DEVELOPMENT OF PHYLLOXERA VASTATRIX LEAF GALL 339 
developed gall. Cook's figure leads me to suspect that he did not 
have a fully developed, normal leaf. The figure which he gives of the 
gall would seem to substantiate this view, since this figure does not 
appear to represent a typical fully developed gall as Cornu (8) or as I 
have found it. However, since gall formation always begins on the 
very young, embryonic bud leaf, it becomes necessary first to con- 
sider the structure of this leaf. Text-figure 3 shows a drawing of a 
cross section of such a young leaf. The diameter of this leaf is about 
one-half that of the fully developed one; the average diameter of the 
mature leaf is about 144 ^ and of the young leaf, about 68 11. A close 
examination of the young leaf figured below shows two epidermal 
layers with as yet no development of cuticle, an abundance of rather 
Fig. 3. A cross section of an embryonic bud leaf, magnified 574 times. 
rigid, unicellular hairs around the margin of the leaf and on the larger 
veins, with a few very sparsely scattered hairs over the lower and upper 
leaf surfaces, an embryonic palisade layer made up of cells not much 
longer than they are wide and showing very little variation in length 
as compared with their width, and an embryonic spongy mesophyll 
made up of 3 layers of cells, so close together that scarcely any air 
spaces can be observed. 
IV. Variations in the Appearance of the Gall 
Here it should be noted that, although superficially the galls ap- 
pear very similar, a closer examination reveals many variations in 
the general size, form, number of hairs, and in convolutions. These 
variations are mainly due to the locality on the leaf selected by the 
insect, as a gall started on a primary vein is quite different from one 
