164: THE OAK. 



cells multiply and form the gall. This is a mere out- 

 line sketch of the matter, however, for the differences in 

 behavior are enormous. Each insect causes the forma- 



FIG. 50. Portion of the spore-bearing hymenium of Merulim lacrymans, 

 the fungus of " dry rot." 



tion 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 



