Whilst walking from Amberley to Arundel in West Sussex, UK, I spotted an abnormal growth on a leaf in a tree near the River Arun... I think it was a horse chestnut (Aesculus hippocastanum), but I do not remember now. There were a collection of these on a few leaves here and there. I suspect one would find some kind of mite inside... I left them undisturbed...
Also on the same tree was a curled leaf...
produced by the gallfly of the family Cynipidae. Can be found all over UK... young Oaks appear to be more susceptible than older. The acorn has been infected by a larvae which alters the normal growth of the acorn. The flesh of the gall is less tough than an acorn would be and is larger and more rounded.