THE PBINCIPLES OF OAK GALL FORMATION. 6 



definite on this point. " What is the first factor in 

 gall formation ? This, the growth of the larva, as has 

 already been pointed out. The second is that the egg- 

 must be laid in, or in contact with, the cambium layer " 

 (' Brit. Phyto. Hymen.,' vol. iv, p. 21). " AVlien a gall- 

 fly has inserted its egg into the neighbourhood of this 

 (cambium ring) tissue, what follows ? In the first 

 place the act of oviposition itself has no eifect. . . . 

 I am convinced that the simple prick of the gall-fly 

 does not set up gall formation ; this, I hold, only begins 

 when the larva emerges from the egg " (' Alternating 

 Generations,' p. 99). 



In comparison with the opinions of such specialists, 

 supported by those of recent observers, and confirmed 

 l)y many who have devoted years of research to the 

 subject, the theories of gall origin advanced by very 

 early writers have but little value. They must not, 

 however, be entirely omitted. It was thought by 

 several of the early observers that the production of 

 cell activity was caused by a glandular secretion of an 

 irritating nature, introduced with the egg at the time 

 of oviposition. Others advanced a theory that the 

 action of oviposition caused irritation to the plant. 

 Another suggestion was made, viz. that the egg pos- 

 sessed a higher temperature than that of the plant, and 

 thus the cells were warmed into activity. Several 

 writers have also maintained that infection, in the 

 form of disease, was the sole cause of galls. 



That a secretion of some kind emanates from the 

 gall-wasp when ovipositing is well known, and it can 

 be seen, especially if a number of imagines develop from 

 galls kept in a glass bottle or glass-topped box. They 

 will place their eggs on the glass, and at the same time 

 the minute drop of fluid can be seen also. Andricns 

 nudus will provide a good example. It w^ould appear 

 that the purpose of the secretion is to seal the hole 

 caused by the ovipositor. That it does not stimulate 

 the plant to action the following will demonstrate. 

 The imagines from galls of Trlc/onnspis crtistalu make a 



