Source, Etc

The alder buckthorn, Rhamnus Frangula, Linne (N.O. Rhamneoe), is distributed generally over Europe. It is distinguished from the common buckthorn (R. catharticus, Linne), the only other indigenous species, by the entire leaves, hermaphrodite flowers with five stamens, absence of thorns, and tree-like habit.

The bark is stripped from the stem and branches, the wood of which was formerly valued for making charcoal for gunpowder. When fresh it has an unpleasant odour and taste, and acts as an emetic, but these properties are lost when the bark is dried and kept: alder buckthorn bark should not therefore be employed medicinally until it has been kept for at least a year.

Source Etc 193Fig. 124.   Alder Buckthorn bark. A, young, B, older bark. Natural size.

Fig. 124. - Alder Buckthorn bark. A, young, B, older bark. Natural size.

Description

The commercial drug occurs in single or double quills varying from 0.5 to 4 cm. in diameter and commonly 15 cm. or more in length. Young bark is usually extremely thin and has a smooth, glossy, dark purplish exterior marked with small, circular or transversely elongated, whitish lenticels. The cork frequently exfoliates, or at least easily separates, disclosing a yellowish brown cortex, but the inner part of the cork is of a dark crimson colour easily seen by gently scraping off the outer cork cells. The inner surface is dark cinnamon-brown in colour and nearly smooth, exhibiting under the lens fine longitudinal striations. The fracture is short in the outer, but rather fibrous in the inner part, groups of bast fibres projecting a short distance beyond the fractured surface.

The traverse section shows under the lens a narrow dark purple cork and yellowish brown cortex and bast.

Older bark is commonly much rougher. It has usually a dull dark purple colour, and is marked with transversely elongated lenticels and shallow longitudinal fissures, the cork exhibiting less disposition to exfoliate. The transverse section shows a similar purple cork; the bast is thicker and allows of very thin medullary rays being distinguished. The absence of sclerenchymatous cells is a character occasionally of value in excluding substitutes.

The bark has no odour and a taste that is scarcely bitter.

The student should observe

(a) The dark purple cork, showing, when scraped, a deep crimson colour, (6) The light-coloured lenticels, (c) The shortly fibrous fracture of the bast.

Fig. 125.   Alder Buckthorn bark. Transverse section. Magnified. (Berg.)

Fig. 125. - Alder Buckthorn bark. Transverse section. Magnified. (Berg).

Microscopical Characters

A transverse section exhibits a cork consisting of narrow cells many of which contain a bright purplish crimson colouring matter. The cortex contains small starch grains, cluster-crystals of calcium oxalate and elongated mucilage cells, but no sclerenchymatous cells. The secondary bast contains numerous tangentially elongated groups of thick-walled bast fibres; the medullary rays are mostly two cells wide. The cells of the medullary rays and bast parenchyma contain a yellowish amorphous substance dissolving in solution of potassium hydroxide with production of a bright purple colour.

Constituents

The active constituents of alder buckthorn bark are but imperfectly known. It contains a glucoside, frangulin, C21H10O9, which crystallises in lemon-yellow needles melting at 228° to 230°, is slowly volatile at ordinary temperature and stains the paper in which the drug is kept; it is soluble in caustic alkalies with purple coloration. Boiled with alcoholic hydrochloric acid it is converted into rhamnose and frangula-emodin. Rhamnose (isodulcite) is a pentatomic alcohol (pentose, pentaglucose), and is produced by the hydrolysis of quercitrin and a number of other glucosides, which are therefore termed rhamnosides. Frangula-emodin, C15H10O5, occurs in reddish yellow crystals melting at 254°. It is a derivative of anthraquinone, viz. trioxymethylanthraquinone, C14H4(CH3)(OH)302, and appears to be identical with rheum-emodin, but different from aloe-emodin and senna- emodin.

Frangula-emodin has been known under various names, viz. frangulin (Casselmann, 1857), frangulinic acid (Faust, 1869, Keussler 1873), avornic acid (Kubly, 1866), rhamnoxanthin (Binswanger, 1850).

Frangulin is said not to be present in the fresh bark, but to be produced from some unknown constituent during the maturing of the bark, the change in the physiological action of the bark occurring simultaneously. The bark also contains free frangula-emodin, chryso-phanic acid and an iso-emodin.

The total quantity of emodin present in the bark either free or in the form of a rhamnoside has been estimated at 1 per cent. in old bark, 2 per cent. in bark of medium age, and 3.8 per cent. in very young bark. This proportion is insufficient to account for the laxative action of the drug, and the assumption is that either other, at present unknown, purgative bodies are present or that the frangulin exists in an unknown more active form.

The presence of emodin in the bark is readily demonstrated by moistening 0.1 gm. of the powdered bark with 10 drops of alcohol and boiling for a few moments with 10 c.c. of water. The cooled decoction is shaken with 10 c.c. of ether, the yellowish ethereal solution separated and shaken with ammonia, which will acquire a reddish colour.

Uses

Alder buckthorn bark has been used as an agreeable laxative, preferable to cascara sagrada on account of its less disagreeable taste.

Substitutes

The bark of R. carniolicus, Kerner, is not unfrequently substituted for that of R. Frangula; the cork contains a dull red, not crimson, colouring matter; medullary rays 4 to 7 instead of 2 to 3 cells wide; outer bark, when formed, contains groups of scleren-chymatous cells.

The bark of Alnus glutinosa, Gaertner (N.O. Betulaceoe) exhibits in transverse section a ring of sclerenchymatous cells. The bark of R. catharticus, Linne, is glossy reddish brown and has very distant lenticels.