Oak apple or oak gall is the common name for a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from 2 to 4 centimetres in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family Cynipidae.
Oak apples on Quercus robur leaf
Oak galls on a Pyrenean oak (Quercus pyrenaica) in León, Spain.
An oak apple on a tree in Worcestershire, England
An oak apple on an oak leaf.
An oak is a hardwood tree or shrub in the genus Quercus of the beech family. They have spirally arranged leaves, often with lobed edges, and a nut called an acorn, borne within a cup. The genus is widely distributed in the Northern Hemisphere; it includes some 500 species, both deciduous and evergreen. Fossil oaks date back to the Middle Eocene. Molecular phylogeny shows that the genus is divided into Old World and New World clades, but many oak species hybridise freely, making the genus's history difficult to resolve.
Oak
Quercus robur habit
Illustration of Quercus lusitanica showing staminate (left) and pistillate flowers, which mature into acorns (right)
Catkins of Quercus alba containing the staminate or 'male' flowers