4 BRITISH GALLS 



For subsequent bibliography the reader is referred to the 

 index of literature. 



Galls arise only from embryonic tissues which have 

 received undue stimiilus. A plant gaU may be defined as 

 abnormal growth induced by the irritation of an animal or 

 fungus parasite. The protoplasm of cells predestined to give 

 rise in ordinary course to the plant or its part has been so 

 affected by the parasite that these cells deviate from their 

 normal mode of growth, become phenomenally active, 

 elaborate a new plan of construction, and give rise to the 

 overgrowths familiarly known as galls. To term the para- 

 site a "gall-maker," as many writers have done, is to 

 convey an utterly erroneous idea of its function. It makes 

 nothing, but induces much. 



It was thought at one time that galls arose solely through 

 the injection of an irritant by an insect, and that each insect 

 had its own peculiar poison. Both Malpighi and R6aumur 

 accounted for the phenomenon by the theory that it was due 

 to an irritant injected by the insect ; the latter observer also 

 thought that the extent of the wound and the heat of the 

 eggs were additional factors in inducing abnormal growth. 

 The infection theory received the support of Darwin, Pro- 

 fessor Riley, and Sir James Paget. The observations of 

 Adler, Cameron, and others, show that in the case of the 

 Cynipidae (gall wasps) no such irritant is introduced by the 

 insect, and that the larva is the cause of the hypertrophy. 

 Cameron remarks that the origin of gall structures cannot 

 be explained comfortably by any one theory, the habits of 

 the insects associated with them being so very diversified. 

 " Even in the Hymenoptera," he writes, " we find two 

 radical distinctions in the habits of the insects; that is to 

 say, in the Tenthreditiidae the gall is already formed before 

 the larva quits the egg, while in the Cynipidae the birth of 

 the larva is synchronous with the formation of the gall. 

 Until the larva is bom and commences feeding, there is no 

 gall formation." 



The observations of Burdon and others on Chermes 



