DEVELOPMENT OF PHYLLOXERA VASTATRIX LEAF GALL 343 
epidermis, perpendicular to the surface of the leaf. Figure 3 and 
higher magnifications of the central part of the same section in figure 4 
show the rapid proliferation of the mesophyll of the lower half of the 
leaf, together with slightly enlarged palisade cells, which only show 
this enlargement as they distance the proboscis. These two figures 
are especially noteworthy since they show the proboscis seemingly at 
work, and extending through the whole width of the leaf. So far as 
I know, they are the only figures of their kind which have yet been 
published. The insect is shown resting in a rather deep cavity instead 
of the shallow depression as it did after twenty-four hours of attack. 
During this three to four days of insect attack the young bud leaf has 
had an opportunity to unfold, being two to four times the size which it 
had at the time the insect first started its attack. It is necessary to 
bear in mind that the pouch-like form of the gall, which is beginning to 
manifest itself at this age, not only represents an excessive local growth, 
but also represents a change in the direction of growth of the tissue 
attacked. Instead of taking the normal direction of growth, the 
portion of the leaf beneath the insect grows downward and takes the 
form of a pouch. This growth downwards, is accentuated by the fact 
that the lower half of the leaf which does most of the proliferating, 
grows downward, in the path of least resistance, while the upper half 
enlarges slightly, without cell divisions. 
It is a peculiar fact that the direction of growth in this gall, as in 
all other galls where the gall producer is situated at one side of a plant 
organ, is always away from the insect, opposite to the direction of the 
application of the stimulus resulting in a sort of negative tropism. 
Kuster (12) says: "The side growing most in a leaf gall is always the 
one away from the gall animal, so that in this rolling of the infected leaf 
area, the gall animal comes to lie within the cavity thus produced." 
However, in this gall, although most of the hyperplastic tissue making 
up the gall comes from the lower half of the leaf, lying beneath the 
insect, the upper half of the leaf, which goes to make up the sides of the 
cavity, grows extensively upward. Pantanelli (22) describes this 
proliferation of the upper epidermis and palisade cells and points to 
the fact that growth due to the excessive proliferation of these cells 
finally forms the "lips" of the gall and almost completely encloses the 
insect. 
The striking histological peculiarity of figures 3 and 4, representing 
3 to 4 days of insect attack, is not the excessive growths of the lower 
