338 
HARRY R. ROSEN 
European galls. Very frequently the lower surface would be com- 
pletely spotted with these wart-like outgrowths. A good-sized gall 
is about as large as a pea. It appears, on close examination, as a 
much furrowed and wrinkled, irregular pouch, with hairy projections, 
the mouth of the pouch opening on the upper surface of the leaf. 
When the gall has reached maturity the mouth is marked by two 
lip-like growths extending 2 to 3 mm. above the upper surface of the 
leaf, while around these lips a profuse growth of glistening, downy 
hairs covers and entirely closes the opening to the cecidial cavity. 
III. Histology of the Normal Leaf 
It is very evident that, in order to understand fully the changes 
which occur in the Vitis vulpina leaf during gall formation, it is neces- 
sary to have a good understanding of the structure of the young and 
of the mature normal leaf. Cook (4) gives a drawing of a normal leaf 
of Vitis vulpina and notes that, as compared with other leaves, the 
palisade is not pronounced, while the mesophyll is more compact. 
Comparing gall structure with the structure of the normal leaf, he 
Fig. 2. A cross section of a normal leaf, magnified about 349 times. 
points out that it is necessary to compare the gall structure with the 
structure of the leaf on which the gall is found, and not with the 
typical leaf. I have found, contrary to Cook, a well-developed pali- 
sade, and a spongy mesophyll with numerous air spaces in mature, 
gall-bearing leaves. Text-figure 2 shows a drawing of a cross section 
of a mature leaf taken from a portion immediately adjoining a fully 
