Bark, in the dissection of plants, is the exterior coat of trees, corresponding to the skin of animals. As these are furnished with a cellular membrane covering all the fleshy parts, and usually replete, with white granulated fat, which can be. liquified only by heat; so are plants surrounded with a bark abounding with oily juices, by means of which, Nature has rendered them inaccessible to cold; because the spiculae of the ice are prevented from fixing and freezing the fluids, which circulate through the vessels. Hence it is, that evergreens continue their verdure at all seasons of the year, because their bark contains an unusual proportion of oil, more than is dissipated by the heat of the sun.
The bark of pants is liable to peculiar diseases, as well as to be preyed upon by insects, which frequently prove destructive to the tree. One of its most common enemies is the lark worm, which infests and perforates its substance; and unless the parts affected be cautiously removed by the knife, and the superficial wounds plastered over with a mixture of wax and turpentine, the stem will in process of time- become cankered, stunted in its growth, and ultimately fall a sacrifice to the disease.
M. Bui-ton has ascertained, by repeated experiments, that trees stripped of their bark the whole length of the stems, do not live longer than three or four years.
It deserves, however, to be remarked, that, when thus deprived of the whole bark, and suffered to die gradually, they afford a more compact, heavy, and more durable timber, than if they had been felled in their healthy state. The reason of this improvement is obvious, as those oily and astringent fluids, which are secreted for the uniform nourishment of the bark, are absorbed and deposited on the fibres of the wood, which, during the progressive dissolution of vegetable life, acquire what Nature had provided for the supply of the external integuments. Yet there is one disadvantage arising from the privation of the bark, perhaps tantamount to the additional value of the timber, namely, that the farther increase, or growth of the tree, is for three or four years effectually checked.
The barking of trees ought, in our climare, to be performed in spring, from about the middle of April to that of May; because at that time the circulating sap facilitates this operation, which, in dry-seasons is not only attended with additional labour, but the bark also will be of inferior value.
With respect: to the extent of stripping the oak-bark from trees, a wide difference of opinion appears to prevail. Some owners of large tracts of wood, and great admirers of timber, cautiously prohibit the removal of the bark nearer than six inches to the ground; about which spot they suppose the tree to be felled : while others enjoin it to be done as near the ground as possible, provided that in this operation there be no part of the foot laid bare. Mr. S. Hayes, the author of an excellent " Practical Treatise on Planting, " price 7s. published in 1796, inclines to the latter opinion; and adds, that the advocates for the former method would, on more accurate investigation, save themselves much unnecessary trouble, to little purpose, if not to their considerable injury.
The inner and more delicate part of the bark, especially that of the ash and lime trees, was used by the ancients, for writing and communicating their sublime ideas to posterity, prior to the invention of paper.
In economy, as well as in many of the practical arts, the utility of different barks is very great and extensive; for instance, that of the oak for tanning leather, and manuring the soil; the Peruvian, cinnamon, quassia, willow-bark, etc. in medicine and tor culinary uses; that of the alder and walnut trees in dyeing; and others again for a variety of purposes, such as the bark of the cork tree, etc. etc. Without detailing the particular and curious processes adopted by foreign nations, for rendering the barks of various trees essentially useful, we shall briefly state, that the Japanese make their beautiful paper of the barK obtained from a species of the mulberry tree, called morus the natives of Otaheite manufacture their cloth of the same tree, as well as the bread-fruit and the cocoa trees ; the Russians and Poles produce their shoes worn by the peasantry, twist rope:-:, and form a variety of other useful articles, of the inner bark of the lime tree; the Germans have, for the last twenty years, convert-ed the bark of the common black and white mulberry trees into excellent Paper.—An analytical account of the last mentioned article, interspersed with many new and curious facts, we propose to give in the sequel.
Bark. - Dr. Darwin considers the bark of the trunks of trees to be similar to that of their roots, of which he conceives it to constitute a part; in as much as it consists of an intertexture of the vessels, that descend from the plume of each individual bud to its radicle, and form its Caudex. The root-bark, however, is provided with lymphatics, for the absorption of water and nutritious juices from the earth, and is covered with a moister cuticle ; while that of the stem has similar vessels for absorbing humidity from the air, and is furnished with a drier cuticle.
Beside the purposes to which the bark of trees may be applied, and which have already been enumerated, there is a considerable quantity of mucilaginous or nutritious matter contained in the inner rind, or bark of the holly, elm, and also according to Dr. D.'s conjecture), in that of the hawthorn, gooseberry, furze, or other trees armed with prickles, for preventing the depredations of animals. This mucilage, he conceives, may be used in times of scarcity, as food, either for man or for cattle, or at for the purpose of fermentation. He remarks, that the inner bark of elm-trees, when stripped off in the spring, and boiled in water, may doubtless be converted into a palatable small-beer, with the addition of yeast.
The quantity of bark on a tree may be increased by pinching off the flower-BUDS, as soon as they appear; but, if the former be wounded by any accident, the edges of the dead rind ought to be carefully cut off, without injuring the living bark; and a mixture of white lead and boiled oil (see vol. i. p. 432) be applied, to preserve the wounded parts from air, moisture, and insects. - The following method of cure, which is stated to have been successfully practised where the bark of a tree had recently been torn off, we give on the authority of Dr. Darwin. It consists simply in again fastening the same piece of bark, or in tying down another piece from a tree, belonging to the same species; the edges of the wound and bark being carefully adjusted ; in consequence of which, the whole will combine in the same manner as the vessels of a scyon unite with those of the bark belonging to the engrafted stock.
A patent was lately granted to Mr. Whitby, for his improved mill, calculated to grind bark for the use of tanners. It is performed by a number of cutting wheels, that are fixed upon axles, and chop the bark to pieces ; which then fall through an eye, and pass between two large cast-iron plates, with grooves or furrows that are cut either hollow, or are bevelled square. The lower plate is made to move in a circular direction, with a view to facilitate the entrance of the bark into the eye. - These plates are set in motion by the mechanism commonly employed in mills.
This machinery, when moved by a horse, grinds 3 cwt. of bark, in one hour ; but as the plates which constitute the chief invention in this mill, may be made of any circumference, according to the power by which they- are impelled, the quantity ground in a certain time, will vary in proportion to their size. - The advantages stated to be derived from Mr. Whitby's contrivance are, a saving of the bark, and greater expedition in the process of tanning: for the rind thus reduced, without being pulverized, spends more rapidly and completely in the pits, than that prepared in mills of the common construction.
Patents were also granted, In February, 1801, to Mr. James WelDoN, in consequence of his improvements on a bark-mill, for which he obtained a privilege in 1798 ; - and, in May, 1801, to Mr. Thomas Bagnall, for a mill designed to chop, grind, riddle, and pound bark, etc. The inquisitive reader will consult the 15th volume of the " Repertory of Arts, " etc. where specifications are inserted, and illustrated with engravings.