15. In the stem of several trees, especially the oak and the locust, we observe lines which radiate from the center of the tree; these are called medullary rays. When they connect the pith to the bark, as at i i, in Fig. 1, they are called primary rays; but where they extend through only a portion of the stem they are called secondary rays.

These medullary rays, generally called silver grain, when exposed on the surface of the cut lumber, consist of a series of vertical plates or sheets. Originally of cellular tissue, they have become flattened by compression until they resemble sheets of mica; they are not, however, continuous vertically, but are buckled and present a serpentine outline, when exposed on the edge as shown in Fig. 2. On the end-cut of the Figure, the medullary rays are marked a, a, while the porous fibers of the zones are marked b, b, and the close or denser ones are marked c, c.

The appearance of the silver grain, or medullary plate, when cut nearly parallel to its direction is shown at a', a'; it is the presence of these medullary rays which gives so much beauty to quartered oak. The rays are prominent also in beech and sycamore, but are not so well defined in birch, chestnut, and maple.

Timber Properties Of Timber Part 2 243

Fig. 2.

16. The structure of the stem, as it presents itself to the observer on examining the end of an oak or ash tree of 13 years' growth, is shown in Fig. 3, in which the porous fibers or sap vessels of the coarser texture are shown at a, a; the closer texture at b, b; the primary medullary rays at i, i, and the secondary ones at j, j; the zone of the bark being shown at c, c. In a temperate climate the process of growth may be thus described: In the spring, the roots extract the juices (generally called crude sap) requisite for vegetable growth from the earth; the crude sap circulates from cell to cell and through the fiber tubes, and ascends and forms the leaves.

At the upper surface of the leaves, under the influence of light, chemical changes take place, the sap absorbing carbon from the air and becoming denser. After the leaves are fully developed, there is no further growth until the autumn, when the so called elaborated sap descends by the under side of the leaves, enters the branches and stem, and continues its descent principally between the wood and the bark, where, by the action of the cambium cells, it builds up a new woody layer or zone for that year, a portion being absorbed by the bark for its nourishment.

During this period the leaves fall off, and when the sap has ceased to circulate, the growth of the tree is suspended. In tropical climates, the circulation of the sap ceases during the dry season.

17. Midsummer and midwinter in temperate climates, and the dry period in tropical climates, are considered to be the best seasons for felling the trees; in both cases it is the time when the sap has ceased to circulate.

Timber Properties Of Timber Part 2 244

Fig. 3.

18. For building purposes, the trees should not be felled until they have attained their mature growth, as already stated; nor should they be used after the tree has exhibited signs of declining vitality. The greatest number of trees arrive at maturity between 50 and 100 years, and commence to decline after 150 or 200 years. When the top of the tree ceases to send forth its full complement of leaves in the spring and bare branches remain, making it appear "stag-headed," it clearly shows a lack of nutritive power, and is a sure sign that it has begun to lose its strength and vitality. The timber then loses its elasticity and firmness and gradually becomes crisp and brittle.

19. Trees growing in the heart of the forest are generally straight and tall, as it is necessary for their leaves to receive sunlight and air sufficient for vitalizing the sap; the lower branches of these trees only last a few years, when they die and fall off. On the edges of the forest, the lower branches of the trees remain alive and active, so that timber cut from such places is knotty and cross-grained, while that cut from the inside trees is clear and straight grained.

20. Where streams are available, the logs are hauled or drawn to them, and floated to the mill; this immersion in the running stream makes the sap more soluble, and tends to make it more responsive to evaporation, during the process of seasoning; but if long immersed, the fiber loses its virtue and elasticity. To save the cost of hauling the waste material, portable engines and saws are frequently transported to the place where the logs are cut.

21. When the lumber is reduced from the dimensions originally cut from the log, it is said to be resawed. After lumber is planed at the mill, it is called dressed lumber, and is assorted into many grades.

The regular thickness of dressed lumber is 5/8, 7/8, 1 1/8, 1 3/8, and 1 7/8 inches, the thickness of the rough sawed material being 1/8 inch greater than these measurements.

Where 1/2-inch dressed material is required, it is usually made by resawing 1 1/4-inch boards.

22. In the body of sound, healthy trees there often occur circular seams, or cracks, where the layers have become separated from each other; these are said to be caused by the action of violent wind storms upon the stem of the tree during the formation of the woody layers, and are called cup shakes. They generally occur near the base of the stem, and in some cases can be detected in the standing timber by an abnormal increase in the bulk of the stem.

Where they occur, as at a, a, Fig. 4, there is much waste in cutting the material; where the shake is short, as at b, the loss is not great. Sometimes circular bands occur in the stem, in which the wood is of a softer and more spongy character than the surrounding layers, and which, in some cases, show signs of incipient decay. This condition is assumed to be caused by the action of sharp frosts upon the rising sap in the newly formed layers. When timber presents this appearance, it should be immediately rejected, as it will be short lived and will soon decay.