Oak apple

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.

[1] The adult female wasp lays single eggs in developing leaf buds.

From the Middle Ages to the early twentieth century, iron gall ink was the main medium used for writing in the Western world.

Tannins belong to a group of molecules known as polyphenols and can be taken from different parts of plants such as leaves, pods, fruits, and gall nuts.

The popular name refers to the event during the English Civil War when Charles hid in an oak tree.