wood in a board is, in whole or in part, oblique to the sides of the board, it is called "cross-grained." An illustration of this is a bend in the fibers, due to a bend in the whole tree or to the presence of a neighboring knot. This bend makes the board more difficult to plane. In many cases, probably in more cases than not, the wood fibers twist around the tree. This produces "spiral" or "twisted" grain.

Often, as in mahogany and sweet gum, the fibers of several layers twist first in one direction and then those of the next few layers twist the other way, Fig. 24. Such wood is peculiarly cross-grained, and is of course hard to plane smooth. But when a piece is smoothly finished the changing reflection of light from the surface gives a beautiful appearance, which can be enhanced by staining and polishing. It constitutes the characteristic "grain" of striped mahogany, Fig. 23. It is rarely found in the inner part of the tree.

Sometimes the grain of wood is "cross," because it is "wavy" either in a radial or a tangential section, as in maple, Fig. 25, and Fig. 26.

"Curly grain" refers to the figure of circlets and islets and contours, often of great beauty, caused by cutting a flat surface in crooked-grained wood. See Fig. 27, curly long-leaf pine, and Fig. 28, yellow poplar. When such crookedness is fine and the fibers are contorted and, as it were, crowded out of place, as is common in and near the roots of trees, the effect is called "burl," Fig. 29. The term burl is also used to designate knots and knobs on tree trunks, Fig. 31. Burl is used chiefly in veneers.

Irregularity of grain is often caused by the presence of adventitious and dormant buds, which may be plainly seen as little knobs on the surface of some trees under the bark. In most trees, these irregularities are soon buried and smoothed over by the successive annual layers of wood, but in some woods there is a tendency to preserve the irregularities. On slash boards of such wood, a great number of little circlets appear, giving a beautiful grain, as in "Bird's-eye maple," Fig. 30. These markings are found to predominate in the inner part of the tree. This is not at all a distinct variety of maple, as is sometimes supposed, but the common variety, in which the phenomenon frequently appears. Logs of great value, having bird's-eyes, have often unsuspectingly been chopped up for fire wood.

The term "grain" may also mean the "figure" formed by the presence of pith rays, as in oak, Fig. 32, or beech, or the word "grain" may refer simply to the uneven deposit of coloring matter as is common in sweet gum, Fig. 33, black ash, or Circassian walnut.

The presence of a limb constitutes a knot and makes great irregularity in the grain of wood, Fig. 34. In the first place, the fibers on the upper and lower sides of the limb behave differently, those on the lower side running uninterruptedly from the stem into the limb, while on the upper side the fibers bend aside making an imperfect connection. Consequently to split a knot it is always necessary to start the split from the lower side. On the other hand it is easier to split around a knot than thru it. The texture as well as the grain of wood is modified by the presence of a branch. The wood in and around a knot is much harder than the main body of the trunk on account of the crowding together of the elements. Knots are the remnants of branches left in the trunk. These once had all the parts of the trunk itself, namely bark, cambium, wood, and pith. Normally, branches grow from the pith, tho some trees, as Jack pine and redwood, among the conifers, and most of the broad-leaf trees have the power of putting out at any time adventitious buds which may develop into branches. When a branch dies, the annual layer of wood no longer grows upon it, but the successive layers of wood on the trunk itself close tighter and tighter around it, until it is broken off. Then, unless it has begun to decay, it is successively overgrown by annual layers, so that no sign of it appears until the trunk is cut open. A large trunk perfectly clean of branches on the outside may have many knots around its center, remnants of branches which grew there in its youth, as in Fig. 34, and Fig. 8, p. 18. The general effect of the presence of a knot is, that the fibers that grow around and over it are bent, and this, of course, produces crooked grain.

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