The study of galls is known as Cecidology and can be a fascinating area of wildlife exploration that can be combined whilst looking for various trees and wild flowers.  Galls are primarily the abnormal outgrowths of plant tissue that are primarily caused by the external influence of an insect, bacteria or fungus.  They come in a variety of shapes and sizes and the agent responsible for triggering their development can be identified without necessarily being present.  Galls can be found on any part of the plant (leaf, trunk/stem, flower, root) with pouch galls, leaf spots, blister galls and erineum galls all common enough.
Insect galls are highly recognisable structures that can be used by the larva as a specific micro-habitat.  Within the gall the larva feeds (sometimes on the nutritious starchy gall itself) whilst gaining protection from numerous predators. The insects which are responsible for the growth of galls can be categorised into
4 groups, these are:-
- these are the original inhabitants of a gall, the ones who trigger the initial growth.
Inquilines - these are secondary users whom use the gall as a place to lays eggs with a view in mind to let their larvae use the gall as a food source. The causer is still in residence.
Parasitoids - these are parasites that bring about the death of the causers, inquilines or other parasites.  The majority are Chalcids whom inject any available larvae with their own eggs.
Followers - the squatters who use old galls as a place for shelter rather than food.   

Plant gall damage is usually only an aesthetic problem and is not noted as a serious condition.  Trees with numerous growths seem to show no signs of injury and it is primarily a case of distortion rather than any long lasting damage.  The amount of galls present are affected by climatic conditions, the proliferation of the causal agent and plant susceptibility.
Galls are still not fully understood and records are sorely under-represented so many opportunities for new discoveries are available to the keen amateur naturalist.  They are indeed a fascinating aspect of the natural world and contribute another vital piece to the overall picture.