An introduction to physiological and systematical botany/Chapter 5
OF THE BARK.
Under the Cellular Integument we find the Bark, consisting of but one layer in plants or branches only one year old, and often not distinguishable from the wood. In the older branches and trunks of trees, it consists of as many layers as they are years old, the innermost being called the liber; and it is in this layer only that the essential vital functions are carried on for the time being, after which it is pushed outwards with the Cellular Integument, and becomes like that a lifeless crust. These older layers, however, are for some time reservoirs of the peculiar secreted juices of the plant, which perhaps they may help to perfect.
In some roots the bark, though only of annual duration, is very thick; as in the Carrot, the red part of which is all bark. In the Parsnep, though not distinctly coloured, it is no less evident. In the Turnip it is much thinner, though equally distinct from the wood or body of the root.
The Bark contains a great number of woody fibres, running for the most part longitudinally, which give it tenacity, and in which it differs very essentially from the parts already described. These woody fibres when separated by maceration exhibit in general a kind of net-work, and in many instances great regularity and beauty of structure. In a family of plants to which the Mezereon belongs, the fibres of the inner bark have a beautiful white shining appearance like silk. In one of this tribe, a native of Jamaica, and called Lace Bark, that part may be separated by lateral extension into an elegant kind of lace.
In the old bark of the Fir tribe, on the contrary, nothing of this kind is discernible. The bark of the Cluster Pine, Pinus Pinaster, some inches in thickness, is separable into thin porous layers, each of them the production of one season, which do really seem to be, according to M. Mirbel's theory, hardened and dried Cellular Integument; but they are rather perhaps that vascular part of the Bark which once contained the secreted fluid, or turpentine, so abundant in this tree.
The bark of Oak trees twenty or thirty years old, if cut and long exposed to the weather, separates into many fine thin layers, of a similar, though less delicate, texture to the Lace Bark of Jamaica. All these layers, in a living state, are closely connected with each other by the cellular texture which pervades the vegetable body in general, as well as by transverse vessels necessary for the performance of several functions hereafter to be mentioned.
In the bark the peculiar virtues or qualities of particular plants chiefly reside, and more especially in several of its internal layers nearest to the wood. Here we find in appropriate vessels the resin of the Fir and Juniper, the astringent principle of the Oak and Willow, on which their tanning property depends, the fine and valuable bitter of the Peruvian Bark, and the exquisitely aromatic oil of the Cinnamon. The same secretions do indeed, more or less, pervade the wood and other parts of these plants, but usually in a less concentrated form.
When a portion of the bark of a tree is removed, the remainder has a power of extending itself laterally, though very gradually, till the wound is closed. This is accomplished by each new layer, added to the bark internally, spreading a little beyond the edge of the preceding layer. The operation of closing the wound goes on the more slowly, as the wood underneath, from exposure to the air, has become dead, and frequently rotten, proving an incumbrance, which though the living principle cannot in this instance free itself from, it has no power of turning to any good account. If, however, this dead wood be carefully removed, and the wound protected from the injuries of the atmosphere, the new bark is found to spread much more rapidly; and as every new layer of bark forms, as will be proved in the next chapter, a new layer of wood, the whole cavity, whatever it may be, is in process of time filled up.
This operation of Nature was turned to great advantage by the late Mr. Forsyth of Kensington gardens, the history of whose experiments is before the public. Under his management many timber trees, become entirely hollow, were filled with new wood, and made to produce fresh and vigorous branches; and pear-trees planted in the time of King William, and become so decayed and knotty as to bear no fruit worth gathering, were by gradual paring away of the old wood and bark, and the application of a composition judiciously contrived to stick close and keep out air and wet, restored to such health and strength as to cover the garden walls with new branches bearing a profusion of fine fruit. These experiments have passed under my own actual observation, and I am happy to bear testimony to the merits of a real lover of useful science, and one of the most honest and disinterested men I ever knew.