PATHOLOGIC CONDITIONS IN AMBROSIA TRIFIDA 
35 
tions are similar both morphologically and histologically so far as has been 
observed by the writer. 
Before entering into the description and consideration of the gall tissue, 
it seems well to describe briefly the structure of the normal stem. In the 
cross section of the gall shown in plate II, figure 8, the fungus has confined 
its activities to one side of the stem, shown on the upper side of the figure, 
so that only a sector is involved in gall formation. The bundles on the 
sides and lower part of the figure are entirely normal. In general there 
are two sorts of bundles in the normal stem: those in which the xylem is 
broadly triangular or somewhat aquiliform in cross section, and those in 
which the xylem forms rather narrow radial segments. The narrow bundles 
are evidently leaf traces, since six of them leave the stem at each node, and 
according to Sinnott (5) the trilacunar condition is characteristic of many 
members of the Compositae. The leaf trace bundles are formed two nodes 
below the point where they leave the stem. There seems to be some dif- 
ference in the structure of the narrow bundles in the young stem, of which 
two kinds can be distinguished; but as the normal anatomy is to be con- 
sidered very briefly, it is beyond the scope of this article to enter into a 
general investigation of the course of the bundles in the stem. The cam- 
bium dips in opposite the narrow bundles (fig. 8), resulting in somewhat 
depressed segments of xylem. It has on this account a somewhat sinuous 
course around the xylem ring. The broader bundles usually alternate 
with the narrow ones, but this is not always true, for sometimes two of the 
broad bundles occur together. Sometimes a ray extends through the 
center of a broad bundle, cutting it in two. There is often a slight depres- 
sion in the center of a broad bundle, causing a notch. A bundle of bast 
fibers is located opposite this notch, shown by the blackened areas in figure 
8. The greater part of the phloem is located just inside the bast fibers. 
The leaf trace bundles are subtended by somewhat larger bundles of bast 
fibers. 
In the young stem the individual bundles are separated from each other 
by broad rays which extend from node to node. As the stems grow older 
and the individual bundles approach each other, the rays become narrower, 
and shorter vertically. Intangential section (fig. i), the rays are fusiform 
and are usually three or four cells wide. They are composed of somewhat 
angular cells which are elongated in a vertical direction. Uniseriate rays 
are uncommon. With the exception of the ray cells and vessels the xylem 
is composed mostly of wood fibers. On the outside of the bast fibers the 
remainder of the bark is composed of several layers of cortical parenchyma 
cells which are elongated tangentially. There are also several layers of 
colenchyma cells, just inside the epidermis, which become thickened oppo- 
site the trichomes. 
A diagram of a cross section of an infected stem is shown in figure 8. 
The bundles on the upper side of the stem have become greatly altered 
