DEVELOPMENT OF PHYLLOXERA VASTATRIX LEAF GALL 349 
glass pins. These were so stuck into the young vine leaves that the 
heads pressed against the surface of the leaves. The pressure of the 
pin head was to take the place of that of the insect body. After the 
leaves had grown to a considerable size, microscopic examination of 
the injured areas showed dead cells which the pins had pierced. A 
slight depression was noticeable where the head of the glass pin had 
pressed against the leaf surface, but nothing was obtained resembling 
the insect cavity, with its fringe of hairs. 
It seems, therefore, that, at the beginning of its work, the insect by 
its sucking has caused a partial collapse of the tissue attacked, or at 
least a cessation of its growth, resulting in a sunken area or hollow in 
which the insect rests. What is the cause of the enlargement of the 
epidermal cells and why do they divide to produce multicellular hairs? 
The production of hairs occurs quite commonly in gall formation. 
Erineum galls are almost entirely given over to the formation of hairs. 
As to the abnormal production of hairs other than in gall formation, 
Haberlandt (13) caused groups of colorless hairs to be formed by de- 
stroying transient glandular hairs of Conocephalus ovatus and C. 
suaveolens. The glandular hairs functioned in eliminating water 
and their removal, according to Haberlandt, resulting in a surplus 
water supply, caused the formation of intumescences consisting of 
bunches of hairs. Ktister (12) points out the similarity between 
epidermal leaf hairs of Erineum galls and root hairs. He cites 
Schwartz as stating that changes in cell turgor may cause abnormally 
large root hairs. Sorauer (26) noted that wooly tufts were produced 
on the inner side of the core of the apple, which he assumes were due 
to an excess of water. This formation, Kiister says, resembles callus 
tissues. 
We note that in these cases a pressure stimulus has been put forth 
by the various investigators, as the initial factor in the abnormal 
production of hairs. The removal of certain organs or a retardation 
of a function of certain parts induces an abnormal pressure on ad- 
joining parts which results in increased growth. 
As stated above and shown in figure 9, the insect causes a de- 
crease in size of the mesophyll cells in the region where the proboscis 
is at work, a decrease which seems to be due to a retardation in growth, 
and which makes this part appear as a depression in the leaf. Around 
the periphery of this depression thus caused, hairs are formed. May 
it therefore not be assumed that these hairs are the result of the 
