Acorn knopper galls can be found on the young oak trees behind Greenwood House and also in the Charlton Down Nature Reserve. They are acorns which have developed in a distorted way because a particular species of small wasps have laid their eggs in the acorn flowers. The irritation as the egg hatches out causes a proliferation of tissue growth on which the larva can feed. Sometimes a whole acorn is transformed this way. Sometimes only part of it. And on some stems a perfectly normal acorn in its cup grows side by side with the gall acorn. The shapes are variable as you can see from the photographs.
If you would like a little more information about these galls, ten years ago I wrote in my other blog about some of the same type of galls that I noticed in Kew Gardens.