Fig. 29.
Redwood Burl (full size).
Fig. 30.
Bird's-eye Maple (full size.)
Fig. 31. Burl on White Oak.
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.
Fig. 32.
Figure Formed by Pith Rays in Oak (full size).
Fig. 33.
Sweet Gum, Showing Uneven Deposit of Coloring Matter (full size.)
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. 19. 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.
Following are the designations given to different knots by lumbermen: A sound knot is one which is solid across its face and is as hard as the wood surrounding it and fixed in position. A pin knot is sound, but not over ¼" in diameter. A standard knot is sound, but not over 1½" in diameter. A large knot is sound, and over 1½" in diameter. A spike knot is one sawn in a lengthwise position. A dead, or, loose knot is one not firmly held in place by growth or position.
(4) Pith. At the center or axis of the tree is the pith or medulla, Fig. 34. In every bud, that is, at the apex of every stem and branch, the pith is the growing part; but as the stem lengthens and becomes overgrown by successive layers of wood the pith loses its vital function. It does not grow with the plant except at the buds. It varies in thickness, being very small,—hardly more than 1/16", in cedar and larch,—and so small in oak as to be hardly discernible; and what there is of it turns hard and dark. In herbs and shoots it is relatively large, Fig. 5, p. 15, in a three-year old shoot of elder, for example, being as wide as the wood. In elder, moreover, it dies early and pulverizes, leaving the stem hollow. Its function is one of only temporary value to the plant.
Fig. 34.
Section Thru the Trunk of a Seven Year Old Tree, Showing Relation of Branches to Main Stem. A, B, two branches which were killed after a few years' growth by shading, and which have been overgrown by the annual rings of wood; C, a limb which lived four years, then died and broke off near the stem, leaving the part to the left of XY a "sound" knot, and the part to the right a "dead" knot, which unless rotting sets in, would in time be entirely covered by the growing trunk; D, a branch that has remained alive and has increased in size like the main stem; P, P, pith of both stem and limb.
THE STRUCTURE OF WOOD.
References:*Roth, Forest Bull. No. 10, pp. 11-23.Boulger, pp. 1-39.Sickles, pp. 11-20.Pinchot, Forest Bull. No. 24, I, pp. 11-24.Keeler, pp. 514-517.Curtis, pp. 62-85.Woodcraft, 15: 3, p. 90.Bitting, Wood Craft, 5: 76, 106, 144, 172, (June-Sept. 1906).Ward, pp. 1-38.Encyc. Brit., 11th Ed., "Plants," p. 741.Strasburger, pp. 120-144 and Part II, Sec. II.Snow, pp. 7-9, 183.
* For general bibliography, see p. 4.
Chapter II.
PROPERTIES OF WOOD.
There are many properties of wood,—some predominant in one species, some in another,—that make it suitable for a great variety of uses. Sometimes it is a combination of properties that gives value to a wood. Among these properties are hygroscopicity, shrinkage, weight, strength, cleavability, elasticity, hardness, and toughness.
THE HYGROSCOPICITY1 OF WOOD.
It is evident that water plays a large part in the economy of the tree. It occurs in wood in three different ways: In the sap which fills or partly fills the cavities of the wood cells, in the cell walls which it saturates, and in the live protoplasm, of which it constitutes 90 per cent. The younger the wood, the more water it contains, hence the sap-wood contains much more than the heart-wood, at times even twice as much.
In fresh sap-wood, 60 per cent. of the water is in the cell cavities, 35 per cent. in the cell walls, and only 5 per cent. in the protoplasm. There is so much water in green wood that a sappy pole will soon sink when set afloat. The reason why there is much less water in heart-wood is because its cells are dead and inactive, and hence without sap and without protoplasm. There is only what saturates the cell walls. Even so, there is considerable water in heart-wood.2
The lighter kinds have the most water in the sap-wood, thus sycamore has more than hickory.
Curiously enough, a tree contains about as much water in winter as in summer. The water is held there, it is supposed, by capillary attraction, since the cells are inactive, so that at all times the water in wood keeps the cell walls distended.
THE