This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
The term 'bark' is commonly employed by pharmacognosists to denote all the tissues of the stem and root of trees and shrubs exterior to the cambium. It comprises, therefore, botanically, such tegumen-tary tissue as may have been formed, primary and secondary cortex, together with primary and secondary bast.
In examining a bark attention should first be directed to the shape of the pieces in which it occurs.
Barks are said to be in flat pieces when they are quite flat; in curved pieces when they present a curved but not deeply concave transverse section; in recurved pieces when the concave surface is the outer portion of the bark; in channelled pieces when the transverse section is deeply concave. Should it be so deeply concave that the edges nearly or quite overlap, a quill is produced, and should both edges be inrolled a double quill is formed. Single or double quills packed inside one another form compound quills.
Next the physical characters should be noted, such as colour odour, taste, nature of the outer and inner surface, etc.
The appearance of the edges when a piece is broken (' fracture' of a bark) often affords useful information; the fracture may be short, granular, splintery, or fibrous - terms which sufficiently explain themselves.
As the bark of any particular tree may be, and frequently is, collected from axes varying in size from a twig to a large trunk, the drug thus produced will exhibit not only corresponding variations in size, but also considerable variation in external appearance caused by the changes in the tegumentary tissue due to the growth of the axis. The internal structure, however, retains its important characteristics unchanged, and the student should therefore carefully examine a smooth transverse section which he will find an excellent and often indispensable guide to the identification of a bark. Before passing to this part of the work he should study the anatomy of the bark in his text-book of botany; the following brief notes may, however, be useful.
Few barks are so young when collected as to retain their epidermis; this tissue has usually been replaced by a cork of varying thickness. Sometimes the development of successive layers of phellogen deeper and deeper in the cortex, and possibly in the secondary bast, has resulted in the formation of corresponding layers of cork, the tissues thus cut off losing their vitality. To the mass of protective tissue (the 'bark' of botanists) thus formed the name of 'outer bark' is commonly assigned by pharmacognosists.
The characters of the cork, influenced in some cases by the nature of the cell contents (cascarilla, alder buckthorn), often afford a valuable means of identifying a bark.
Phelloderm is seldom formed in quantity sufficient to be diagnos-tically important. Canella bark alone contains a sclerenchymatous phelloderm easily visible under a lens.
This tissue, often termed 'primary,' or 'middle' bark by pharmacognosists, is the tissue extending from the epidermis, cork, or phelloderm, as the case may be, up to and including the endodermis. Beyond the presence or absence of groups of sclerenchymatous cells, of oil-cells, oil-glands, or calcium oxalate, it seldom affords any well-marked characters.
The primary bast being seldom distinguishable even under a microscope, this tissue is practically composed of secondary bast. It comprises the tissue extending from the endodermis to the cambium, and corresponds to the inner bark of many pharmacognosists. Its structure should be carefully examined, as it is frequently of great diagnostic importance.
It occasionally happens that the cells of the pericycle thicken and lignify, and thus form a band of sclerenchymatous tissue, readily visible under a lens, in the mature bark (sassy bark, nux vomica bark); otherwise the extent of the secondary bast is not readily fixed; it continues at least as far as the medullary rays extend.
The chief feature of importance in the bast, and visible under a lens, are the presence or absence of: (i) sclerenchymatous cells; (ii) bast fibres; (iii) oil-cells or glands; (iv) calcium oxalate; (v) mucilage.
Sclerenchymatous cells commonly occur in rounded, less often in tangentially elongated, groups exhibiting a uniform semi-translucent appearance. They impart hardness to the bark and render the fracture short and granular (sassy bark).
Bast fibres may occur scattered or in groups, which are then often tangentially elongated and arranged in tangential lines. They impart to the bark a tough and fibrous character; from the fractured surface of such a bark the fibres usually project (oak, elm, &c).
If the bast fibres approach sclerenchymatous cells in nature the the fracture becomes splintery (quillaja).
Oil-glands or oil-cells usually appear as minute yellowish, reddish or brownish translucent points; their presence is often very-characteristic (canella bark).
Calcium oxalate is generally visible as colourless points or lines (cusparia bark); if present in large quantities the tissue itself may appear white (canella).