An introduction to physiological and systematical botany/Chapter 6
CHAPTER VI.
OF THE WOOD.
When the bark is removed, we come to the substance of the wood, which makes the principal bulk of the trunk or branch of a tree or shrub. When cut across, it is found to consist of numerous concentric layers, very distinct in the Fir, and other European trees in general. Each of these circular layers is externally most hard and solid. They differ however among themselves in this respect, as well as in their breadth on the whole. It often happens that all the layers are broadest towards one side of the tree, so that their common centre is thrown very much out of the actual centre of the trunk.
The wood owes its strength and tenacity to innumerable woody fibres, and consists of various vessels running for the most part longitudinally; some having a spiral coat, others not. Of these vessels, some in their youngest state convey the sap from the root to the extremities of the branches and leaves; others contain the various peculiar or secreted juices; others perhaps contain air. The whole are joined together by the cellular substance already described.
Linnæus and most writers believe that one of the abovementioned circular layers of wood is formed every year, the hard external part being caused by the cold of winter; consequently, that the exact age of a sound tree when felled may be known by counting these rings. It has even been asserted that the date of peculiarly severe winters may be found in the harder more condensed rings formed at those periods; and moreover, that the north side of a tree may always be known by the narrowness and density of the rings on that side. All this is controverted by Mirbel, chiefly on the authority of Du Hamel, who nevertheless scarcely says enough to invalidate the ancient opinion on the whole. It is very true that there may be occasional interruptions in the formation of the wood from cold or fickle seasons, and that in some trees the thin intermediate layers, hardly discernible in general, which unite to form the principal or annual ones, may, from such fluctuation of seasons, become more distinct than is natural to them. Such intermediate layers are even found more numerous in some trees of the same species and age than in others. But as there is always a most material difference between summer and winter, so I believe will there always be a clear distinction between the annual rings of such trees as show them at all. Trees of hot countries indeed, as Mahogany, and evergreens in general, have them but indistinctly marked; yet even in these they are to be seen. With regard to their greater compactness on the north side of a tree, Du Hamel justly explodes this idea. In fact, there is most wood formed, and consequently these circles are broadest, on the side most favourable to vegetation, and where there are most branches and leaves. This in a solitary tree is generally towards the south; but it is easy to perceive the occasional variations which must arise from local exposure, soil, moisture and other causes.
In some trees, a number of the outermost rings differ greatly in colour from the innermost, and are called by workmen the sap. In the Laburnum the former are yellow, the latter brown. In the Oak and many other trees a similar difference, though less striking, is perceptible, and in most the external rings are much less firm, compact and durable than the rest, retaining more vital principle, and more of the peculiar juices of the plant. Such rings are all comprehended by Du Hamel under the name of Aubier, alburnum; and he rightly observes that this difference often extends to a greater number of rings on one side of a tree than on another. It seems that the more vigour there is in a tree, or side of a tree, the sooner is its alburnum made perfect wood. By this term, however, is properly understood only the layer of new unhardened wood of the present year. When the word alburnum is used in the following pages, it applies to this part only.
Physiologists have long differed and do still differ about the origin of the wood. Malpighi and Grew thought it was formed by the bark, and the best observations have confirmed their opinion. Hales supposed the wood added a new layer to itself externally every year. Linnæus had a peculiar notion, that a new layer of wood was secreted annually from the pith, and added internally to the former ones. Truth obliges us to confess that the latter theory is most devoid of any kind of proof or probability.
Du Hamel, by many experiments, proved the wood to be secreted or deposited from the innermost part of the bark or liber. He introduced plates of tinfoil under the barks of growing trees, carefully binding up their wounds, and, after some years, on cutting them across, he found the layers of new wood on the outside of the tin. His original specimens I have examined in the public museum at Paris.
Dr. Hope, the late worthy Professor of Botany at Edinburgh, instituted an experiment, if possible more decisive, upon a branch of Willow three or four years old. The bark was carefully cut through longitudinally on one side for the length of several inches, so that it might be slipped aside from the wood in the form of a hollow cylinder, the two ends being undisturbed. The edges of the bark were then united as carefully as possible, the wood covered from the air, and the whole bound up to secure it from external injury. After a few years, the branch was cut through transversely. The cylinder of bark was found lined with layers of new wood, whose number added to those in the wood from which it had been stripped, made up the number of rings in the branch above and below the experiment. For an account of this experiment I am indebted to Dr. Thomas Hope, the present Chemical Professor at Edinburgh.
Du Hamel engrafted a portion of the bark of a Peach-tree upon a Plum. After some time he found a layer of new wood under the engrafted bark, white like that of the Peach, and evidently different from the red wood of the Plum. Moreover, in this and other experiments made with the same intention, he found the layers of new wood always connected with the bark, and not united to the old wood. See his Physique des Arbres, vol. 2. 29, &c. It deserves also to be mentioned, that by performing this experiment of engrafting a portion of bark at different periods through the spring and summer, the same accurate observer found a great difference in the thickness of the layer of new wood produced under it, which was always less in proportion as the operation was performed later in the season.
That the bark or liber produces wood seems therefore proved beyond dispute, but some experiments persuaded Du Hamel that in certain circumstances the wood was capable of producing a new bark. This never happened in any case but when the whole trunk of a tree was stripped of its bark. A Cherry-tree treated in this manner exuded from the whole surface of its wood in little points a gelatinous matter, which gradually extended over the whole and became a new bark, under which a layer of new wood was speedily formed. Hence Mirbel concludes, vol. 1, 176, that the alburnum and the wood are really the origin of the new layers of wood, by producing first this gelatinous substance, or matter of organization, which he and Du Hamel call cambium, and which Mirbel supposes to produce the liber or young bark, and at the same time, by a peculiar arrangement of the vascular parts, the alburnum or new wood. His opinion is strengthened by the observation of a tribe of plants to be explained hereafter, Palms, Grasses, &c. in which there is no real bark, and in which he finds that the woody fibres do actually produce the cambium. Dr. Hope's experiment will scarcely invalidate this opinion, because it may be said the cambium had already in that case formed the liber.
This matter will be better understood when we come to speak of Mr. Knight's experiments on the course of the sap.