This section is from the book "A Text Book Of Materia Medica, Being An Account Of The More Important Crude Drugs Of Vegetable And Animal Origin", by Henry G. Greenish. Also available from Amazon: A Text Book of Materia Medica : Being an Account of the More Important Crude Drugs of Vegetable and Animal Origin.
Galls are excrescences on the twigs of Quercus infectoria, Olivier (N.O. Cupuliferoe), resulting from the deposition of the eggs of Cynips galloe tinclorioe, Olivier.
Under the generic term of galls a large variety of excrescences and other abnormal formations are included, that are produced not only upon plants, but also in a few instances upon animals, the exciting cause being either a plant or an animal. The plants that induce the formation of galls are exclusively fungi, but a variety of animals are capable of producing them, the principal being species of Cynips and Aphis. Thus the true oak-apple, the bedeguar of the rose, the oak gall, oak spangles, etc, are all varieties of galls produced by various insects.
The official drug is a particular variety of gall produced by a particular insect upon a particular plant. This variety of gall is known commercially as ' Aleppo ' galls or ' Turkey ' galls; they are collected in Asiatic Turkey, especially in the province of Aleppo.
The wasp that produces these galls is Cynips galloe tinctorioe, Olivier. Of this insect the female only is known, reproduction taking place parthenogenetically. By means of her ovipositor the wasp deposits an egg between the rudimentary leaves near the growing-point of young shoots of the oak. The larva emerges from the egg and wounds the delicate tissue with its mandibles, simultaneously introducing into the tissue a secretion that stimulates a rapid development of tissue. The new tissue thus formed assumes an concentric arrangement and within this the larva lives, feeding upon starch produced by the cells. Arrived at maturity the larva passes into the pupa stage, from which the wasp emerges and, boring through the gall with its mandibles, escapes. The gall is therefore probably to be regarded as a metamorphosed shoot. Very remarkable is the fact that no development of tissue takes place until a glandular secretion passes from the mandibles of the larva into the surrounding tissue, which is always merismatic in nature; this formation of tissue continues as long as the exciting substance is supplied, but should the larva perish it at once ceases and the growth of the gall is arrested.
Galls should be collected preferably before the insect escapes; after that has happened they become lighter in weight and are less esteemed.
Aleppo galls are nearly spherical in shape and vary from 12 to 20 mm. in diameter. They are hard and heavy, and bear, especially in the upper portion, short, bluntly pointed projections. They are of a bluish green or olive green colour externally, yellowish or brownish white within. There is usually a small cavity in the centre, in which the remains of the larva or of the gall-wasp may be found. They have no odour, but an intensely astringent taste followed by a slight sweetness.
Galls contain as principal constituent from 50 to 70 per cent. of tannic acid, which, to distinguish it from other varieties of tannic acid, is termed gallotannic acid. They contain also a little gallic acid (2 to 4 per cent.), ellagic acid, cyclogallipharic acid, sugar and starch.
Gallotannic acid, the 'tannic acid' of the British Pharmacopoeia, is a pale yellow amorphous substance yielding bluish black precipitates with solutions of ferric salts. Its aqueous solution darkens when exposed to the air with simultaneous formation of gallic acid, C7H605,H10, and sometimes also of ellagic acid, C14H608,2H10.
Galls are used medicinally as a local astringent chiefly in the form of a suppository or ointment. They find an extensive application technically in tanning and dyeing, in the manufacture of ink, etc.
Fig. 232. - Galls. A, showing interior; B, exterior, both with hole bored by the insect. (Vogl).
White galls are the galls collected after the escape of the gall-wasp; they are rather larger than the 'blue' galls, rather lighter in weight, and yellowish in colour. They are less esteemed, and are considered to contain less gallotannic acid, which, however, does not from analyses appear always to be the case.
Chinese and Japanese galls are produced by Aphis chinensis, Bell., on Rhus semialata, Murray (N.O. Anacardiaceoe); they are of a very irregularly lobed shape, reddish brown in colour, hollow and covered with a thick, grey, velvety down. They are largely used in the manufacture of gallotannic acid, of which they contain about 70 per cent.