374 Transactions of the Canadian Institute [vol. ix 



Summary. 



The idea that the gall-producing stimulus must of necessity be 

 applied directly to the cambium layer is not true in all cases, as any 

 actively growing tissue will respond to a producer's influence. 



The effect of this stimulus is operative on tissue at a considerable 

 distance from the centre of application. 



Certain inquilines in Cynipid galls possess the gall-producing power 

 but to a less extent than the real producer. 



Cynipid producers and probably others secrete an amylalytic ferment 

 that pre-digests food for the larva and may indirectly stimulate cell 

 proliferation by storing the nutritive zone with an unusually large quan- 

 tity of available nourishment which can diffuse to all parts of the gall. 



The gall-producing stimulus renders the protoplasm of the host 

 more active and awakens in it dormant characteristics, but apparently 

 does not endow it with power to produce entirely new structures. This 

 has been demonstrated in the case of glands, trichomes and aeriferous 

 tissue. 



The red colour of galls is perhaps a dormant characteristic that may 

 be reinstated by enzymatic action but there are other possible inducing 

 factors such as the light relations and in sawfly galls mechanical injury 

 by the act of oviposition. 



The shape of galls is controlled partly at least by the direction of the 

 stimulus and the location of the egg of the producer. In galls such as 

 the Lepidopterous types, where the larva burrows into the tissues after 

 leaving the egg, this feature has no effect. 



The relation of the various zones in the Cynipid galls is influenced 

 in some cases by the early differentiation of a cambium layer. 



