BROOK LIFE. 



i6i 



These cone-shaped objects belong to that class of 

 vegetable growths termed galls. There are very 

 many kinds of these ; and upon some plants, as oaks, 

 for example, they are very abundant. 



Galls are produced by insects in this way. 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 poison, 

 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. Evidently if there is a poison 

 in such cases it must be secreted by the larva. The 

 explanation of why galls grow is not yet clear ; but 

 we know this much, that each species of gall-making 

 insect makes a par- 

 ticular kind of gall. 

 Hence one versed 

 in this subject can 

 tell by the form 

 and structure of a 

 gall what species 

 of insect produced 

 it. The gall serves 

 as a home and food 

 for the larva de- 

 veloped within it. 



Let us return to 

 the pine-cone wil- 

 low-gall (Fig. 133). 



This differs in Fig. 133.— The pine-cone wUlow-gall. 



shape and in the manner of its growth from most 

 galls. (Other types of galls will be described in later 



