gall (plant material) en

Material made of excrescences produced on trees, usually the oak species Quercus infectoria in Europe and the Near East, and on the sumac species Rhus semialata in China and Japan, by the action of insects, chiefly of the genus Cynips. Galls are formed when gall-wasp eggs are deposited on tree branches in nut-like pockets; the gall grows until the larvae are completely enveloped. The mature insect bores a hole through the gall in order to escape. All galls contain 36-60% tannins; the bluish-green galls, gathered before the insect escapes, are richest in gallotannic acid. The tannins have been used for centuries for vegetable tanning of leather. When galls are combined with ferrous sulfate, a black dye is produced; alone, they produce a gray dye. Gall extract was also used as a mordant in the preparation of writing inks.