DEVELOPMENT OF CERTAIN PACHYPSYLLA GALLS 279 
A still older stage (i mm. long diameter) of a P. mamma gall (median 
section) is shown in figure 9. The cover-cone has attained almost its 
maximum size (in the adult gall it is a minute papilla), and the characteristic 
sub-hemispheric form of the gall has definitely appeared. The occurrence 
on the very young galls of the large trichomes is worthy of note. These 
are highly evanescent, never being seen on the adult structure. A presen- 
tation of the histology of the nearly mature gall is given in an earlier paper 
of the writer's (8). 
The early histogenesis of a P. asteriscus gall is different from that of the 
P. mamma gall. The differences are of course related to the specificity of 
the galls. 
Figure 5 shows the outline of a median section of a very young P. 
asteriscus gall (i mm. long diameter). At this early stage the fundamental 
form of the mature gall (fig. 7) is appearing in the prominent cover-cone, 
the circular outgrowth beneath which in section is exhibited as two lateral 
processes. The thickness of the region beneath the insect is definitely less 
than that of the normal leaf as seen to left and right, indicating a marked 
inhibition of growth in those tissues. 
In figure 5<x are presented the tissue conditions of a critical portion of 
the section shown in figure 5. Here, as in the case of the P. mamma gall, 
a meristem-like layer is well defined. The details of these two layers and 
other cellular features are shown in figure 5&, which presents the region 
immediately surrounding the mouth parts. This region, it will be noted, 
with the exception of a small portion of it directly beneath the sheath- 
enclosed mouth parts, has maintained the original five-layered condition of 
the young leaf. Though slightly excessive nuclear division has taken 
place in the zone of growth inhibition, wall formation has not occurred,, 
the result being the appearance of multinucleate or "giant" cells, the 
largest of which is that one in which the end of the proboscis terminates. 
The presence of such a giant cell as the one last mentioned, is also noted in 
the P. mamma galls (fig. 6). Further discussion of this giant cell situation 
will be given later. 
Rosen (7) found an inhibition of growth in the floor of the grape Phyl- 
loxera leaf gall but reports no multinucleate condition. 
Differing in still another character from the P. mamma gall, that pro- 
duced by P. asteriscus shows the very early differentiation of a sclerenchyma 
layer through the deposition on the walls of the lower hypodermal cells of 
bands of lignin, simulating the scalariform type of lignification (fig. ^h). 
The curious condition of the cell in the upper right-hand corner of the 
section (fig. 5^), the cell being almost empty and a rivet-shaped plug of 
deeply staining matter inserted in the wall, is undoubtedly due to mechanical 
puncture by one of the two stout bristles with which each leg of the nymph is 
armed. More injury of this character does not occur probably because of 
the extreme passivity which characterizes the insect throughout the period 
of gall formation. 
