Galls

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Gall
Gall, also cecidium, swelling or excrescence of plant tissues caused by the action of parasites. Such swellings, which may occur on any part of a plant but are usually found in regions of active cell division and growth, are initiated by the chemical action of secretions of infesting organisms, usually insects or fungi. Galls assume a great variety of forms; each form is characteristic of the parasite that causes it, and usually each species of gall-forming parasite infects a specific organ of a specific plant. The growth of the gall may result either from a tremendous swelling of the individual cells or from the rapid division of cells near the point of attack by the parasite. The gall provides a protective capsule for the parasite while it feeds on the plant.

The most striking galls are caused by insects that begin their invasion by laying eggs in plant tissues. The resultant enlargement of surrounding tissue is caused either by growth-promoting stimulation furnished by the eggs or by secretions of the larvae. The gall wasps, a family of small hymenopterous insects, Cynipidae, include the greatest number of species of gall-forming insects. These species restrict their attacks to about 20 genera of plants included in 6 families, especially the oaks; various species produce galls on roots, trunks, leaves, buds, flowers, or acorns. Several cynipids of the genera Amphibolips and Dipolepis stimulate production of galls known as oak apples. Specialization of function is so great in the cynipids that reproduction in some has been altered. Plagiotrochus punctatus, for example, produces in the summer a normal generation, the larvae of which cause blister galls on oak leaves. A second generation, entirely female, is produced in the spring; its larvae cause irregular swellings on oak twigs. The unfertilized eggs of the second generation produce gall wasps of both sexes, which repeat the cycle.

See also: Diseases of plants

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