Gall
Gall, an abnormal growth or swelling on a tree or other plant. Galls may occur on the leaves, bark, stems, or roots. Many galls damage plants or cause them to die by cutting off their normal food and water supplies. Galls range in size from small swellings to growths larger than a baseball. Some galls are hard or spongy spheres, others have a scaly appearance. Still others are hairy. Galls also differ greatly in their interior structure. Some are resinous, others corky, cobwebby, or leathery in texture.
Galls are usually caused by parasites, such as fungi, bacteria, or insects. Galls may also form where two branches rub together. Most galls are caused by insects, such as gallfies and gall wasps, that puncture the bark or leaf by biting, then lay their eggs in the wound. The young larvae feed on the gall that is formed. Each species of gall-producing insect lays its eggs in the same kind of place on a particular species of plant. For example, one kind of insect causes galls on the stalk of the goldenrod; other kinds attack the leaves and shoots of willows.
Some galls are used as a source of dyes, tanning materials, and medicines. The Aleppo galls, or gallnuts—produced by wasplike insects on several species of oaks in eastern Europe and Asia Minor—have been used for centuries as a tonic, astringent, and antidote for certain poisons, and are also used in manufacturing permanent ink. Other galls have been used for dyeing fabrics and as tattoo dye.