All trees which provide us with material for the carpenter and joiner's use are what are called outward-growing trees - that is to say, those whereof the gradually increased size is brought about by successive layers, which might be said to be formed around the circumference of the stem or trunk, and close underneath within the bark. Each year generally adds one of these layers or rings, and consequently they are called annual rings; but each ring is, as it were, double, being made up of two distinct layers of a different nature and colour. The darker layer is more apparent in certain kinds of timber, and is usually a source of weakness in a varying degree; inasmuch as being, for the most part, composed of resin, it is of a soft nature, with very little power of resistance to either compression or tension; while, on the other hand, the lighter-coloured layer is that which possesses the greater tenacity.
The growth-processes, of which these annular rings are the result, are briefly as follows : The tree, during the autumn and winter, having been to all appearances dead, when spring comes round absorbs, by means of its roots, a considerable amount of moisture out of the soil; and this, in the form of sap, ascends the cellular tubes of the tree, and causes the leaves and signs 'of life to appear. As summer advances the leaves become stronger and of more substance, owing to the carbon extracted by them from the air; and as autumn again draws near the sap, in its more substantial state, begins to descend, leaving a layer of wood immediately under the bark; then the leaves, owing to the loss of nourishment, drop off. This action takes place each year, so that the age of a tree (in ordinary climates) can be ascertained by the number of its annular layers or rings. In cold climates, where of course trees do not grow so fast, the rings are not so thick, and the inner or "heart" wood is more compressed; so that timber of slow growth is considered to be of greater strength. The "heart" wood or "duramen" is the strongest, best, and most durable part of the tree, the outside or sappy part being considerably weaker, and practically unfit for use.
In addition to the annual layers or rings just explained, trees have what are called medullary rays, which are thin and generally broken lines, radiating from the centre or pith to the bark, and vice versa, as fig. 261. These medullary rays or transverse septa are not very easily seen in some woods, but in some kinds of oak they are very apparent; and if the wood is cut obliquely they form a very beautiful figure, as it is commonly called, but also known as felt or silvered grain, from its light-coloured, and hard, polished appearance.
It may be as well to mention that trees generally should be felled in the middle of the winter, when the sap has descended and the tree is at rest There is also a proper age for the felling of trees, as the young ones are full of sap, and the old ones decayed at the heart; but only experience will enable one to judge of the proper time to fell, as the period at which a tree may be said to be at its prime varies from 50 to 150 years.