The stem of a gum-bearing Astragalus cut transversely, exhibits concentric annual layers which are extremely tough and fibrous, easily tearing lengthwise into thin filaments. These inclose a central column, radiating from which are numerous medullary rays, both of very singular structure, for instead of presenting a thin-walled parenchyme, they appear to the naked eye as a hard translucent gum-like mass, becoming gelatinous in water. Examined microscopically, this gummy substance is seen to consist not of dried mucilage, but of the very cells of the pith and medullary rays, in process of transformation into tragacanth. The transformed cells, if their transformation has not advanced too far, exhibit the angular form and close packing of parenchyme-cells, but their walls are much incrassated and evidently consist of numerous very thin strata.
That these cells are but ordinary parenchyme-cells in an altered state, is proved by the pith and medullary rays of the smaller branches which present no such unusual structure. Mohl was able to trace this change from the period in which the original cell membrane could be still easily distinguished from its incrusting layers, to that in which the transformation had proceeded so far that it was impossible to perceive any defined cells, the whole substance being metamorphosed into a more or less uniform mucilaginous mass.
The tension under which this peculiar tissue is held in the interior of the stem is very remarkable in Astragalus gummifer which one of us had the opportunity of observing on the Lebanon in 1860.[710] On cutting off a branch of the thickness of the finger, there immediately exudes from the centre a stream of soft, solid tragacanth, pushing itself out like a worm, to the length of ¾ of an inch, sometimes in the course of half an hour; while much smaller streams (or none at all) are emitted from the medullary rays of the thick bark.
Production—The principal localities in Asia Minor in which tragacanth is collected are the district of Angora, the capital of the ancient Galatia; Isbarta, Buldur and Yalavatz,[711] north of the gulf of Adalia; the range of the Ali Dagh between Tarsous and Kaisariyeh, and the mountainous country eastward as far as the valley of the Euphrates. The drug is also gathered in Armenia on the elevated range of the Bingol Dagh south of Erzerum; throughout Kurdistan from Mush for 500 miles in a south-eastern direction as far as the province of Luristan in Persia, a region including the high lands south of lake Van, and west of lake Urumiah. It is likewise produced in Persia farther east, over an area 300 miles long by 100 to 150 miles broad, between Gilpaigon and Kashan, southward to the Mahomed Senna range north-east of Shiraz, thus including the lofty Bakhtiyari mountains.
As to the way in which the gum is obtained, it appears from the statements of Maltass, that in July and August the peasants clear away the earth from around the stem of the shrub, and then make in the bark several incisions, from which during the following 3 or 4 days the gum exudes and dries in flakes. In some localities they also puncture the bark with the point of a knife. Whilst engaged in these operations, they pick from the shrubs whatever gum they find exuded naturally.
Hamilton,[712] who saw the shrub in 1836 on the hills about Buldur, says “the gum is obtained by making an incision in the stem near the root, and cutting through the pith, when the sap exudes in a day or two and hardens.”
Formerly the peasants were content to collect the naturally exuded gum, no pains being taken to make incisions, whereby alone white flaky gum is obtained. We have in fact heard an old druggist state, that he remembered the first appearance of this fine kind of tragacanth in the London market. According to Professor Haussknecht, whose observations relate chiefly to Kurdistan and Persia, the tragacanth collected in these regions is mostly a spontaneous exudation.
Tragacanth is brought to Smyrna, which is a principal market for it, from the interior, in bags containing about 2 quintals each, by native dealers who purchase it of the peasants. In this state it is a very crude article, consisting of all the gatherings mixed together. To fit it for the European markets, some of which have their special requirements, it has to be sorted into different qualities, as Flaky or Leaf Gum, Vermicelli and Common or Sorts; this sorting is performed almost exclusively by Spanish Jews.
Description—The peculiar conditions under which tragacanth exudes, arising from the pressure of the surrounding tissues and the power of solidifying a large amount of water, will account to some extent for the strange forms in which this exudation occurs.
The spontaneously exuded gum is mostly in mammiform or botryoidal masses from the size of a pea upwards, of a dull waxy lustre, and brownish or yellowish hue. It also occurs in vermiform pieces more or less contorted and very variable in thickness; some of them may have exuded as the result of artificial punctures. It is this form that bears the trade name of Vermicelli. The most valued sort is however the Flake Tragacanth, which consists of thin flattish pieces or flakes, 1, 2, 3 or more inches in length, by ¼ to 1 in width.[713] They are marked on the surface by wavy lines and bands, or by a series of concentric wave-marks, as if the soft gum had been forced out by successive efforts. The pieces are contorted and altogether very variable in form and size. The gum is valued in proportion to its purity and whiteness. The best, whether vermiform or flaky, is dull white, translucent, devoid of lustre, somewhat flexible and horny, firm, and not easily broken, inodorous and with scarcely any or only a slight bitterish taste.