Fig. 2. - Showing manner of growth in needle - leaved trees.
* The terms needle-leaved trees and broad-leaved trees used throughout this book may be taken as practically synonymous with Conifers and Dicotyledonous trees. - Trs.
Narrow annual layers betoken good wood in needle-leaved trees ; but the opposite holds good in the case of broad-leaved trees with large pores, e.g., the oak, the ash, and the elm. Here broad annual layers are characteristic of a good quality of wood, because the pores which render the wood open in the grain occur chiefly in that portion of the layer which is formed in early spring, and are less numerous in the closer tissue of the autumn wood. See Fig. 3.
Close and loose timber.
Narrow layers, hard resinous timber.
Broad layers, loose fibred timber.
Narrow layers, loose fibred porous timber.
Broad layers, hard timber.
Vessels or Air-tubes.
When a cross-section of a stem is carefully examined a number of minute holes or pores are seen. These are the mouths of vessels or air-tubes, which penetrate the whole substance of the wood, parallel with the fibres. Their function is to enable the air to circulate in the stem, and they are found even in wood of the closest grain, rendering it porous. Vessels are most numerous in the wood formed early in spring, and very few are found in autumn wood, a circumstance which helps to make the annual layers more distinct. According to the size of these vessels wood is said to be fine or coarse-grained.
Each kind of tree has something peculiar to itself in the manner of distribution, the number, and the size of its vessels. They are most marked in the oak, the ash, and the elm, giving to the wood of these trees, when seen in vertical section, its striped or streaked appearance. In a number of trees on the other hand, e.g., the birch, the vessels are hardly visible, and they are distributed pretty equally over the concentric annual layers, making it difficult to distinguish consecutive layers.
The porous-ness of wood.
Needle-leaved trees have no air vessels, but have channels filled with resin, i.e., resin-canals. These occur chiefly in the autumn wood, to which they give a darker colour.
Heart-wood and Sap-wood.
In many kinds of trees, when the stem is sawn across, a considerable difference may be observed between the appearance of the inner and older, and the outer and younger concentric annual layers. The inner layers are usually firmer and closer in texture and darker in colour than the outer, which are less compact, lighter in colour, and full of sap.
The firmer, darker wood is called heart-wood or duramen ; the looser, lighter wood, sap-ivood or alburnum. As a rule the latter forms a comparatively narrow ring round the former, which constitutes the greater portion of the stem, and which, when sound, is the valuable portion on account of its firmer texture and greater durability.
The proportion which the heart-wood bears to the sap-wood varies in different kinds of trees. For example, in the case of broad-leaved trees, the proportion is largest in the oak, the ash, and the elm; least in the birch, the maple, the alder, the hornbeam, etc. In needle-leaved trees, it is greatest in the larch and the fir; least in the pine. The resin in these trees is found chiefly in the heart-wood. It greatly increases its closeness and durability, and darkens its colour.
The most striking example of the difference in appearance between heart-wood and sap-wood is presented by ebony, in which the former is black and the latter white.
The Heart-wood, the valuable part of the stem.
The Pith and the Medullary Rays.
The pith forms a column in the central part of the stem, and the medullary rays radiate from the pith towards the bark.
The pith is looser in texture, and is composed of shorter cells than the wood. The shape and size of the column vary considerably in different trees. In some, e.g., the yew, it is very thin; in others, e.g., the elder, it occupies a considerable space.
The medullary rays or " transverse septa " are composed of flat cellular tissue, which forms thin vertical plates radiating towards the bark. During the first year of the growth of the tree, these rays originate in the pith, divide the patches of wood and bast, and reach as far as the bark. In subsequent years they are formed in connection with the new wood, not with the pith, and they extend into the bark. The medullary rays are the medium by which the pith and the wood are brought into communication with the bark. They also divide the wood into wedge-shaped bundles. They are seldom so straight and regularly disposed as is represented in the diagram (Fig. 1), but are generally more or less curved, and they often branch out obliquely. They vary considerably both in number and appearance in different trees, and thus, like the vessels, they serve as a guide to the recognition of different kinds of wood. For example, oak is easily known by the smoothness and glossiness of its broad medullary rays when these are seen in radial section. This gives to oak timber the beautiful figured appearance called " silver grain." The beech has also long, broad medullary rays. The maple is distinguished by the fineness and number of its medullary rays.
In the greater number of loose-fibred, broad-leaved trees, the rays are very narrow, and scarcely distinguishable by the naked eye. This is also the case with needle-leaved trees, the rays of which are extremely numerous.
Different kinds of wood known by the character of the medullary rays.
The medullary rays affect to a considerable extent the ease or difficulty with which wood may be split. As a general rule, timber is easily split if it has broad rays like the oak and the beech, or if the rays, though numerous, are straight and narrow like those of the fir and the pine. Other circumstances, however, may determine the greater or less resistance which any given timber presents to cleavage.
The cleavage of wood.
Next to the wood the sap is the most important element in timber. Its chief constituent is water, which holds in solution various organic and inorganic substances, but its composition undergoes changes in the course of circulation through the different parts of the tree.
The sap materials are absorbed by the roots, and as crude, or ascending sap, are carried by the still active cells of the sap-wood to the leaves. Here, through the influence of light and air, the crude sap is changed and made fit for the nourishment and growth of the tree, and is called elaborated sap. From the leaves it descends in the bast tubes to the cambium, where the new wood and bast are formed.
Amongst the organic substances which the sap holds in solution may be named, starch, sugar, colouring matter, tannic acid, and albuminoids. The latter render it very liable to fermentation, and when this takes place the wood decays. This is the reason why timber, felled when the sap is circulating, and allowed to lie unbarked, readily becomes " sour." It also explains why sap-wood decays more quickly than heart-wood.
When wood is burnt the inorganic constituents remain in the ashes.
Sap also contains substances which are not required for the growth of the tree, but which occupy space and channels in the wood. Amongst these substances are the volatile oils, which are found chiefly in needle-leaved trees, and of which turpentine is the most important. The resin or gum found in needle-leaved trees is also formed from these oils. Tannic acid is found in a great many trees, especially in the bark. It is known by its acrid taste, and it abounds chiefly in the oak, the fir, and the alder. When fresh timber in which there is a great deal of tannic acid is split or sawn, the acid makes the polished edge of the tool become blue-black in colour.
The organic constituents of the sap.
Turpentine, resin, gum, and tannic acid.
The destructive effect of the albuminoids of the sap is counteracted by the turpentine, resin, and tannic acid.
The sap, as stated above, consists chiefly of water; and, as it circulates in the sap-wood, it follows that the latter contains more water than the heart-wood, and more in spring than in the height of summer. As a general rule the water contained in unseasoned wood is about 40 to 50 per cent, of the weight of the wood. In unseasoned ash and beech it is 20 to 30 per cent.; in loose-grained oak, hornbeam, maple, elm, Scotch fir, and spruce fir, 30 to 40 per cent.; in the looser fibred trees in which sap abounds, e.g., the alder, the lime, the willow, and the aspen, 40 to 50 per cent.
The presence of water has generally a hurtful effect upon timber, as is shown in what follows.