Cecidology is the scientific study of plant galls. Galls are induced by many different organisms, such as gall wasps, gall midges, leafroller moths, gall mites, fungi, bacteria and viruses. They serve for reproduction or protection against weather and predators. Plant galls develop in different ways. In insects, plant tissue is altered by the injection of cytokinins, producing characteristic gall forms. These injections can stimulate, inhibit or regenerate growth. Histoids and organoids are distinguished. Organoid galls are characterised by changes in individual plant organs, such as changes in shape, anomalies of leaf arrangement and branching, and the formation of new organs. Histoid galls are recognised by changes in plant tissue, such as felt, pith, covering or blister galls, and leaf rolls or folds. Plant galls may also serve as refuges for inquilines (animal species living in abandoned but still inhabited galls, mines, galleries or nests of other species) and parasites, which usually ultimately results in the death of the gall inducer.

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