438. Wood is that substance which forms the principal part of the roots, trunks, and branches of trees and shrubs; and its usefulness in the art of construction is well known.
The woods of different trees differ much in strength, hardness, durability, and beauty; and, consequently, in their fitness for the various purposes to which they are applied. The wood which is felled and seasoned for the purpose of building is called timber; and in stating the properties of woods, those only shall be considered which are fit for timber.*
439. If the stem or trunk of a tree be cut across, the wood is found to be made up of numerous concentric layers or rings; very distinct in some trees, but less so in others. One of these layers is usually formed every year; consequently their number corresponds nearly with the age of the tree. Each layer consists, in general, of two parts; the one solid, hard, heavy, and dark coloured; the other of a lighter colour, porous, and soft; which renders the lines of separation between the annual layers distinct. Scarcely any two layers of the same tree are precisely alike, either in the proportion of the hard part, or in the thickness of the layers;
* Timber is derived by Dr. Johnson from the Saxon timbrian, to build; hence the above definition: but it is more probable that the term is derived from the French timbrien, to mark or stamp - denoting trees marked for being cut. The legal definition of timber is restricted to particular species of wood, and custom varies in different counties as to the species ranked among the timber trees.
as the layers vary in thickness according to the degree of vegetation which took place in the years of their formation: and also in the same tree they vary in thickness, either according to the situation of the principal roots, or the aspect; the annual layers being always thicker on that side of the tree which has been most favourable to the growth of the roots, or that which has had the advantage of a good aspect.
Wood appears to be composed of various vessels, which, in the living tree, convey the fluids necessary to its growth; between those vessels there are cells interposed. There is nothing of the character of solid fibres in wood, except the thin membraneous coats of the cells and vessels, which adhere so slightly together in recently formed wood, that it is easy to separate them. The vessels in the growing tree are intended to convey a watery fluid, called the sap, from the roots to the leaves; when it arrives at the leaves it undergoes some changes and returns through the bark;* and the bark being expanded by this accession of moisture, rises from the wood, and leaves a cavity that becomes filled with the proper sap, which gradually hardens and forms a new layer of wood. The rising sap flows chiefly through the annual rings next the bark; and from the experiments of Mr. Knight, † it appears that the sap during its ascent dissolves some portion of a substance that had been deposited in the vessels of the wood during the preceding winter, for the nourishment of the buds, leaves, and young wood; hence the flowing sap is more dense in the upper than in the lower part of a tree. Dr. Darwin draws a like conclusion from the debarked oaks producing leaves.‡
* Art. Anatomy, Vegetable,' Encyclopaedia Britannica.' †'Philosophical Transactions,' 1805.
‡' Phytologia,' p. 159. The fact may be owing entirely to the fluid being reduced by evaporation.
In trees, as the leaves expand the sap ceases to flow, and the bark again adheres to the wood; and from the middle of June to the middle of August there appears to be a pause in vegetation; but after this period the sap again begins to flow, and the bark which adhered so closely in the preceding months may be separated almost as easily as in the spring.
440. The sap which rises through the wood from the roots is very different in its nature from that which descends through the bark to form the new layer of wood. That which ascends is nearly as liquid as water, and is called the common sap. It has in general a sweetish taste, and contains sugar and mucilage. It always contains an acid, sometimes in a free state, sometimes combined with lime or potash. When this sap is left to itself, it soon ferments and becomes sour; and when the proportion of sugar is considerable, it will undergo the vinous fermentation.*
The descending sap, called the proper sap, differs so considerably in different trees, and is so difficult to procure in a separate state, that its properties have not been closely examined. It is always less liquid, and contains a much greater proportion of vegetable matter than the common sap. It is also very probable that trees of the same kind produce proper sap of different qualities in different climates, as we find that the facts established respecting timber grown in one climate are not applicable to the same species of timber grown in another.
