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cells multiply and form the gall. This is a mere outline sketch of the matter, however, for the differences in behavior are enormous. Each insect causes the formation

Fig. 50.—Portion of the spore-bearing hymenium of Merulius lacrymans, the fungus of "dry rot."

of a specific kind of gall, differing in shape, size, color, and other characters from those caused by other gall-insects. There are many kinds, and only a few can be mentioned here. Each species of oak may have its