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 stimulus. A plant gall 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 Reaumur 
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 Tenthredinidae 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 born and commences feeding, there is no 
gall formation.” 
The observations of Burdon and others on Chernies 
