10. _A. Parnassi_ Boiss., var. _cyllenea_, a small shrub found in
abundance on the northern mountains of the Morea, which is stated by
Heldreich[705] to be the almost exclusive source of the tragacanth
collected about Vostizza and Patras.
=History=—Tragacanth has been known from a very early period.
Theophrastus in the 3rd century B.C. mentioned Crete, the Peloponnesus
and Media as its native countries. Dioscorides, who as a native
of South-eastern Asia Minor was probably familiar with the plant,
describes it correctly as a low spiny bush. The drug is mentioned
by the Greek physicians Oribasius, Aëtius, and Paulus Ægineta (4th
to 7th cent.), and by many of the Arabian writers on medicine. The
abbreviated form of its name “Dragantum” already occurs in the book
“Artis veterinariæ, seu mulomedicinæ” of Vegetius Renatus, who lived
about A.D. 400. During the middle ages the gum was imported into Europe
through the trading cities of Italy, as shown in the statutes of
Pisa,[706] A.D. 1305, where it is mentioned as liable to impost.
Pierre Belon, the celebrated French naturalist and traveller, saw and
described, about 1550, the collecting of tragacanth in the northern
part of Asia Minor; and Tournefort in 1700 observed on Mount Ida in
Candia the singular manner in which the gum is exuded from the living
plant.[707]
=Secretion=—It has been shown by H. von Mohl[708] and by Wigand[709]
that tragacanth is produced by metamorphosis of the cell membrane, and
that it is not simply the dried juice of the plant.
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.
[705] _Nutzpflanzen Griechenlands_, Athen, 1862. 71.
[706] Bonaini, _Statuti inediti della città di Pissa dal xii. al xiv.
secolo_, iii. (1857) 106. 114.
[707] _Voyage into the Levant_, Lond. (1718) 43.
[708] _Botanische Zeitung_, 1857. 33; _Pharm. Journ._ xviii. (1859) 370.
[709] Pringsheim’s _Jahrbücher f. wissenchaftl. Botanik_, iii. (1861)
117.
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.
[710] Hanbury, _Science Papers_, 29.
[711] _Pharm. Journ._ xv. (1856) 18.
[712] _Researches in Asia Minor, Pontus and Armenia_, i. (1842) 492.
=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.
The tragacanth of Kurdistan and Persia shipped from Bagdad, which
sometimes appears in the London drug sales under the incorrect name of
_Syrian Tragacanth_, is in very fine and large pieces which are rather
more translucent and ribbon-like than the selected tragacanth imported
from Smyrna: in fact, the two varieties when seen in bulk are easily
distinguishable.
The inferior kinds of tragacanth have more or less of colour, and are
contaminated with bark, earth and other foreign substances. They used
formerly to be much imported into Europe, and were frequently mentioned
during the past centuries as _black tragacanth_.
=Microscopic Structure=—The transformation of the cells into tragacanth
is usually not so complete, that every trace of the original tissue
or its contents has disappeared. In the ordinary drug, the remains
of cell-walls as well as starch granules may be seen, especially if
thin slices are examined under oil or any other liquid not acting on
the gum. Polarized light will then distinctly show the starch and the
cell-walls. If a thin section is imbued with a solution of iodine in
iodide of potassium and then moistened with concentrated sulphuric
acid, the cell-walls will assume a blue colour as well as the starch.
[713] In the Museum of the Pharmaceutical Society in London, there is
some Flake Tragacanth remarkable for its enormous size, but in other
respects precisely like the ordinary kind. The ribbon-like strips are
as much as 2 inches wide and ³/₁₀ of an inch thick, and the largest
which is several inches long weighs 2¾ ounces. Professor Haussknecht
has informed us that he has seen in Luristan stems of _Astragalus
eriostylus_ Boiss. et Haussk. more than 6 feet in height and 5 inches
in diameter, and bearing tragacanth. It is probable that the specimen
of gum we have described was produced by some species attaining these
extraordinary dimensions. Among the Kurdistan tragacanth, there occur
curious cylindrical vermiform pieces, about ⅕ of an inch in diameter,
coated with a network of woody fibre. We are told by Professor H. that
they are picked out of the centre of cut off pieces of stem, split open
by rapid drying in the sun.
=Chemical Composition=—When tragacanth is immersed in water it swells,
and in the course of some hours disintegrates so that it can be
diffused through the liquid. So great is its power of absorbing water
that even with 50 times its weight, it forms a thick mucilage. If one
part of tragacanth is shaken with 100 parts of water and the liquid
filtered, a neutral solution may be obtained which yields an abundant
precipitate with acetate of lead, and mixes clearly with a concentrated
solution of ferric chloride or of borax,—in these respects differing
from a solution of gum arabic. On the other hand, it agrees with the
latter in that it is thrown down as a transparent jelly by alcohol,
and rendered turbid by oxalate of ammonium. The residue on the filter
is a slightly turbid, slimy, non-adhesive mucilage, which when dried
forms a very coherent mass. It has received the name of _Bassorin_,
_Traganthin_ or _Adraganthin_, and agrees with the formula C₁₂H₂₀O₁₀.
Tragacanth is readily soluble in alkaline liquids, even in ammonia
water and at the same time assumes a yellow colour; heated with ammonia
in a sealed tube at 90° C. it blackens.
The drug loses by drying about 14 per cent. of water, which it absorbs
again on exposure to the air. Pure flake tragacanth incinerated leaves
3 per cent. of ash.
=Commerce=—Tragacanth is shipped from Constantinople, Smyrna and
the Persian Gulf. The annual export of the gum from Smyrna has been
recently stated[714] to be 4,500 quintals, value 675,000 Austrian
florins (£67,500); and the demand to be always increasing.
=Uses=—Though tragacanth is devoid of active properties, it is a very
useful addition to many medicines. Diffused in water it acts as a
demulcent, and is also convenient for the suspension of a heavy powder
in a mixture. It is an important ingredient for imparting firmness to
lozenges and pill-masses.
=Adulteration=—The fine quantities consisting of large distinct pieces
are not liable to adulteration, but the small and the inferior kinds
are often sophisticated. At Smyrna, tragacanth is mixed with gums
termed respectively _Mosul_ and _Caramania Gum_. The former appears
to be simply very inferior tragacanth; the latter which is sometimes
called in the London market _Hog Gum Tragacanth_ or _Bassora Gum_,[715]
is said to be the exudation of almond and plum trees. It occurs in
nodular masses of a waxy lustre and dull brown hue, which immersed in
water gradually swells into a voluminous white mass. To render this gum
available for adulteration, the lumps are broken into small angular
fragments, the size of which is adjusted to the sort of tragacanth with
which they are to be mixed. As the Caramania Gum is somewhat dark,
it is usual to whiten it by _white lead_, previous to mixing it with
_Small Leaf_ or _Flake_, or with the _Vermicelli_ gum.
By careful examination the fraud is easily detected, angular fragments
not being proper to any true tragacanth. The presence of lead may be
readily proved by shaking suspected fragments for a moment with dilute
nitric acid, which will dissolve any carbonate present, and afford a
solution which may be tested by the ordinary reagents.
[714] C. von Scherzer, _Smyrna_, Wien, 1873. 143.
[715] It is sometimes shipped from Bussorah.