An oak apple or oak gall is the common name for round, vaguely apple-like, galls formed on many species of oak, they’re usually and inch or two in diameter. They grow when a female wasp of the family Cynipidae (commonly in Europe, the wasp Biorhiza pallida) lays a single egg in a developing leaf bud. The larva that emerges secretes chemicals that cause the tree to pump nutrients into the “infected” leaf bud and the larva then feeds on the gall tissue.
There were numerous oak apple growing on this tree (Quercus robur, commonly known as the pedunculate oak or English oak) growing in the centre of Rampton Pocket Park. Oak Apple Day (or Royal Oak Day) used to be a public holiday in England held on 29th May to commemorate the Restoration of King Charles II in 1660, alluding to him purportedly hiding in an oak tree during the English Civil War.