Examples of Pupils' Work 169 



Why this peculiar growth should take place we cannot 

 say; but authorities have told us how a gall is formed. 

 Comstock says, "The female gall-producing insect stings 

 the plant and lays an egg in the wound. It is believed 

 that in some cases there is deposited with the egg a drop 

 of a poison (as in the gall, which we shall study presently) 

 which causes the growth of the gall. But in other cases 

 the gall does not begin to develop until the larva hatches 

 from the egg and begins to feed upon the tissue of the plant. 



FIG. i. Sage-brush with Gall. 



Evidently if there is a poison in such cases it must be 

 secreted by the larva. Though the explanation .of why galls 

 grow is not clear, we know this much, that each species of 

 gall-making insect makes a particular kind of gall. Hence, 

 one versed in this subject can tell by the form and structure 

 of a gall w r hat species of insect produced it." 



