CORTEX QUERCUS.

Oak Bark; F. Ecorce de Chêne; G. Eichenrinde.

Botanical Origin—Quercus Robur L., a tree, native of almost the whole of Europe, from Portugal and the Greek Peninsula as far north as 58° N. lat. in Scotland, 62° in Norway, and 56° in the Ural Mountains.

There are two remarkable forms of this tree which are regarded by many botanists as distinct species, but which are classed by De Candolle[2225] as sub-species.

Sub-species I. pedunculata—with leaves sessile or shortly stalked, and acorns borne on a long peduncle, and acorns either sessile or growing on a short peduncle.

Sub-species II. sessiliflora—with leafstalks more or less elongated.

Both forms occur in Britain. The first is the common oak of the greater part of England and the lowlands of Scotland. The second is frequently scattered in woods in which the first variety prevails, but it rarely constitutes the mass of the oak woods in the south of England. In North Wales however, in the hilly parts of the north of England, and in Scotland, it is the commoner of the two forms (Bentham).

History—The astringent properties of all parts of the oak[2226] were well known to Dioscorides, who recommends a decoction of the inner bark in colic, dysentery and spitting of blood. Yet oak bark seems at no time to have been held in great esteem as a medicine, probably on account of its commonness; and it is now almost superseded by other astringents. For tanning leather it has always been largely employed.

Description—For medicinal use the bark of the younger stems or branches is collected in the early spring. It varies somewhat in appearance according to the age of the wood from which it has been taken: that usually supplied to English druggists is in channelled pieces of variable length and a tenth of an inch or less in thickness, smooth, of a shining silvery grey, variegated with brown, dotted over with little scars. The inner surface is light rusty-brown, longitudinally striated. The fracture is tough and fibrous. A transverse section shows a thin, greenish cork-layer, within which is the brown parenchyme, marked with numerous rows of translucent colourless spots. The smell of dry oak bark is very faint; but when the bark is moistened the odour of tan becomes evident. The taste is astringent and in old barks slightly bitter.

Microscopic Structure—The outer layer of young oak bark consists of small flat cork-cells; the middle layer of larger thick-walled cells slightly extended in a tangential direction, and containing brown grains and chlorophyll. This tissue passes gradually into the softer narrower parenchyme of the inner bark, which is irregularly traversed by narrow medullary rays. It exhibits moreover a ring, but slightly interrupted, of thick-walled cells (sclerenchyme) and isolated shining bundles of liber-fibres.

Groups of crystals of calcium oxalate are frequent in the middle and inner bark, but the chief constituents of the cells are brown granules of colouring matter and tannin. As the thickness of the bark increases the liber is pushed more to the outside, the middle cortical layer being partly thrown off by secondary cork-formation (rhytidoma, see pp. 354 and 538). Hence the younger barks, which alone are medicinal, are widely different from the older in structure and appearance.

Chemical Composition—The most interesting constituent is a peculiar kind of tannin. Stenhouse pointed out in 1843 that the tannic acid of oak bark is not identical with that of nutgalls; and such many years afterwards was proved to be the case.

The first-named substance, now called Querci-tannic Acid, yields by destructive distillation pyrocatechin, and according to Johanson (1875) very little pyrogallol. By boiling it with dilute sulphuric acid querci-tannic acid is split up into a red derivative and sugar. A solution of gelatine is precipitated by querci-tannic acid as well as by gallo-tannic acid; yet the compound formed with the latter is very liable to putrefaction, whereas the tannin of oak bark, which is accompanied by a large amount of extractive matter, furnishes a stable compound, and is capable of forming good leather.

As querci-tannic acid has not yet been isolated in a pure state, the exact estimation of the strength of the tanning principle in oak bark has not been accomplished, although it is important from an economic as well as from a scientific point of view. The method of Neubauer (1873) depends upon the amount of permanganate of potassium decomposable by the extract of a given weight of oak bark. Neubauer found in the bark of young stems, as grown for tanning purposes, from 7 to 10 per cent. of querci-tannic acid, soluble in cold water.

Braconnot (1849) extracted from the seeds of the oaks under notice a crystallized sugar, which was shown in 1851 by Dessaignes to be a peculiar substance, which he termed Quercite. Prunier proved (1877-1878) that it agrees with the formula C₆H₇(OH)₅ + 4 OH₂, and is closely allied to kinic acid, C₆H₇(OH)₄COOH (see page 363). Quercite gives off water at 100°, melts at 225° C., and again losing water yields a crystallized anhydride. In the oak bark extremely small quantities of quercite appear also to be present, as pointed out by Johanson.

