2 BRITISH OAK GALLS. 



of fibro-vasciilar l)imdles wliicli corresponds to the 

 inner bark. 



The activity of the cambium layer begins in the 

 spring and increases throughout the summer, the cells 

 dividing continually, until hj the autumn the layer is 

 ten or twelve times thicker than at the commencement 

 of activity. The result of the multiplication of. these 

 cells, which are formative, is to produce a new ring of 

 xylem on its inner and a new layer of l^ark on its 

 outer surface. The whole life of the plant springs 

 from this cambium layer ; it is the part in which 

 active metabolism takes place, and it alone possesses 

 the necessary conditions for gall production. 



One essential condition for gall formation is that 

 the egg must be laid in, or in closest contact with, the 

 cambium layer. Another condition is the birth and 

 growth of the larva. 



It should be borne in mind that the act of oviposition, 

 ■/. e. a wound caused by a simple puncture, does not of 

 necessity give the impetus to gall formation. There 

 are, however, exceptions to this. They are supplied 

 by several species of the aculeate Hymenoptera. To 

 name but one instance, the gall caused by Aulax 

 luipochoeridis in the flower stalk of the Cat's-ear 

 (Hi/jwcliwris radicata, L.) is well developed l)efore the 

 larvse hatch. 



It is somewhat difficult to understand why this 

 should be so. If it were merely a question of providing 

 a sufficiency of food for the larvae before they hatch, 

 then the same reason Avould be expected to hold good 

 with regard to the gall of Apliilotliric raclicis ; the 

 more so, because the number of larva; of the latter 

 species (normally aliout sixty), is about five times 

 greater than that of the former. 



But as regards galls on the oak, it is only when the 

 larva emerges from the egg that active metal^olism 

 begins and the seat of increase of plant tissue is mani- 

 fested. Both Cameron and Adler, the two principal 

 writers on galls, whose works are in English, are very 



