264 
The Ohio Naturalist. 
[Vol. II, No. 7 , 
papers now in preparation: A. Stem galls; B. Leaf galls; 
C. Bud galls, a. Terminal buds, b. Lateral buds ; D. Root galls. 
Leaf galls may in many eases 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 layers 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. 1), Ulmus aviericana L. (Fig. 4), and Tilia americana L. 
(Fig. 6) may be considered as typical and yet in themselves show 
minor differences. In Vitis vulpina L. (Fig. 3) the palisade is 
not so pronounced as in the preceding and the mesophyll is more 
compact. In Quercus alba L. (Fig. 7) and in Acer saccharinum 
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. 1) and Salix cordata (Fig. 2), members of two 
related families, are as great as those differences frequently found 
between a normal leaf and the galls occurring upon it, e. g., H. 
ovata (Fig. 1) 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 either 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 
