Oak marbles (Andricus Kollari)  

These spherical growths seen on oak trees known as oak marbles are a type of gall created by the parasitic wasp (Andricus kollari)

The larvae of the wasp live inside the growth before burrowing out creating a small hole as seen in some of the pictures here.

Oak galls were used for their tannins to create dark coloured inks throughout the medieval period. The acidity of the ink created from galls eventually corrodes through the paper or parchment it's used on, leaving holes in the shape of the original text.