164 THE OAK 



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

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

 behaviour are enormous. Each insect causes the forma- 



FlG. 50. Portion of the spore-bearing hymenium of Merulius 

 lacrymans, the fungus of ' dry rot.' 



tion of a specific kind of gall, differing in shape, size, 

 colour, 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 



