Astragalus Verus. Tragacanth.
Nat. Ord. — Fabaceae. Sex. Syst. — Diadelphia Decandria.
The Concrete Juice.
Description. — This is a small shrub about two or three feet in hight, with a stem about an inch in thickness, and numerous, very closely crowded branches, covered with imbricated scales, and spines which are the remains of former petioles. The leaves are a little more than half an inch long, and consist of from sixteen to eighteen opposite, villous, stiff, pointed, linear, hispid leaflets, with a midrib terminating in a sharp yellowish point; stipules at first downy, afterward smooth. The flowers are small, yellow, axillary, in clusters of from two to five, sessile, papilionaceous, and furnished with cottony bracts. Calyx tomentose, obscurely five-toothed.
History. — Tragacantha or Tragacanth is obtained from several species of plants, belonging to the genus Astragalus. The A. Tragacantha of Linnaeus, now the A. Massiliensis of Lamarck, a tree growing in southern Europe and northern Africa, and which yields no gum, was formerly referred to as affording the drug, but this is now known to be incorrect- The greater portion of the species from which tragacanth is obtained, are natives of Asia, having rigidly persistent petioles, forming spines. Most of the spinous species, furnish the peculiar exudation, which is known in commerce, as Gum Tragacanth, especially those inhabiting warm regions, but Botanists have not positively determined as to the particular plants. The A. Verus, a low and very hispid shrub, is stated by Olivier to afford the largest proportion of the gum sent to Europe. It is found growing naturally in Persia, Armenia and Asia Minor. The gum exudes from the stem naturally, or after incisions, hardening as it exudes ; and is gathered from July till September. It is forwarded to India through Bagdad and Bassoro, then to Russia, and from thence to Aleppo. The other species which are admitted to furnish the gum, are A. Gummifer, a native of Syria and Koordistan, which is said to yield the white or best variety ; A. Creticus, growing in Crete, and A. Aristatus, a native of Greece and the south of Europe, and reputed to afford some of the tragacanth imported from Smyrna and Marseilles. Professor Lindley has likewise described another species, the A. Strobiliferus, growing in Koordistan, and furnishing the dark-colored, or inferior gum, which is mingled with the commercial article; but this requires confirmation. The plants which yield tragacanth, resemble each other so closely, that much confusion has existed among Botanists in distinguishing them.
Tragacanth presents the appearance of very thin, pale-grayish, or grayish-yellow, almost parchment-like plates or scales, marked by spiral or circular ridges. It is semitransparent, or translucent, resembling horn in appearance, hard, more or less fragile, but difficult of pulverization, unless exposed to a freezing temperature, or thoroughly dried, and powdered in a heated mortar, tasteless and inodorous. Its powder is very fine and white. Sometimes pieces of a slightly reddish color are met with, of a roundish or irregularly oblong shape. The specific gravity of tragacanth is 1.384. Cold or boiling water converts it into mucilage ; introduced into cold water, it absorbs a portion of that fluid, swells very much, and forms a paste without being dissolved. By boiling, its solution is nearly completed. The mucilage is changed to a blue color by the addition of iodine, owing to the presence of a small proportion of starch. Tragacanth is insoluble in alcohol, and is composed of a substance soluble in water, and a substance not soluble, but which swells when placed in water. The soluble portion resembles gum arabic, but differs from it by not yielding any precipitate with silicate of potassa or sesquichloride of iron ; the insoluble portion is called Tragacanthin, constitutes 43 per cent, of the drugs, and is supposed to be identical with bassorin, with a small quantity of insoluble starch. The composition of tragacanth is not satisfactorily settled by chemists ; M. Guerin considers it to be a compound of 53.3 parts of arabin or soluble gum, 33.1 of bassorin and insoluble starch, 11.1 of water, and when burned 2.5 of ashes.
Properties and Uses. — Tragacanth is nutritive and demulcent ; seldom used except for the suspension of heavy, insoluble powders, to impart consistence to troches and lozenges, and to form paste for the druggists, with which to label their prescriptions.