May, 1903 .] Galls and Insects Producing Them. 
421 
thus forming a bulb-like compound gall. On the inner surface 
of the base of each petiole is a cavity containiug the larva. The 
stem remains short but the outer leaves are fully developed in 
most cases. 
(5) Pachypsylla celtidis-gemma Riley (Fig. 38) is evidently a bud 
gall very similar to the preceding. Only advanced stages of this 
gall were collected, and therefore its development could not be 
observed. From the specimens collected it appeared that each 
scale and undeveloped bud formed a pocket for the insect, there 
being a single insect under each scale. 
CONCLUSIONS. 
Bud galls are subject to considerable variation due to the fact 
that they are produced by insects of different orders and that 
these insects attack different parts of the buds and different 
tissues in these parts. I11 all cases except the fourth the demands 
of the insect are so great as to cause a very pronounced change in 
the bud. In the fourth the modifications are not so pronounced 
as in the other four types. 
Part IV. Stem Galls. 
Stem galls, according to my definition, include only those galls 
which cause a swelling of the stem and with the larva placed in 
or near the center, thus affecting the stelar and fibro-vascular 
parts of the stem. This definition may not be as broad as it 
should be, but I hesitate to make it include other forms until I 
have had an opportunity to make a more careful examination of 
the questionable forms. The fact that such galls as H. globulus 
(Fig. 34, a, b, c), which is frequently mentioned as a stem gall, 
are in reality bud galls, leads me to be doubtful of the origin of 
galls which have similar locations. Many of the so-called stem 
galls may be in reality bud galls and this point can be determined 
only by a study of their development and structure. 
Some galls occur on both leaves and stem, but in these cases 
the gall affects only the outer layers of the cells of very young 
twigs and these cells at this time resemble the leaf cells in both 
structure and functions. Phyl/oxa a carya-spinosa Shimer (Part 
I, Fig. 19) and Phylloxera caryae-caulis Fitch (referred to in 
Part V) are good examples of leaf galls affecting stems. 
The Lepidopterous galls are usually stem galls and may be 
either solid or hollow and are most common on Solidago. In 
studying such galls it is necessary' to examine first a normal stem. 
The stem of Solidago (Fig. 39) shows the ordinary dicotyledo- 
nous character. The epidermal cells (e p) are firm and rather 
hard. Just below these cells is the parenchyma zone (p a) of 
closely-fitted cells and few intercellular spaces. Below the par- 
enchyma zone are the fibro-vascular bundles (p. v. b.), which 
