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. Fach insect causes the forma- 
Fic. 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. Hach species of oak may have its 
