Whilst on a walk through a local churchyard I noticed these fine old Turkey Oaks (Quercus cerris) in the grounds. I saw all these strange knobby growths on the acorns. I took these series of photographs, and although I knew they were a plant gall of some kind, I later identified them as the Knopper Gall, which is caused by a tiny wasp. The asexual generation of Andricus quercuscalicis develops on the acorns causing the formation of the galls. Green and sticky to begin with, the galls eventually flush red and then turn brown and woody.
The galls fall from the trees in late summer, and the adult gall wasps will emerge the following spring, although some may remain within the galls for up to four years. Eggs are then laid in Turkey Oak buds which result in tiny cone-shaped galls on the male catkins. The Knopper Gall arrived in Britain in the 1960s, and it did cause some alarm at first. It can be extremely abundant in some years (the trees I saw were completely covered) but there are usually enough acorns left for the trees to survive.
Photographs taken August 2013, local churchyard, Staffordshire.