Essays on the Microscope / Containing a Practical Description of the Most Improved Microscopes, a General History of Insects, etc., etc.

by George Adams

OF THE BARK.

The bark lies next within the rind, and differs but little from it in construction, though it holds a more important office in the scale of vegetation, the growth and qualities of the tree being in a great measure connected with it. It is, therefore, found to differ considerably in substance, quantity, and quality, in various kinds.

It is originally the outer membrane, covering the lobes of the seed. Even there, as in the branch of a tree, it appears in the form of a kind of spunge, or like a crust of bread, composed of flatted bladders.

Its spunge-like nature may be further inferred from the contraction of its pores when dry, and the ease with which they dilate when in water. Grew has called it a most curious and exquisitely fine, wrought spunge. In the course of its growth, the outer ranges of these bladders drying, it becomes what we call the rind; for the rind was once bark, and has only suffered a slight change in separating from it.

By the bark the tree is fed with a continual supply of moisture, protected from external injuries, and defended from the excesses of heat and cold; for these purposes it is variously disposed in different trees. In the hardy and slow growing, as the oak and chesnut, it is thin; in the quick growing, as willow, poplar, and the like, it is thick. And what is more particularly to be attended to is, that in some its inner verge is radiated. There are some trees, and a great many herbaceous plants, in which this part is continued inward, in form of rays, through the blea into the wood, and seems to form so many green wedges, that split as it were the substance of both those parts;[135] a circumstance which accounts for the vegetation of some particular trees, which are known to live when deprived of the bark; because they have rays of the same substance within which answer the purpose, and this in a degree answering to the nature of their life.

[135] Hill’s Construction of Timber, p. 118. Ibid. p. 120,

The bark appears to be formed, first, of longitudinal fibres, which Du Hamel considers as so many lymphatic vessels; secondly, by a sort of a filmy cellular tissue, which has been considered as a kind of bladders by some, or as parenchymatous by others; thirdly, of the vasa propria interiora, or interior juice vessels.

The longitudinal fibres are disposed in strata, which lie one over the other. In that stratum which is next the rind, or rather the cellular coat, we perceive a net of longitudinal fibres, the meshes of which are large and easily distinguished, particularly when the cellular tissue that fills up the interstices is removed. To do this, the branches should be macerated for a considerable time; some require to be kept in this state for years. It will then be easy to separate first the rind, then the cellular coating, and afterwards this pulpy matter. It may sometimes be easily removed after the branches have been boiled.

The most exterior stratum, when examined by the naked eye, seems to be formed of simple fibres, which graft, solder, or inosculate one with the other; but when examined by a microscope, each of these fibres will be found to be a bundle of filaments, which may be easily separated from each other.

Grew says, that each filament, like the nerves in animals, consists of twenty or thirty small contiguous tubes, which run uniformly from the extremity of the root, without sending off any branches, or suffering any change in their size and shape. Hence the bark may be torn or divided lengthwise, with greater ease than in an horizontal direction; when macerated, they are capable of a very great degree of subdivision.

The filaments of a cortical vessel are to be looked on, agreeable to what we have already observed, as so many little bundles placed near together, and at first growing parallel to each other; but soon quitting this direction, the filaments of one fascicle parting from that to which they originally belonged, and inclining more or less obliquely towards another, sometimes uniting with it, at others, bending backwards, and uniting again with that from which it proceeded, or with some one that it meets with. In this manner new fascicles are often formed, while other parcels are increased or diminished by the additions of new filaments; by these means, a kind of irregular net is formed, and the fibres proceed in a serpentine line from the top to the bottom of the tree.

The thickness of the bark is entirely formed of strata of these longitudinal fibres, which lie one over the other; each of these strata is similar to the exterior one, only the meshes are smaller, and the fibres finer, in proportion as they are more interior, insomuch that at last the meshes are almost annihilated, and the fibres seem to lie quite parallel to each other.

There are some trees, however, where the meshes are not visible, and in which the fibres lie quite in a straight direction. There are many other circumstances in which they vary in different trees; in some the meshes of each stratum correspond with each other, diminishing gradually in size as they are more interior, and forming as it were so many conical cells.

We may, I think, conclude from what has been said, that the bark is composed of several thin membranes, which extend over the whole exterior surface of the tree. The most exterior membrane is the rind; under this is what Du Hamel calls the cellular coat; next to this the cortical stratum or true bark of the tree, which is formed of lymphatic vessels ranged more or less in a reticular form, and of the vasa propria interiora. The meshes are so constituted as to form large cavities next the rind, and small ones near the wood. These cavities are filled with a parenchymatous substance or the cellular tissue, which being continued from the wood to the rind, joins and unites the cortical stratum, and afterwards spreading on the outside thereof, forms what has been termed the cellular coat.