Timber is the substantial substance of all trees. Woods differ in their properties; some being tough and hard, while others are brittle or soft. They are, therefore, of value proportional to the kind of work for which they are required. A great variety of opinions exists in regard to the manner in which wood is formed. All are, however, agreed, that the trunk and branches of trees are composed of three parts, - the bark, the wood, and the pith.

The Bark is a covering which incases the entire wood, and is composed of three distinct parts, - the Epidermis, the Cellula, and the Liber.

The Epidermis is a thin skin, being the extreme outer covering.

The Cellula is the organic matter next inside the Epidermis. It answers to the flesh of animals, and is formed into an infinite number of tubes.

Properties Of Timber

The Liber is the inner or newly formed bark. The Epidermis and the Liber together form what is called the Cutis, or outer bark, the Liber being the inner.

The Wood is the material that exists between the pith and the bark, and is of two kinds, - the heart-wood, or Duramen; and the sap-wood, or Alburnum.

The Heart-wood is the hard and dark part next the pith.

The Sap-wood is that which is between the heart-wood and the bark.

The Pith is the soft and spongy substance which is enclosed by the heart-wood at its centre.

In all new shoots, the pith and bark are in contact, without wood between them; but, as the shoot extends, it enlarges by the deposit of a secretion called cambium, which lies in a cylindrical ring between the pith and the bark. The deposit thus made is of two kinds. One is formed into bark, and the other ultimately hardens into wood. A deposit of this nature is made annually; and, if the trunk of a tree be cut off across the fibres of the wood, the surface will present a series of consecutive layers or rings, so that one is enabled to determine by their number the age of the tree in which they exist

It is unusual to find any two rings that are alike, either in regard to their whole thickness, or the proportion of the solid part to the porous; the dimensions and proportion being governed by the amount and nature of the deposit, some years being more favorable to each respectively than others.

So exact are the laws by which this is governed, that the part of the rings on the north side of trees is thinner, making the heart-wood nearer the north side. This results from the fact, that, the south side being more exposed to the action of the sun, the pores are expanded, and a larger quantity of sap is transmitted through that side.

The wood of no tree is entirely solid, but is filled with tubes, or pores; and the only substance of a solid nature that exists is that which forms the walls of the cells before named. These vessels are designed for the conveyance of a fluid called sap, which is absorbed by the roots, and passes up through the pores of the wood to the leaves, where it undergoes a chemical change, and is then returned through the cellula, or porous part of the bark.

Sap, when it leaves the roots, is very limpid, being nearly as thin as water: but, as it passes up through the pores, it either meets with a substance which it dissolves and carries along with it, or, when it arrives at the most distant parts, is condensed; for, on its return, it is thickened; and entirely changed in its nature.

As it passes downward through the cellula, it gradually deposits a large proportion of the material it contains; so that, when it arrives at the roots, it is as thin as when it started to pass upward.

As soon as the leaves are developed, sap ceases to flow. The deposit gradually hardens; and thus is formed a new laver of material for wood and bark. From this period till near autumn, vegetation ceases; but, after this, the sap is again in motion, and, as it passes up, deposits along the pores of the wood the substance which the ascending sap of the next spring will dissolve and carry along for the formation of the new wood and leaves. As the tree increases in diameter, the wood at the centre is compressed by the growth of the new wood; and, becoming more solid, the pores decrease in size, and hence but little sap will flow through them. The part nearer the bark, being less compressed, is soft and porous; and, as the larger part of the sap passes through it, it takes its name sap-wood.

Those parts of the tree which need to be elastic and porous are continually receiving new substance of a proper nature for its replenishment; while, at the same time, those which are compressed into hard wood serve to give the requisite and additional support, or back-bone, to the increased tree. It has been well remarked, that the life of a tree is like that of a man, and may as properly be divided into three periods, - infancy, maturity, and old age.

During the whole of the first period, the tree continues to increase. Through the second, it simply maintains itself, and neither loses nor gains. As soon, however, as the heart-wood begins to decay, the second period ends, and signs of old age soon appear: and the comparison is not then inapt; for like an old man who seems to be still fresh and vigorous, but whom one storm of disease may break and sweep away, so often does a venerable and revered oak, clinging still to life, as if loath to die, put on, with each returning spring, "its youthful robes anew." But, its heart diseased, and vitality expended, being engaged in some tempestuous hour in an unequal contest, it falls to rise no more.

The timber of all trees partakes, to a greater or less degree, of the nature of the soil on which it grows. Trees grown on soft and spongy soil usually produce wood that is comparatively soft and irregular in fibre. Therefore, if oak be grown on dry and good land, the wood will be solid and tough; but, if grown on soft and wet land, it will be proportionally poor, and of less value. This fact is true of all timber-trees.

The wood of trees which stand alone, or where there are but few, and those scattered, is better than that grown in the middle of a forest, where it is not exposed to the sun and air. Hence, for building purposes, those trees which stand alone are to be first selected.

It may be well to mention here, that if the softwood trees are very large (as is often the case with some of the pines), and most of the branches are near the top, the wood near the base of the trunk is sometimes found to be shaky. This defect is produced by the action of heavy winds on the top of the tree, which wrenches or twists the but, and thus cleaves apart the fibres of the wood.