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Galls are abnormal growths, swellings, pustules or discolourations produced by a plant or other host under the influence of another organism, involving the enlargement and/or proliferation of host cells and the provision of both shelter and food or nutrients for the invading organism. Galls provide a home for the larvae or grubs of certain invertebrates, where they can feed and develop, and each type of gall-producer or causer is specific to a particular kind of plant. Galls come in many shapes including spheres, knobs, lumps, warts or blisters, each being characteristic of the causal organism, and can have a range of colours. Galls can be found on the stems, leaves, flowers, fruits, seeds and roots of plants. Although many varieties of plants can be affected, oaks and willows are particularly rich in galls. Oaks are said to be host to more than 500 different types of galls. Weather, plant susceptibility, and pest populations affect the occurrence of plant galls.

Certain galls are documented to have been used in the production of ink since at least the time of the Roman Empire, and iron gall ink was the main medium used for writing in the Western World from the Middle Ages to the early 20th century. Oak marble galls > Link for instance, were introduced in the early 19th century in an attempt to grow our own supply of galls for making ink. In Chinese medicine, oak galls are used as a remedy called moshizi, which is used for treating dysentery, ulcers and hemorrhoids among other things. Native Americans used poultices of ground gall nuts on sores, cuts and burns. The high content of tannic acid also makes oak galls a good source for tanning and dyeing.

Galls cause little permanent injury and rarely kill the infested plant.

For more detailed information go to the British Plant Gall Society's website at Link