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.
(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 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.
(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.
Fig. 18.
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.
The cells of conifers are called tracheids, meaning "like tracheæ." 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 "tracheæ," 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 constitute an open tube. Such tubes are known as pores, or vessels, or "tracheæ," 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 (tracheæ). 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 tracheæ, trachæids, 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.
Fig. 20.
Ring-porous woods. Looking thru the microscope at a cross-section of ash, a ring-porous wood, Fig. 20:
(1) The large round or oval pores or vessels grouped mostly in the spring wood first attract attention. Smaller ones, but still quite distinct, are to be seen scattered all thru the wood. It is by the number and distribution of these pores that the different oak woods are distinguished, those in white oak being smaller and more numerous, while in red oak they are fewer and larger. It is evident that the greater their share in the volume, the lighter in weight and the weaker will be the wood. In a magnified cross-section of some woods, as black locust, white elm and chestnut, see Chap. III, beautiful patterns are to be seen composed of these pores. It is because of the size of these pores and their great number that chestnut is so weak.
(2) 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.
(3) The lines of the pith rays are very plain in some woods, as in oak. No. 47, Chap. III.
(4) The irregular arrangement and
(5) 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 (tracheids) as in the conifers.
Looking at the radial section, Fig. 20:
(6) If the piece is oak, the great size of the medullary rays is most noticeable. Fig. 32, p. 37. 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.
(7) 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:
(8) The pattern of the