Wood and Forest Part 5

The most common type of structure among the broad-leaved trees contains trache, trachids, 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.

[Ill.u.s.tration: 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. 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.

(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 grain is seen to be marked not so much by the denseness of the summer wood as by the presence of the vessels (pores).

(9) The ends of the pith rays are also clear.

In _diffuse porous woods_, the main features to be noticed are: In the transverse section, Fig. 21:

(1) The irregularity with which the pores are scattered,

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

(3) The radiating pith rays,

(4) The irregular arrangement and,

(5) The complex structure.

In the radial section, Fig. 21:

(6) The pith rays are evident. In sycamore, No. 53, Chap. III, they are quite large.

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

In the tangential section, Fig. 21:

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

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

[Ill.u.s.tration: Fig. 21.]

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 coa.r.s.e 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 coa.r.s.e 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 "coa.r.s.e grained,"

if narrow, "fine grained," so that of two trees of the same species, one may be coa.r.s.e grained and the other fine grained, depending solely on the accident of fast or slow growth.

The terms coa.r.s.e 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 coa.r.s.e and fine textured. When such coa.r.s.e 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" (or better, fine textured,) 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 coa.r.s.e 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 ill.u.s.tration 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. (See some of the logs in Fig.

107, p. 254.) This produces "spiral" or "twisted" grain.

[Ill.u.s.tration: Fig. 22. Straight Grained Long-leaf Pine (full size).]

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 const.i.tutes the characteristic "grain" of striped mahogany, Fig.

23. It is rarely found in the inner part of the tree.

[Ill.u.s.tration: Fig. 23. Mahogany, Showing Alternately Twisted Grain (full size).]

[Ill.u.s.tration: Fig. 24. Spiral Grain in Cypress.

_After Roth._]

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.

[Ill.u.s.tration: Fig. 25. Planed Surface of Wavy-Grained Maple (full size).]

[Ill.u.s.tration: Fig. 26. Split Surface of Wavy-Grained Maple (full size).]

"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 k.n.o.bs on tree trunks, Fig. 31. Burl is used chiefly in veneers.

[Ill.u.s.tration: Fig. 27. Curly Grained Long-leaf Pine (full size).]

[Ill.u.s.tration: Fig. 28. Curly Yellow Poplar (full size).]