340 
HARRY R. ROSEN 
started between the veins. A primary vein in a very young leaf is 
made up of two epidermal layers with a large number of unicellular 
and a few multicellular hairs, which arise from both layers, a mass of 
parenchyma cells which make up a large part of the vein, surrounding 
the vascular elements proper and later on becoming thick-walled sup- 
porting cells; the xylem consists of several vessels usually having close 
and loosely woven spiral thickenings, while the phloem consists of a 
few (about 30), elongated sieve cells. One of the striking features 
with reference to the arrangement of galls on the leaf is the fact that 
they so frequently occur along the veins, especially along the largest 
veins. The mouth of the gall, which is the hairy opening on the upper 
surface of the leaf, usually is not circular but has a longer diameter 
parallel to the axis of the vein. This seems to be due to the fact that 
the insect places herself with her longitudinal axis parallel to that of 
the vein. Figure 2 shows the insect in this position, although she is 
Fig. 4. A nymph attacking a primary vein, resting in the depression. 
partly contracted away from the surface. Text-figure 4, however, 
leaves no doubt concerning her orientation, the insect being shown 
with her longitudinal axis parallel to the direction of the primary vein, 
which she is vigorously attacking. 
The growth of the gall keeps pace with that of the young leaf. 
Should anything retard the growth of the leaf, that of the developing 
gall is correspondingly inhibited. Likewise, should anything happen 
to the insect, the gall ceases to develop. This has been observed by 
