Part 4 (2/2)
(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 ba.s.s, 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 cla.s.ses 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.
[Ill.u.s.tration: 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.
[Ill.u.s.tration: Fig. 16. Cross-section of Ring-porous Wood, White Ash, Full Size (top toward pith).]
[Ill.u.s.tration: 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, acc.u.mulates, 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.
(6) 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 ca.n.a.l, 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.
(7) 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:
(8) The straightness and overlapping of the cells is to be seen again, and
(9) 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.
[Ill.u.s.tration: Fig. 18.]
The cells of conifers are called tracheids, meaning ”like _trache_.”
They are cells in which the end walls persist, that is, are not absorbed and broken down when they meet end to end. In other words, conifers do not have continuous pores or vessels or ”_trache_,” and hence are called ”non-porous” woods.
But in other woods, the ends of some cells which meet endwise are absorbed, thus forming a continuous series of elements which const.i.tute an open tube. Such tubes are known as pores, or vessels, or ”trache,” 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 (trache).
Parenchyma const.i.tutes the pith rays and other similar fibers, retains its protoplasm, and becomes filled with starch in autumn.
[Ill.u.s.tration: Fig. 19. Isolated Fibers and Cells. _a_, four cells of wood parenchyma; _b_, two cells from a pith ray; _c_, a single cell or joint of a vessel, the openings, x, x, leading into its upper and lower neighbors; _d_, tracheid; _e_, wood fiber proper. _After Roth._]
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