DEVELOPMENT OF PHYLLOXERA VASTATRIX LEAF GALL 357 
growths may be made to appear on the upper or lower surface of the 
leaf, depending on whether the leaf is sprayed on the upper or lower 
side. 
In no case are depressions or cavities produced at the point of 
contact between the leaf tissue and the chemical applied. The 
growth response is always in the mesophyll tissue immediately below 
the point where the spray was applied and not on the side furthest 
away from the appHcation of the, spray. Sprays with commercial 
diastases on young cauliflower leaves likewise produced intumescences, 
but I feel that this latter experiment was not done on a large enough 
scale to warrant any conclusions. It is possible that other substances 
in the commercial diastase were factors in the production of the in- 
tumescences. If any conclusion may be drawn from these experiments 
it is that excessive growth takes place in those cells in which the ap- 
plied chemicals are at their greatest concentration, and not at a dis- 
tance from the center of application, where the concentration would 
be less. 
If we compare these artificially produced intumescences with the 
Phylloxera gall which has been described above, it will be seen that in 
the intumescences produced by the application of chemicals, the place 
of application is the place of excessive growth, and in the Phylloxera 
the place of application is the place of hindrance of growth. From 
these experiments the burden of proof becomes more difficult for 
those who adopt the "chemical" theory of gall production for sucking 
insects. This is especially true in the case of the Phylloxera gall. 
VIII. Summary 
1. The Phylloxera vastatrix leaf gall starts to develop on em- 
bryonic bud leaves. In twenty-four hours the insect produces a de- 
pression at the periphery of which hairs are formed on the upper sur- 
face of the leaf. The depression is due to a lessened growth of the 
attacked mesophyll. 
2. After three to four days of insect attack the lower half of the 
leaf tissue which surrounds the portion in which the proboscis is in- 
serted has proliferated enormously. The whole thickness of the leaf 
in the region immediately around the proboscis shows no proliferation. 
That portion of the leaf which is beneath the insect does not proliferate, 
but the upper half at the sides of the insect grows upwards and forms 
the walls of a large insect cavity. Upper epidermal cells and several 
