DEVELOPMENT OF PHYLLOXERA VASTATRIX LEAF GALL 355 
gall production is the result primarily of the sucking by the insect. 
His experiments on this point are very scanty and some of his reasons 
against chemical stimulation are open to question. Cornu noted that 
the sucking and puncturing stops the development of certain cells and 
as a result, he says, tensions are set up which cause neighboring cells 
to become dilated up to a certain size and then to divide. On the leaf 
the tensions occur on the underside, so the growth is in that direction, 
while on the rootlets the tension is felt on the sides, so the growth is at 
the sides of the insect. 
Cook (5) studied the mouth parts of several Hemiptera gall pro- 
ducers, among which was Phylloxera vastatrix. He says: "So far as 
I have been able to determine, the insects do not remain attached to 
any one point for a great length of time." From this he concludes 
that the modification of plant tissue to form the gall is purely mechan- 
ical, being a continuous effort on the part of the plant to heal the 
wound produced by the repeated puncturing of the cells by the insect. 
Cornu (8) however, finds that the insect in the Phylloxera vastatrix 
leaf and root gall remains immovable, and fixed by its proboscis to the 
bottom of the cavity. My observations substantiate those of Cornu 
in that the insect, as soon as it fixes itself on the upper epidermis, 
seems to remain fixed for at least a considerable period. A proof 
for this is the frequency with which I have found the proboscis in the 
tissues in many of my prepared slides. But the immobility of the 
insect manifests itself in several other ways. Text-figure 2 shows how 
closely the insect fits into the cavity, so close in fact, that it could 
hardly move around in it, and in this case the insect has remained 
attached to the leaf from twenty-four to forty-eight hours. Again, 
cross sections of young and old galls show the proboscis usually at one 
fixed place, which place may be marked by a greater depth of the in- 
sect cavity, and by the broken-up epidermal and mesophyll cells, 
which are very few and are only found where the proboscis has pene- 
trated. We may therefore conclude that Cook's hypothesis of a 
mechanical stimulus playing the part in gall production is not founded 
on observed facts, since the insect remains fixed and does very little 
puncturing. 
The following facts stand out markedly in the formation of the 
Phylloxera gall: The young nymph attaches itself to the upper sur- 
face of a bud leaf. Within twenty-four hours, or less, the attacked 
portion shows a depression, the edge of which is bordered by hairs. 
