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a gall formed on young branches and leaves of certain species of oak and sumac. Gallnuts form at the sites where the larvae of gall wasps or aphids develop. They are spherical or somewhat elongated excrescences measuring 1.5–2 cm or more in diameter. Gallnuts contain tannins (up to 70 percent of the dry mass), which are used for tanning leather and making pharma ceuticals (for example, astringents).