Galls or cecidia are outgrowths on the surface of lifeforms. Plant galls are abnormal outgrowths of plant tissues and can be caused by various parasites, from fungi and bacteria, to insects and mites. Plant galls are often highly organized structures and because of this the cause of the gall can often be determined without the actual agent being identified. This applies particularly to some insect and mite plant galls. In pathology, a gall is a raised sore on the skin, usually caused by chafing or rubbing.
Causes of plant galls 
Insect galls are the highly distinctive plant structures formed by some herbivorous insects as their own microhabitats. They are plant tissue which is controlled by the insect. Galls act as both the habitat and food source for the maker of the gall. The interior of a gall can contain edible nutritious starch and other tissues. Some galls act as "physiologic sinks", concentrating resources in the gall from the surrounding plant parts. Galls may also provide the insect with physical protection from predators.
Insect galls are usually induced by chemicals injected by the larvae or the adults of the insects into the plants, and possibly mechanical damage. After the galls are formed, the larvae develop inside until fully grown, when they leave. In order to form galls, the insects must seize the time when plant cell division occurs quickly: the growing season, usually spring in temperate climates, but which is extended in the tropics.
The meristems, where plant cell division occurs, are the usual sites of galls, though insect galls can be found on other parts of the plant, such as the leaves, stalks, branches, buds, roots, and even flowers and fruits. Gall-inducing insects are usually species-specific and sometimes tissue-specific on the plants they gall.
It is worth noting that the fungus Ustilago esculenta associated with Zizania latifolia, a wild rice, produces an edible gall highly valued as a food source in the Zhejiang and Jiangsu provinces of China.
Bacteria and viruses 
Agrobacterium tumefaciens is an example of a gall-causing bacterium.
Other plants 
Mistletoe can form galls on its hosts
Galls are rich in resins and tannic acid and have been used in the manufacture of permanent inks (such as iron gall ink) and astringent ointments, in dyeing, and in tanning. A high-quality ink has long been made from the Aleppo gall, found on oaks in the Middle East; it is one of a number of galls resembling nuts and called "gallnuts" or "nutgalls".
The gall of Rhus chinensis, Galla chinensi, has long been considered to possess many medicinal properties.
Gall on a Maple leaf
Rose bedeguar gall on a wild rose in summer.
Oak artichoke gall (Andricus fecundator)
Eucalyptus leaf gall
Andricus kollari oak gall
Cola-nut galls (Andricus lignicola) on Pedunculate Oak
Oak marble galls, one with a Gall fly exit hole and another with Phoma gallorum fungal attack.
Red-pea gall (Cynips divisa) on Pedunculate oak.
Developing Pineapple pseudocone galls on Norway Spruce
Lime nail galls (Eriophyes tiliae tiliae)
Gall of peach tree leaves, found at Beijing
Andricus kollari gall
See also 
- Bush coconut
- Chirosia betuleti
- Mulga apple
- Oak apple
- Oak Marble gall
- Knopper gall
- Oak artichoke gall
- Rose bedeguar gall
- Pineapple gall
- Cola-nut gall
- Neuroterus quercusbaccarum Common Spangle and Currant galls
- Witch's broom
-  Merriam-Webster Online Dictionary, accessed Nov. 16, 2007 ("an abnormal outgrowth of plant tissue usually due to insect or mite parasites or fungi and sometimes forming an important source of tannin")
- Larson, K. C., and T. G. Whitham. 1991. Manipulation of food resources by a gall-forming aphid: the physiology of sink-source interactions. Oecologia 88, P.15 – 21.
- Weis, A. E., and A. Kapelinski. 1994. Variable selection on Eurosta’s gall size. II. A path analysis of the ecological factors behind selection. Evolution 48, P.734 – 745.
- Graham N. Stone and Karsten Schonrogge (2003) The adaptive signiﬁcance of insect gall morphology. TRENDS in Ecology and Evolution 18(10):512-522
- Terrell, E.E. and L.R. Batra. Zizania latifolia and Ustilago esculenta, a grass-fungus association. Economic Botany 36(3):274-285.
- Zhang J, Li L, Kim SH, Hagerman AE, Lü J. 2009. "Anti-cancer, anti-diabetic and other pharmacologic and biological activities of penta-galloyl-glucose." Pharm Res 26: 2066–2080.
|Wikimedia Commons has media related to: Galls|
- Goldenrod Gall Fly
- All Galls Are Divided Into Three Parts (At Least In Goldenrod), Solidago
- "Gall". Infoplease encyclopedia. Retrieved March 2006.
- "Common oak gall". University of Kentucky Entomology. Retrieved September 11, 2006.