The Common Oaks of this country are much affected by galls. They occur sometimes on the leaves, where they form the socalled 'Oak-apples,' sometimes on the shoots, where they do great mischief by checking and distorting the growth of the tree. The young larva that hatches from the eggs feeds upon the tissues of the plant and secretes in its mouth a peculiar fluid, which stimulates the cells of the tissues to a rapid division and abnormal development, resulting in the formation of a gall. The larva thus becomes completely enclosed in a nearly spherical mass, which projects from the twig, furnishing it with a supply of starch and other nutritive material. The best Aleppo galls, collected in Asiatic Turkey, principally in the province of Aleppo, are collected before the insects escape.
Galls are also largely imported from Persia and to a lesser extent from Greece.
If collected after the insects have escaped, galls are of a pale, yellowish-brown hue, spongy and lighter in weight, perforated near the centre with a small hole. These are known in commerce as white galls. Galls have no marked odour, but an intensely astringent taste, and slightly sweet after-taste. English Oak Galls, or Oak Apples, are smooth, globular, brown, usually perforated and much less astringent than Aleppo Galls, containing only 15 to 20 per cent of gallotannic acid. They have no commercial value.