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
| C₆H₂(OH)₂COOH |  | O, |
| C₆H₂(OH)₂CO |
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.