441. That part of the wood next the bark is called sap-wood, because it is through it chiefly that the sap ascends; and as it is shown by Mr. Knight to contain some vegetable matter to be expended in forming leaves and buds, it is reasonable to suppose from analogy that the sap-wood must be more prone to decay than the internal part of the tree, called the heart-wood.
* The properties of the different kinds of sap that have been examined are given in Dr. Thomson's 'System of Chemistry,' iv., 209-213.
As trees increase in size the oldest part of the sap-wood gradually loses all vegetable life, and the more fluid parts of it are either absorbed by the new forming sap-wood or evaporated; its vessels and cells become closed by the pressure of the new-forming wood, and it ceases to perform any other part in the growth of a tree than to support it. When these changes have taken place it is found to be more compact, and generally of a darker colour; and also contains only a small proportion of vegetable matter besides that kind which is called the woody fibre by chemists. It is then heart-wood, or wood in its most perfect state.
The sap-wood is softer and generally lighter coloured than the heart-wood, and contains a considerable portion of vegetable matter, which partakes of the nature of the sap which ascends through it. It is found to decay rapidly, and is also very subject to the attack of worms. The reason is obvious, for it contains the food upon which they live, the most of which is absorbed or evaporated from the heart-wood.*
The proportion of sap-wood in different trees varies considerably; Spanish chestnut has a very small proportion of sap-wood, oak has more, and fir a still larger proportion than oak; but the proportions vary according to the situation and soil. Three specimens of a medium quality gave the following: -
Chestnut, whole age 58 years, 15 1/2 in. diam., 7 years sap-wood, 3/8 in. thick.
* In many woods the line of demarkation between the heart and sap-wood is so strongly defined, that, as a friend well remarks, " the change seems to take place, as it were, per saltum:" it certainly is a singular phenomenon, and merits more of the attention of physiologists than it has received.
Therefore, if the diameter be unity, or 1, that part of it which is sap-wood will be, in the chestnut, 0.1; in the oak, 0.294; and in the Scotch fir, 0.416. The Scotch fir was the produce of the Mar Forest.*
442. The life of trees, like that of men, has been commonly-divided into three stages, infancy, maturity, and old age. In the first, the tree increases from day to day; in the second, it maintains itself without sensible gain or loss; but in the third, it declines. These stages vary in every species according to the soil, the aspect, the climate, or the nature of the individual plant.
Sir H. Davy states† that oak and chestnut trees decay sooner in a moist soil than in a dry and sandy one, and their timber is less firm. The sap vessels being expanded with moisture wanting a sufficient quantity of nourishing matter, the general texture necessarily becomes less firm. Such wood splits easily, and is very liable to shrink and swell with the changes of the weather.
Trees of the same kind arrive at the greatest age in that climate which is best adapted to their nature. The common oak, fir, and birch thrive best towards the northern; the ash and the olive tree thrive best towards the southern parts of Europe.
We find, says Mirbel, the ash-trees of Calabria and Sicily to be longer lived than those of Prussia or Great Britain. ‡ Oak and chestnut trees, under favourable circumstances, sometimes attain an age of about 1000 years; beech, ash, and sycamore, seldom arrive at half that age.
The decline of trees appears to be caused by the decay of the heart-wood; it is this, as Sir H. Davy remarks, § which seems to constitute the great limit to the age and size of trees; and the long period the central parts are preserved by the cooling influence of the living trees is truly wonderful.
* These measures were taken from specimens in the collection of William Atkinson, Esq., Surveyor to the Board of Ordnance, † ' Agricultural Chemistry,' p. 255, 8vo edit. ‡ 'Journal of Science, etc.,' vol. iv., p. 11. § 'Agricultural Chemistry,' p. 220, 4to edit.
In trees that have not arrived at maturity, the hardness and solidity of the wood are greatest at the heart, and decrease towards the sap-wood; but in the mature or perfect tree the heart-wood is nearly uniform; while that of a tree on the decline is softer at the centre than it is next the sap-wood. These observations were made by Buffon in the course of his numerous experiments, and also by Duhamel.