Oak apple due to the superficial resemblance and the preponderance of the oak marble gall in the wild. Other galls found on oak trees include the oak artichoke gall and the acorn cup gall, but each of these has its own distinctive form. Some common oak-apple-forming species are the "Biorhiza pallida" gall wasp in Europe, "Amphibolips confluenta" in eastern North America, and "Atrusca bella" in western North America. Oak apples may be brownish, yellowish, greenish, pinkish, or reddish. Oak galls have been used in the production of ink since at least the time of the Roman Empire. From the Middle Ages to
Oak Apple Day to God, for the great Blessing and Mercy he hath been graciously pleased to vouchsafe to the People of these Kingdoms, after their manifold and grievous Sufferings, in the Restoration of his Majesty..." The public holiday was formally abolished in the Anniversary Days Observance Act 1859, but the date retains some significance in local or institutional customs. It is, for example, still observed as Founder's Day by the Royal Hospital Chelsea, which was founded by Charles II in 1681. Traditional celebrations to commemorate the event often entailed the wearing of oak apples (a type of plant gall, possibly known in