An oak apple or oak gall is a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from 2 to 4 centimetres (1 to 2 in) in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family Cynipidae.
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Quercus robur in the context of Royal Oak
The Royal Oak was the English oak tree within which the future King Charles II of England hid to escape the Roundheads following the Battle of Worcester in 1651. The tree was in Boscobel Wood, which was part of the park of Boscobel House. Charles told Samuel Pepys in 1680 that while he was hiding in the tree, a Parliamentarian soldier passed directly below it. The story was popular after the Restoration, and is remembered every year in the English traditions of Royal Oak Day.
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