22 THEOEIES OF GALL OEIGIN. 



being stopped, provided they are kept in the open air 

 and supplied plentifully with moisture. 



When the larval irritation sets in there is clearly a 

 flow of sap to the affected part, and that, too, in 

 greater abundance than to those parts not affected. 

 The irritation continues with the growth of the gall, 

 and the gall assumes very rapidly a definite form. It 

 is conceivable that the mode of irritation may not be 

 uniform in all species, and obviously the results pro- 

 duced cannot be alike when applied to different parts 

 of a plant. We cannot conceive of galls formed on 

 catkins, or young or even old leaves, having the same 

 complicated structure as those in direct relationship 

 with the woody tissues. In other words, the seat of 

 origin of the gall has necessarily a direct relation to 

 its morphology and form. I do not think that the 

 form of the ovipositor has anything to do with the 

 matter ; for although we do find considerable diversity 

 in the egg-laying apparatus, yet that diversity has 

 relation to the position in which the eggs are to be 

 laid — those ovipositing in buds, for example, requiring 

 a longer ovipositor to enable them to pierce the scales 

 than a species laying eggs in leaves. There is also a 

 difference in the form of the egg ; yet that also has 

 reference to the position in which they are placed, as 

 explained when treating of the eggs. Of any dif- 

 ference in the form of the larva we have no evidence ; 

 and certainly the wound made by the ovipositor, 

 per se s cannot be held accountable for the growth of 

 the gall, for the latter may take weeks or even months 

 after being in the plant before it grows. 



As for there being any evidence of a chemical or 

 molecular difference in the fluid which the gall-flies 

 undoubtedly secrete, there is none whatever. Lacaze- 

 Duthiers, arguing from the unquestionable fact that 

 the venom of wasps and bees differed in strength as 

 shown by its different degrees of virulence, concluded 

 that a like diversity existed in the gall-fly venom. It 

 must, however, be remembered that the function of 



