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 containing the larva. The 

 stem remains short but the outer leaves are fully developed in 

 most cases. 



(5) Pachypsylla celtidis-gcmma 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. In 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 onh' 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 luitil I 

 hav^e 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 

 onl}' by a study of their development and structure. 



Some galls occur on both leaves and stem, but in these cases 

 the gall affects onh' the outer layers of the cells of very 3'oung 

 twigs and these cells at this time resemble the leaf cells in both 

 structure and functions. P/iy/loxeia carya-spinosa Shinier (Part 

 I, Fig. 19) and Phylloxera caryac-caiiUs Fitch (referred to in 

 Part V) are good examples of leaf galls affecting stems. 



The I,epidopterous 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 (pa) of 

 closely-fitted cells and few intercellular spaces. Below the par- 

 enchyma zone are the fibro- vascular bundles (p. v. b.), which 



