264 The Ohio Naturalist. [Vol. II, No. 7, 



papers now iu preparation: A. Stem galls; B. Leaf galls; 

 C. Bud galls, a. Terminal buds, d. Lateral buds ; D. Root galls. 

 Leaf galls may in many cases be classed as bud galls if we con- 

 sider that the egg in some orders of insects is deposited while the 

 leaf is in the bud, but in the above classification the term applies 

 to the developed gall, and the ' bud gall ' applies to a distortion 

 of the entire bud. 



1. The Normal Leaf Structure and Its Variations. 

 The normal leaf structure may be said to consist of a single layer 

 of epidermis on the upper and lower surfaces of the leaf ; next to 

 the upper epidermis is the usually single layer of palisade or 

 columnar cells, placed with their long axis at right angles to the 

 surface of the leaf ; between the palisade cells and the lower epi- 

 dermis is the mesophyll, made up of many la3'ers of irregular 

 cells, between which are the large air spaces connected with -the 

 outside by the stomata in the lower epidermis ; running through 

 the leaf are the fibro-vascular bundles noticable to the naked eye 

 as the venation. 



Although the above may be said to be a description of a typical 

 leaf, it must be kept in mind that leaves are subject to great 

 variation and this must be taken into consideration in a discus- 

 sion of the variation of the gall structure from the normal leaf. 

 The structure of the gall must be compared with the structure 

 of the normal leaf of the plant on which the gall is found, not 

 with the typical leaf. 



A brief study of the normal leaves of the plant will serve to 

 emphasize the preceding points. Hicoria ovata ( Mill. ) Britton 

 (Fig. i), Ubmts avicricana L. ^'^g- 4), and Tilia amcricana L. 

 (Fig. 6) may be considered as t3'pical and yet in themselves show 

 minor differences. In Vitis zndpina Xi- (Fig- 3) the palisade is 

 not so pronounced as in the preceding and the mesophyll is more 

 compact. In Qiierais alba L. (Fig. 7) and in Acer saccharhiiivi 

 L. (Fig. 5) the palisade is typical, but the mesophyll is very 

 compact. In Salix cordata Muhl, (Fig. 2 ) the mesophyll while 

 distinct from the palisade has assumed palisade characters. 



The differences in structure between the normal leaves of 

 Hicoria ovata (Fig. i) and Salix cordata (Fig. 2), members of two 

 related families, are as great as those differences frequentl}' found 

 between a normal leaf and the galls occurring upon it, e. g., H. 

 ■ovata (Fig. i) and the simpler Phylloxera galls (Figs. 16-20). 



2. Phytoptus Galls. This discussion is based not only on 

 the four galls described below, but from observations of several 

 others. However, the following will illustrate all the points 

 observed : 



The Phytoptus galls are small and may extend on cither or 

 both sides of the leaf. The outer surface of the galls show the 

 normal epidermis and below this cells which are not palisade but 