A colourless, crystallizable, bitter substance, soluble in water, but not in absolute alcohol or ether, was extracted from oak bark in 1843 by Gerber, and named Quercin. It requires further examination: Eckert (1864) could not detect its existence in young oak bark.

Uses—Occasionally employed as an astringent, chiefly for external application.

GALLÆ HALEPENSES.

Botanical Origin—Quercus lusitanica Webb, var. infectoria (Q. infectoria Oliv.),[2227] a shrub or rarely a tree, found in Greece, Asia Minor, Cyprus and Syria. It is probable that other varieties of this oak, as well as allied species, contribute to furnish the Aleppo galls of commerce.

History—Oak galls are named by Theophrastus, and were well known to other ancient writers. Alexander Trallianus prescribed them as a remedy in diarrhœa.[2228]

The earliest accurate descriptions and figures of the oak and the insect producing the galls are due to Olivier.[2229] Pliny[2230] mentions the interesting fact that paper saturated with an infusion of galls may be used as a test for discovering sulphate of iron, when added as an adulteration to the more costly verdigris: this, according to Kopp, is the earliest instance of the scientific application of a chemical reaction.[2231] For tanning and dyeing, galls have been used from the earliest times, during the middle ages however they were not precisely an article of great importance, being then, no doubt, for a large part replaced by sumach.

Nutgalls have long been an object of commerce between Western Asia and China. Barbosa in his Description of the East Indies[2232] written in 1514 calls them Magican,[2233] and says they are brought from the Levant to Cambay by way of Mekka, and that they are worth a great deal in China and Java. From the statements of Porter Smith[2234] we learn that they are still prized by the Chinese.

Formation—Many plants are punctured by insects for the sake of depositing their eggs, which operation gives rise to those excrescences which bear the general name of gall.[2235]

Oaks are specially liable to be visited for this purpose by insects of the order Hymenoptera and the genus Cynips, one species of which, Cynips Gallæ tinctoriæ Olivier (Diplolepis Gallæ tinctoriæ Latreille), occasions the galls under notice.

The female of this little creature is furnished with a delicate borer or ovipositor, which she is able to protrude from the extremity of the abdomen; by means of it she pierces the tender shoot of the oak, and deposits therein one or more eggs. This minute operation occasions an abnormal affluence to the spot of the juices of the plant, the result of which is the growth of an excrescence often of great magnitude, in the centre of which (but not as it appears until the gall has become full-grown) the larva is hatched and undergoes its transformations.

When the larva has assumed its final development and become a winged insect, which requires a period of five to six months, the latter bores itself a cylindrical passage from the centre of the gall to its surface, and escapes.

In the best kind of gall found in commerce, this stage has not yet arrived, the gall having been gathered while the insect is still in the larval state. In splitting a number of galls, it is not difficult to find specimens in all stages, from those containing the scarcely distinguishable remains of the minute larva, to those which show the perfect insect to have perished when in the very act of escaping from its prison.

Description—Aleppo galls[2236] are spherical, and have a diameter of ⁴/₁₀ to ⁸/₁₀ of an inch. They have a smooth and rather shining surface, marked in the upper half of the gall by small pointed knobs and ridges, arranged very irregularly and wide apart; the lower half is more frequently smooth. The aperture by which the insect escapes is always near the middle. When not perforated, the galls are of a dark olive green, and comparatively heavy; but after the fly has bored its way out, they become of a yellowish-brown hue, and lighter in weight. Hence the distinction in commerce of Blue or Green Galls, and White Galls.

Aleppo galls are hard and brittle, splitting under the hammer; they have an acidulous, very astringent taste followed by a slight sweetness, but have no marked odour. Their fractured surface is sometimes close-grained, with a waxy or resinous lustre; sometimes (especially towards the kernel-like centre) loosely granular, or sometimes again it exhibits a crystalline-looking radiated structure or is full of clefts. The colour of the interior varies from pale brown to a deep greenish yellow. The central cavity, sometimes nearly ¼ of an inch in diameter, which served as a dwelling for the insect, is lined with a thin hard shell. If the insect has perished while still very young, the central cavity and the aperture contain a mass of loose starchy cellular tissue, or its pulverulent remains: if the insect has not been developed at all, the centre of the gall is entirely composed of this tissue.

