What Is a Plant Gall?

The modern definition according to Redfern et al. (2002: 208) is as follows: “A gall is an abnormal growth produced in a plant or other host under the influence of another organism. It involves enlargement and/or multiplication of the host cells and provides the gall inducer with both shelter and food or nutrients.” Another definition, from Meyers Großes Konversations-Lexikon (1907: Vol. 7, Leipzig, pp. 280–281), states: “Galls (cecidia) are pathological modifications of plants caused by plant parasites (phytocecidia) or animal parasites (zoocecidia). The permanent or temporary presence of the parasite distinguishes galls from similar deformities of plants caused by injury or other factors. A stimulus originating from the parasite, or a substance secreted by it (a growth enzyme), causes the tissue at the affected site to develop abnormally, eventually resulting in a more or less clearly defined transformation of the plant part concerned. Galls can therefore develop only on young plant parts that are still undergoing development, such as tender buds, young roots, stems, shoots and leaves.”
Incidentally, certain galls, known as oak apples, were already known in antiquity. These oak apples were used as medicines by the Greeks, Romans, Byzantines and Arabs, and have also been used in Central Europe since the Middle Ages.