GALLS PRODUCED BY TWO CEDAR RUST FUNGI 243 
of the same character. The central part of the leaf is occupied by a 
single fibro-vascular bundle of the collateral type. This is composed 
of a small group of scalariform tracheids and a group of phloem cells 
about equal in size. Just back of this bundle near the base of the leaf 
is a resin duct. The remainder of the tissue of the leaf is made up of 
parenchyma cells. The parenchyma cells near the upper or inner 
surface below the stomata are globose in shape and are loosely ar- 
ranged, forming a tissue similar to mesophyll in appearance. The 
outer layers of cells are elongate, the long axis being perpendicular to 
the surface forming a palisade tissue. The structure of the very 
young stems which bear the young cedar apples is only slightly dif- 
ferent from that of the leaf except of course that the fibrovascular 
system consists of a medullated central cylinder which is split up into 
several collateral bundles by the presence of leaf gaps. The cortical 
tissue of the stem and the parenchyma cells of the leaf are so much 
alike that it is impossible to distinguish between them. The parts of 
the stem not covered by leaves are protected by an epidermal layer 
similar to that of the leaf. 
One of the first and most conspicuous things which may be observed 
in a longitudinal section of a stem bearing a young gall (PI. XII, Fig. 4) 
is the position of the gall as compared with that of the opposite leaf. 
It is evident in every case that the gall occupies a position identical 
with that of the leaf on the opposite side of the stem. There is no 
sign of an axillary structure of any kind. Usually the leaf whose 
position the gall occupies and on which it develops becomes distorted 
beyond recognition except that there is evident a portion of its tip. 
A section through the leaf bundle at the base of the gall shows clearly 
that the vascular bundles of the gall arise from this leaf bundle. This 
is best studied in galls which have originated some distance from the 
axil of the leaf as shown in Plate XIII, Figures i and 2. In these figures 
it will be seen that the gall has been formed by the production of a 
large number of parenchyma cells from the parenchyma of the leaf, 
and by the vascular bundles which have arisen from the leaf-trace 
bundle. Examination of serial sections of such a gall precludes the 
possibility of the existence of any separate vascular bundle in the leaf 
from which the gall bundles might have arisen. In cases where the 
gall lies at or near the base of the leaf and from external appearances 
might possibly be axial in nature, serial sections show no vascular 
supply derived from the stele except the normal small leaf trace or its 
