Fig. 23. - a, the cortex or bark; b, the cambium; c, the annual rings; d, the medulla or pith; e, the medullary rays : these are drawn in a darker tint, to bring them prominently into notice. They are usually lighter coloured than the remainder of the wood.

These are the medullary rays.

Fig. 23 is the representation of a piece of wood showing the transverse section, and two vertical sections at right angles to each other. This illustration shows the general features alluded to, and by showing several sections of the medullary rays, makes their real shape apparent. The wedge-like column of cells forming a medullary ray, it will be seen, extends much more in a vertical direction than tangentially to the curve of the annual rings. In shape and arrangement the medullary rays may be said to resemble, in some degree, the spokes of a wheel.

Thus the medullary rays or 'silver grain' act in the economy of the tree as ducts to convey moisture and nutrition more easily and rapidly through the tree, for the cells here, as in the vertical tracheides, are provided with lateral valves for the percolation of liquid matter.

The medullary rays also have a very strengthening effect on the tree, as they bind the outside to the inside; and being opposed vertically to crushing strains applied to a baulk or log laterally, make the resisting power of the log very much greater.

The thickness of annual rings varies in different kinds of trees, and specimens of the same species will also have rings of greatly different thicknesses. Soil, situation, and the atmospheric conditions are the chief causes of difference in growth, wide rings being formed in favourable seasons and narrow ones in seasons when the weather, or any other circumstance is not so advantageous to the growth of the tree.

Accordingly, in tropical climates, where no material checks from changing seasons take place, the formation of wood progresses frequently without any apparent difference between spring and autumn wood, giving a level, even-coloured appearance, as if no annual ring existed. Peculiarities of this and other kinds occur in every possible combination, and it is really only possible to give here the particular characteristics of certain well-known woods, which will probably be found useful in manual training.

Felling

A tree should always be felled at a time when the sap is at a standstill - the height of the summer, when it has ceased to run up the tree, but has not yet begun to return; or the winter, when it has completely run down. The winter is much the better time, however, in temperate climates, as there is then the least amount of sap, and probably of moisture, in the tree, and less loss of weight and less contraction from drying and seasoning will occur.

Timber, if containing much sap, is not so durable, and is more liable to warps, twists, and shakes in seasoning.

Trees are sometimes barked in the early spring, especially in the case of such trees as the oak, which has valuable bark. When this is done the tree is felled in the following winter, the debarking being said to improve the timber.

The exact time for felling timber varies according to the special characteristics of each kind of tree; but the great thing to be attended to is the condition of the sap, which must be at rest.

Seasoning

The sap in felled timber will, by the presence of the organic substances in it, afford an opportunity for the successful attack of parasitic fungi, and the timber will, if this happens, be quickly destroyed or rendered useless. It is, therefore, essential to remove the sap together with the moisture, which frequently forms some 40 or 50 per cent., or even more, of the total weight. To effect this, the log, when stripped of its bark, is submitted to certain courses of treatment for varying periods.

It is clear that the removal of moisture from a baulk, or log, of timber will reduce it in bulk and prevent further shrinkage after it is wrought to a particular shape, when perhaps the consequences might be disastrous. Paint, or varnish, too, cannot be put upon wet or unseasoned timber, as it will almost certainly come off in flakes, and in any case will lose its virtue.

The most common form of seasoning is the 'natural,' and, generally speaking, it is the best. This plan is to stack the stripped logs, split in halves or sawn into planks, either on end or horizontally in racks. This stacked timber should be protected by a roof from sunshine and rain, but each piece must be so placed as to allow a free circulation of air round it.

On no account should the ends of the planks or logs rest on the ground, as moisture will then probably ascend through the pores, ultimately causing dry rot.

No vegetation should exist near seasoning timber, as it may transmit the germ of dry rot, and timber yards are frequently strewn with ashes to prevent its growth.

The usual plan of stacking timber to ensure a free current of air when in a horizontal position is to place thin strips of wood at intervals between the planks.