A wooden house

Even though these strange growths are not the tree's regular organs, they are actually part of it. Galls, as they are known, are tumours produced by the tree but caused by insects, which remodel part of the tree to meet their own needs. The galls provide them with shelter and, occasionally, food as well. Room and board, an all-inclusive package! These insects don't always get the best deal, though, as they frequently act as hosts to other pests.

Oak trees, which are often home to galls, are recognized for the biodiversity they support. Part of that biodiversity is the direct result of these galls.