I found a small oak tree today, no more than 1.5 metres high, that was covered in these marble galls. I've never seen so many on one small tree. The next branch had another 10-12 galls, but not so tightly clustered. These lovely structures are produced by abnormal cell development that is triggered by a small wasp, Andricus kollari. Each gall provides a home for a developing wasp larva as well as a range of other invertebrate lodgers, known as inquilines, and a number of parasites feeding on members of this community. The larger holes are escape-holes of the mature wasp and smaller holes mark the exit points of some of the inquilines or parasites. This wasp is not native to the UK, but is now very widespread after being introduced in the 19th century when the galls were much sought after as a source of tannin for dyeing and ink-making.