Forest Preserve District of Cook County, Illinois
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Nature Bulletin No. 83   September 14, 1946
Forest Preserve District of Cook County 
Clayton F. Smith, President
Roberts Mann, Supt. of Conservation

Galls are those abnormal growths you see on plants, caused by parasites. Some of the conspicuous galls in this region are the swellings on the stems of golden-rod and the twigs of wild roses, the pine-cone galls on willows, the cockscomb-like galls on elm leaves and on poplars, and the galls on oak trees. Some of the oak galls are spheres the size of golf balls -- some green and smooth; some red and velvety. Some are warts on the upper side of leaves; some are on the under side. Occurring in several colors -- green, brown, red, yellow and blue -- these oak galls formerly were used for dyes, tanning leather, and making ink. There are several hundred kinds of tiny gall wasps, each producing its own peculiar kind of gall on oaks.

Almost all forms of plant life have galls. The parasite may be bacteria, mold, fungus, a mite, an insect, or combinations of these: the "witches- broom" on hackberry trees being caused by the combined action of a mite and a fungus. The bacterial galls on the roots of clover, alfalfa, locust trees and other legumes are Nature's most important means of enriching the soil by making nitrogen from the air available for plant use.

Insects of a great many kinds are responsible for as many kinds of galls. In addition to the gall wasps: saw flies, gall gnats, leaf rollers and leaf miners also produce galls, but most important to man are the aphids (plant lice). One of these, which produces galls on the roots of grape vines, was accidentally carried from eastern U. S. to Europe in 1859 and to California in 1874, where it did tremendous damage, destroying 2 1/2 million acres of vineyards in France alone. European grape vines were grafted onto the roots of American wild grapes and certain of our cultivated strains to produce a vine resistant to the gall-aphid.

Nobody knows why some galls are very simple and some very complex; nor why the gall made by each kind of insect is different from that made by any other. Nor does anybody know how the gall is made, except that apparently it is made by the plant itself and not by the parasite. Apparently the plant appeases the parasite.

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