2 BRITISH OAK GALLS. 
of fibro-vascular bundles which 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 by 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 bark 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, 
7,é, 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 
hypochevidis in the flower stalk of the Cat’s-ear 
(Hypocheris radicata, L.) is well developed before the 
larvee hatch. 
It is somewhat difficult to understand why this 
should be so. Ifit were merely a question of providing 
a sufficiency of food for the larve before they hatch, 
then the same reason would be expected to hold good 
with regard to the gall of Aphilothria radicis; the 
more so, because the number of larvz of the latter 
species (normally about 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 metabolism 
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 
