Wood and Forest. William Noyes

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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 medullary rays. While most of the elements in wood run longitudinally in the log, it is also to be noted that running at right angles to these and radially to the log, are other groups of cells called pith rays or medullary rays (Latin, medulla, which means pith). These are the large "silver flakes" to be seen in quartered oak, which give it its beautiful and distinctive grain, Fig. 32, p. 37. They appear as long, grayish lines on a cross-section, as broad, shining bands on the radial section, and as short, thick lines tapering at each end on the tangential section. In other words, they are like flat, rectangular plates standing on edge and radiating lengthwise from the center of the tree. They vary greatly in size in different woods. In sycamore they are very prominent, Fig. 13. In oak they are often several hundred cells wide (i.e., up and down in the tree). This may amount to an inch or two. They are often twenty cells thick, tapering to one cell at the edge. In oak very many are also small, even microscopic. But in the conifers and also in some of the broad-leaved trees, altho they can be discerned with the naked eye on a split radial surface, still they are all very small. In pine there are some 15,000 of them to a square inch of a tangential section. They are to be found in all exogens. In a cross-section, say of oak, Fig. 14, it can readily be seen that some pith rays begin at the center of the tree and some farther out. Those that start from the pith are formed the first year and are called primary pith rays, while those that begin in a subsequent year, starting at the cambium of that year, are called secondary rays.

      Fig. 13. Tangential Section of Sycamore, Magnified 37 Diameters. Note the large size of the pith rays, A, A (end view).

      The function of the pith rays is twofold. (1) 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 (2) 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" (Greek, beside, to pour), 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.

      Fig. 14 Cross-section of White Oak. The Radiating White Lines are the Pith Rays.

      If a cambium cell is opposite to a pith ray, it divides crosswise (transversely) 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.

      Now suppose that laid among the ordinary thin-walled tubes were quite large tubes, so that one could tell the "ring" not only by the thin walls but by the presence of large tubes. That would represent the ring-porous woods, and the large tubes would be called vessels, or tracheæ. Suppose again that these large tubes were scattered in disorder thru the layers. This arrangement would represent the diffuse-porous woods.

      By holding up to the light, thin cross-sections of spruce or pine, Fig. 15, oak or ash, Fig. 16, and bass or maple, Fig. 17, these three quite distinct arrangements in the structure may be distinguished. This fact has led to the classification of woods according to the presence and distribution of "pores," or as they are technically called, "vessels" or "tracheae." By this classification we have:

      

      (1) Non-porous woods, which comprise the conifers, as pine and spruce.

      (2) Ring-porous woods, in which the pores appear (in a cross-section) in concentric rings, as in chestnut, ash and elm.

      (3) Diffuse-porous woods, in which (in a cross-section) the rings are scattered irregularly thru the wood, as in bass, maple and yellow poplar.

      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.

      Fig. 15. Cross-section of Non-porous Wood, White Pine, Full Size (top toward pith).

      Non-porous woods. In examining thru the microscope a transverse section of white pine, Fig. 18:

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

      (2) 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.

      

      Fig. 16. Cross-section of Ring-porous Wood, White Ash, Full Size (top toward pith).

      Fig. 17. Cross-section of Diffuse-porous Wood, Hard Maple, full size (top toward pith).

      

      (3) Running radially (up and down in the picture) 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.

      (4) Scattered here and there among the regular cells, are to be seen irregular gray or yellow dots which disturb the regularity of the arrangement. These are resin ducts. (See cross-section of white pine, Fig. 18.) They are not cells, but openings between cells, in which the resin, an excretion of the tree, accumulates, oozing out when the tree is injured. At least one function of resin is to protect the tree from attacks of fungi.

      Looking now at the radial section, Fig. 18:

      (5) 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.

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