Gall, abnormal growth on trees caused by parasites such as fungi and insects. Fungal galls are usually irregular in shape, whereas insects produce galls that are often spherical, like the oak apple. The adult insect lays its eggs in the plant's tissue, and the larvae cause the growth of the gall. They then feed on the deformed tissues and eventually bore their way out as adults. Gall insects include certain small flies and many small wasps. It is often possible to identify the insect by the shape of its gall. Galls are often a serious nuisance and destroy many acres of valuable crops. Some, however, are useful. The oak apple, for example, is a source of tannin, which is used in tanning leather.