GALL-FLIES. 



327 



with sucli a gluten. In either of these two cases, the gluten 

 will prevent the sap that flows through the puncture from 

 being scattered over the leaf and wasted ; and the sap, being 

 thus confined to the space occupied by the eggs, will ex- 

 pand and force outwards the pellicle of gluten that confines 

 it, till becoming thickened by evaporation and exposure to 

 the air, it at length shuts up the puncture, stops the further 

 escape of the sap, and the process is completed. This 

 explanation will completely account for the globular fomi 

 of the galls alluded to ; that is, supposing the egg of the 

 gall-fly to be globular, and covered or coated with a pellicle 

 of gluten of uniform thickness, and consequently opposing 

 uniform resistance, or rather uniform expansibility, to the sap 

 pressing from within. It will also account for the remark- 

 able uniformity in the size of the gall apples ; for the 

 punctures and the eggs being uniform in size, and the 

 gluten, by supposition, uniform in quantity, no more than 

 the same quantity of sap can escape in such circumstances. 



F.edegnar Gall of the Rose, produced hy Cynips Rosa. 



But though this explanation appears to be plausible, it is 



