DEVELOPMENT OF PHYLLOXERA VASTATRIX LEAF GALL 345 
base of the insect cavity collapse and their individuality is lost. These 
are the most noteworthy histological developments seen at the end of 
three to four days of insect attack. 
At the end of five to six days of cecidial development, the cells of 
the palisade layer at the base of the insect cavity can be distinguished 
from the rest of the mesophyll, while at the sides of the cavity their 
identity is lost and they assume the characters of rapidly dividing 
thin-walled parenchyma cells, sometimes enlarging enormously and 
becoming isodiametric or irregularly ovoid in shape. The rest of 
the mesophyll below the bottom of the cavity, except in the region of 
the proboscis, continues to proliferate enormously into a mass of thin- 
walled cells. As to the epidermal cells, some of them enlarge in the 
plane perpendicular to the surface of the leaf, as was shown in figure 7 
for the earlier gall development, but at the upper part of the cavity 
they give rise to a large number of multicellular hairs. The lower 
epidermis likewise produces many such hairs. 
The daily growth in size of the gall is very marked. In twenty- 
four hours it may increase 0.3 mm., in its long axis, so that proliferation 
and enlargement is very rapid. At the end of twelve to fifteen days, 
when the gall reaches maturity and the normal leaf has attained its 
maximum size, a section through the side of the gall, cut perpendicu- 
larly to the leaf sjrface, reveals an enormous mass of thin-walled, 
partly empty parenchyma cella, some of which are .ereatly elongated. 
This is the most striking feature in the mature gall. 
The histological structure of the mature gall is as follows : Starting 
with the lower epidermis, we note the small number of stomata, while 
the epidermal cells show very little cuticular development. The 
epidermal cells are usually smaller or the same in size as the normal 
epidermal cells, although they sometimes appear elongated and narrow, 
running parallel to the surface of the gall. The corrugations or 
striations of the cutinized layer of the normal leaf are absent in the 
epidermal cells of the gall, except where it bounds vein parenchyma, 
in which case the striations are very evident. The mesophyll is made 
up of a mass of cells, many of which are rather undersized, as com- 
pared with normal mesophyll cells, while others are usually elongated 
perpendicularly to the surface of the gall as noted by Cook (4). How- 
ever, they sometimes turn sharply and run parallel to the surface of the 
gall. This condition is brought out in figure 5. The arrow is pointing 
to groups of such elongated cells which are bent at right angles to the 