Microscopic Structure—The cellular tissue of the gall is formed in the middle layer of large spherical cells with rather thick porous walls, becoming considerably smaller towards the circumference. The outermost rows are built up of cells having but a very small lumen and comparatively thick walls, so that they form a sort of rind. Here and there throughout the entire tissue, there occur isolated bundles of vessels which pass through the stalk into the gall. Towards the kernel, the parenchyme gradually passes into radially-extended, wider, thin-walled cells, the walls of which are marked with spiral striæ. The hard shell of the chamber[2237] is composed of larger, radially-extended, thick-walled cells, with beautifully stratified porous walls. On the inner side of this shell there are found, after the escape of the insect, the remains of the starchy tissue already mentioned, which originally filled the chamber and had been consumed by the insect as nourishment.

The parenchyme-cells outside the shell contain chlorophyll and tannin; the latter is in transparent, colourless, sharp-edged masses, insoluble in benzol, but dissolving slowly in water, quickly in alcohol. Thin slices soaked in glycerin appear after some time covered with beautiful crystals of gallic acid. The thick-walled cells (stone-cells) and the neighbouring striated cells, are rich in octahedra of calcium oxalate. The tissue of the gall situated within the shell of thick-walled cells contains starch in large, compressed, mostly spherical granules; also isolated masses of brown resin. Besides these, there appears to be in this part of the tissue an albuminoid compound.

Chemical Composition—The rough taste of galls is due to their chief constituent, Tannic or Gallo-tannic Acid, C₁₄H₁₀O₉, or

the type of a numerous family of substances to which vegetables owe their astringent properties. Tannic matter was long supposed to be of one kind, namely that found in the oak gall, but the researches of later years have proved the tannin of different plants to possess distinctive characters: hence the term gallo-tannic acid to distinguish that of galls, from which it is principally derived. It was however shown by Stenhouse as far back as the year 1843, again in 1861, as well as by still more recent unpublished experiments, that the tannic acid found in Sicilian sumach, the leaves of Rhus Coriaria L., is identical with that of oak galls. Löwe in 1873 came to the same conclusion. The best oak galls yield of this acid, from 60 to 70 per cent.

Gallic Acid is also contained in galls ready-formed to the extent of about 3 per cent. Free sugar, resin, protein-substances, have also been found. Neither gum nor dextrin is present.

Commerce—The introduction into dyeing of new chemical substances, and the increased employment of sumach and myrobalans, have caused the trade in nutgalls to decline considerably during the last few years. The province of Aleppo which used to export annually 10,000 to 12,000 quintals, exported in 1871 only 3000 quintals.[2238] A staple market for the galls which are collected in the mountains of Kurdistan is Diarbekir, whence they are sent to Trebizond for shipment. Galls are also shipped in some quantity at Bussorah, Bagdad, Bushire, and Smyrna.

There were imported into the United Kingdom from ports of Turkey and Persia during 1872, 6349 cwt. of galls, valued at £18,581.

Uses—Oak galls in their crude state are seldom used in medicine unless it be externally; but the tannic and gallic acids extracted from them are often administered.

Other kinds of Gall.

Chinese or Japanese Galls—The only kind of galls, besides those of the oak, which are of commercial importance. They are described at page 167.

Pistacia Galls—The genus Pistacia, which belongs to the same order as Rhus, is very liable to the attacks of Aphis, which produce upon its leaves and branches excrescences of exactly the same nature as Chinese galls. In the south of Europe, horn-like follicles, often several inches long,[2239] are frequently met with on the branches of Pistacia Terebinthus (page 165). These Gallæ vel Folliculi Pistacinæ, in Italian Carobbe di Giudea, were formerly used in medicine and in dyeing.[2240] They were noticed in 1555 by Belon, but already well known as early as the time of Theophrastus.

Another much smaller gall of different shape is formed (by the same insect?) on the ribs of the leaves of Pistacia Terebinthus; P. Lentiscus (page 161) affords also a similar small excrescence.

Again, another growth of the same character constitutes the small and very astringent galls known in the Indian bazaars by the names of Bazghanj and Gule-pistah, the latter signifying flower of pistachio; they have been termed in Europe Bokhara Galls. They were imported by sea into Bombay in the year 1872-73, to the extent of 184 cwt., chiefly from Sind;[2241] and are also carried into North-western India by way of Peshawar and by the Bolan Pass. Occasionally a package finds its way into a London drug sale.

Tamarisk Galls—These are roundish knotty excrescences of the size of a pea up to ½ an inch in diameter, found in North-western India on the branches of Tamarix orientalis L., a large, quick-growing tree, common on saline soils. The galls are used in India in the place of oak galls, and are mentioned as “non-officinal” in the Pharmacopœia of India, 1867. We are not aware that they have been the subject of any particular chemical research; their microscopic structure has been investigated by Vogl.[2242]