A plant gall is an abnormal plant growth caused by a parasite such as certain aphids, wasps, and bacteria. Insects produce galls by depositing their eggs in the plant. As the insect larvae grow inside, they irritate the plant tissue and cause it to swell. Parasites tend to favor particular plants, producing galls of identifiable shapes.
Easily identifiable galls are common on pistachio trees (see Pistachio) and oak trees (see Oak).
Pistachio galls are produced by orange aphids; they are harvested as a natural dye. Oak galls are produced by the wasp Cynics calicos; they are mixed with iron salts to make high-quality black ink.