404 
ALBAN STEWART 
distance. A large resin cavity occurs in this region of the leaf. A 
single bundle enters the upper part of the leaf, xylem above and 
phloem below. On each side of this bundle are peculiar tracheids 
with bordered pits and bar-like thickenings (text fig. I, xxx). For 
further consideration of these tracheids see De Bary (2). 
It is sometimes difficult to determine in the young stem which of 
the cells belong to the leaf and which to the outer bark of the stem 
as the leaf and stem tissue blend together more or less at the point of 
union of leaf and stem. Between the attachment of the opposite 
leaves, however, the stem is exposed and is covered with epidermal 
cells similar to those which occur on the inner side of the leaf. There 
is a single layer of rather thick-walled parenchyma cells inside the 
epidermis, followed farther in by one or more layers of large, thin- 
walled, oval-shaped parenchyma cells. The inner bark is composed 
of concentric rings of bast fibers between each two of which there are 
three layers of cells. According to De Bary (2), two of these layers 
are of sieve tubes which are separated from each other by a layer of 
parenchyma cells. 
The xylem of the young stem is composed of small triangular 
groups of tracheids, separated from similar adjacent groups by uni- 
seriate rays. The rays extend outward into the inner bark, thus 
dividing the phloem and bast region into segments. The rays are 
broader opposite where the leaf trace bundles leave the xylem portion 
of the' central cylinder. This broadening comes about usually by 
an enlargement of the ray cells, but sometimes there is an inter- 
polation of additional ray cells. As two leaf traces leave the stem 
about opposite each other, the gaps caused by the broader rays 
divide the woody portion of the central cylinder into two nearly 
symmetrical halves. A small V-shaped depression occurs in the 
xylem where the leaf trace leaves it. A small group of pith cells 
occupies the center of the stem. 
A diagrammatic drawing of a very young cedar apple gall and the 
stem that bears it is shown in figure i. This gall has arisen from the 
axil of a leaf, a portion of which is shown to the right of the figure. 
I have examined a large number of young galls and have been unable 
to find any which were similar to the two figured by Coons (i, right 
side of fig. i), which according to the figure have arisen evidently 
from the surface of the leaf. I have also been unable to find any 
young galls which were terminally located, and which might have 
