GALLS. In plants, deformities caused by the presence of foreign living organisms in the tis sues, by substances which these organisms have produced, or by both causes combined. The tech nical term `cecidium' has been proposed as a sub stitute for 'gall,' with the purpose of using it with prefixes to indicate origin, e.g. myco-cecidium for gall produced by fungi; diptero-cecidium for gall due to gnats, etc. The organism producing the gall is usually either an insect or a fungus, although some galls are due to the attack of slime-molds (Alyxomyeetes), algae, bacteria, or worms. Galls show a great diversity of form. The deformity is due primarily to an exag gerated growth of the tissues normally present in the plant organ affected, although the tissues frequently have their cells much altered in form and size; secondarily the gall is due to the development of special tissues peculiar to galls_ The development of the gall is sometimes due to the substance injected by the insect at the time of biting or egg-laying, but more commonly it depends upon the presence of the egg or larva, whose excreta. or movements supply the
necessary stimulus. Attempts to produce galls artificially by injuries or injections have failed.
Galls produced by parasitic plants are usually due to the presence of the organism, and since these are immobile the initial cause must be looked for in the excreta of the attacking plant (Fig. 1). Examples of galls produced by plants are the wens and tumors of various sizes and forms common upon leaves and stema infested by parasitic fungi (Fig. 2). They are not easily distinguished in form from similar galls produced by insects. The club-root (q.v.) of cabbage and turnips, due to the attack of slime-molds, and the tubercles upon the roots of Leguminosn, due bacteria-like parasites, are examples of root-galls (Fig. 3). See GALL-INSECTS.