BARK, in vegetable anatomy, a term Which denotes the exterior part of vege table bodies; which is separable from the other parts of the plant during the sea son of vegetation, but at other times re quires maceration in water, or boiling ; and when detached by any of these means, the fine connections which unite it to the wood are destroyed. When bark is thus separated, and seen by means of the microscope, it exhibits par. s differing much in structure and use. These have been divided into the cuticle or epider mis, the cellular envelope or parenchy ma, and the cortical layer and fiber. The epidermis is a thin transparent membrane,, which covers all the outside of the bark. It is pretty tough. When inspected with a microscope, it appears to be composed of a number of slender fibres, crossing each other, and forming a kind of net work. It seems even to consist of diffe rent thin retiforrn membranes, adhering closely together. This, at least, is the ease with the epidermis of the birch, which Mr. Duhamel separated into six layers. The epidermis. when rubbed off, is reproduced. In old trees it cracks and decays, and new epidermis are succes sively formed. This is the reason that the trunks of many old trees have a rough surface. The parenchyma lies immedi ately below the epidermis ; it is of a deep green colour, very tender and succulent. When viewed with a microscope, it seems to be composed of fibres which cross each other in every direction, like the fibres which compose a net. Both in it and the epidermis there are numberless interstices, which have been compared to so many small bladders. The cortical layers form the innermost part of the bark, or that which is next to the wood. They consist of several thin membranes, lying the one above the other ; and their number appears to increase with the age of the plant. Each of these layers is com posed of longitudinal fibres, which sepa. rate and approach each other alternately, so as to form a kind of net-work. The meshes of this net-work correspond in each of the layers ; and they become small er and smaller in every layer as it ap proaches the wood. These meshes are filled with a green-coloured cellular stub stance, which has been compared by ana tomists to a number of bladders adhering together, and communicating with each other.
The matter of the parenchyma, and the juices which exist in barks, vary extreme ly, and probably occasion most of the dif ferences between them. Some, as oak bark, are characterized by their astrin gency, and contain tannin ; others, as cin namon, are aromatic, and contain an es sential oil ; others are bitter, as Jesuit's bark ; some are chiefly mucilaginous, others resinous, &c.
1. Bark of the cinchona floribunda, or quinquina of St. Domingo. This bark is in rolled pieces, six or seven inches long, and three or four lines in thickness. Its colour is greyish green externally, but within it exhibits different shades of green, purple, white, brown, &c. 'Its taste is bitter and disagreeable ; its odour strong and unpleasant. It gives out nearly half its weight to water, provided it be boiled in a sufficient quantity of that liquid. The residue possesses the properties of woody fibre. The decoction of the bark has a reddish brown colour, and an ex tremely bitter taste. It deposits on cool ing a blackish substance, soft and tenaci ous, which does not dissolve in cold wa ter, though it is soluble in hot water and in alcohol. More of this substance pre cipitates as the liquor is evaporated. When the inspissated juice, freed from its precipitate, is mixed with alcohol, a quantity of gummy matter separates. When the black matter, which precipi tates as the decoction cools, is treated with hot alcohol, the greatest part of it is dissolved; but a fine red powder re mains mixed with some mucilage, which is easily separated by water. When the alcoholic solution is exposed to the air, it deposits light yellowish crystals of a saline nature. When mixed with water, white flakes are thrown down which possess the properties of gluten ; but the greatest part remains in solution. Thus the soluble part of the bark may be separated into five distinct substances ; namely, gum, gluten, a red powder, a saline matter, and a brownish bitter substance, retained in solution by the diluted alcohol. The last is by far the most abundant To it the peculiar qualities of the decoction of this bark are to be ascribed.