CHAPTER IL.
CHAPTER IL.
TIMBER.
The Source of Timber. —The wood used by the carpenter and joiner is obtained from the plants known as trees. In tropical countries palms and grasses (e.g. bamboo) grow to great size, but the stems of these plants are unsuitable for timber. In temperate climates most forest trees are of a quite different type, belonging either to the class which includes the oak, ash, beech, birch, etc., or to that known as the conifers, among which are the pines and firs. It is such trees alone which yield the timber used by wood-workers.
The Structure of the Stem of the Oak.—If a branch or young stem of an oak tree be cut across, it will be seen to consist of (a) a central pith; (6) layers of wood; (c) bark, consisting of an outer part composed of corky and dead layers, and an inner part of bast which can be torn off in shreds ; and (d) a thin layer between the bast and the wood called cambium which, by dividing, forms new layers of wood on its inner side and new layers of bast on its outer side. The cambium is most active in spring and early summer, and the new wood then formed is of open texture. As autumn approaches the activity of the cambium decreases, and the wood it forms is close in texture and small in amount. In the winter the division of the cambium stops altogether, to be renewed in spring by the formation of more open-textured wood. The difference in the appearance of the autumn wood of one year and the spring wood of the next is so marked that it is easy to distinguish the
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2 A MANUAL OF CARPENTRY AND JOINERY.
limits of the wood formed in one year. The layer of wood formed in one year is called an annual ring. The bast is soft and becomes squeezed up under the bark so that it is not at all conspicuous. ,
In such a cross section as that described may be seen, stretch- ing from the pith to the bark, a number of radial lines of tissue
Fia. 1.—Sketch showing the appearance of silver grain in a radial longitudinal section of Oak.
which are called medullary rays. A comparison of such a trans- verse section with wood cut in other directions shows that the medullary rays are really thin lath-like plates arranged radially. In a radial longitudinal section of the wood (Fig. 1) the
Pith.
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BARK:
Fic. 2.—Cross section of the trunk of an Oak tree.
medullary rays show as silvery patches on the surface, giving the appearance known as silver grain. In most kinds of wood the medullary rays—though really present—are not distinguish- able by the naked eye; they are most clearly seen in the oak and the beech. In a section of an older oak (Fig. 2) it will be seen that the wood consists of two well-marked parts: an inner heartwood, dark in colour and hard; and an outer sapwood,
TIMBER. 3
lighter in colour and of somewhat spongy texture. Their
difference is explained by the fact that the heartwood is dead and of no use to the tree except as a mechanical support ; while the sapwood is still ac- tively alive and con- veying up the trunk the water and mineral food which the roots take up from the soil.
The heartwood, al- though dead so far as the life of the tree is
concerned, is the only part of the tree which is suitable for use for constructional purposes.
When a tree begins to decay, the heartwood, being the oldest, is naturally the first to suffer. Trees cut down before they have attained maturity are likely to have an over-abundance of sap- wood.
Sapwood is unsuitable for use on account of its soft spongy texture, and its liability to absorb moisture.
The Timber of other Trees. —Although the above descrip- tion of the oak applies also, in its general features, to other timber- producing trees, there are many respects in which a marked difference is observable. In the pines, firs, and lJarches (Fig. 3), the annual rings are
more clearly defined, and the wood is perforated with small canals which contain resin (Fig. 4). Compared with oak the
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annual rings,
Fic. 4.—Portion of a four-year-old stem of the Pine, cutin winter. 1, 2, 3, 4, the successive annual rings of the wood; b, bast; br, bark ; ¢, cambium ;
4, spring wood; hk, resin canals; junction of wood of successive years m, pith; ms, medullary rays; s, autumn wood. (Four times natural size.)