PATHOLOGIC CONDITIONS IN AMBROSIA TRIFIDA 
39 
there are sometimes callus-like masses, but whether these are due to the 
wound stimulus or to the gall stimulus can not be determined. Irregularly 
shaped cavities sometimes form in the bark and in other parenchyma 
portions, evidently due to a breaking down of cells. Somewhat similar 
cavities also occur in uninfected stems. 
A diagrammatic drawing of a tangential section of one of the galls is 
shown in figure 3. This section shows about average conditions; greater 
tissue disturbances occur in some galls, in others less. By comparing this 
figure with that of the normal stem (fig. i) it is seen that the most striking 
changes brought about in the gall are the broadening of the rays and a re- 
duction in the width of the bands of fibers and other elements between the 
rays. There is also a vertical shortening of some of the rays, but this is 
not especially pronounced. The cells which compose the rays are in general 
^ more nearly isodiametric than those in the normal rays, many of whose 
cells are lengthened vertically. The vessels pursue a more irregular course 
than in the normal stem, and the vessel segments are much shorter. A 
sharp turning of the fibers from their usual upright course, which is pro- 
nounced in the traumatic wood of this species (fig. 12), occurs but seldom. 
There are, however, occasional bundle ellipses (fig. 9), which in their more 
complex form consist of several vessel segments so bent as to form a more or 
less oval or elliptical body enclosing short tracheids. It is probably true 
that the larger segments are more nearly like tracheids, as distinct mem- 
branes often separate one segment from another. Some of the segments in 
figure 9 show this condition. Bundle ellipses do occur which are much 
simpler in structure than the one figured. These sometimes consist of as 
few as three segments which do not enclose still other cells at the center. 
Irregularly shaped masses of short segmented vessels also occur variously 
wound and twisted together. 
One gall was found which had been severely wounded. A longitudinal 
slit had been formed in it extending to within a short distance of the 
pith. On each side of the wound, parallel with its edges, there are strands of 
fibers, parenchyma, and short segmented vessels. These have been turned 
over at right angles to their usual position and extend outward in a radial 
direction. A small cavity at the inner end of the wound has become sur- 
rounded by callus tissue, and a similar modification has taken place along 
the edges of the wound in the outer bark. Tangential sections through this 
wounded gall show broad ray-like masses of parenchyma with suggestions of 
fiber inclusions similar in some respects to the condition found in traumatic 
wood (fig. 13). Resting as these observations do on a single specimen, it 
is hardly safe to draw any general conclusions from them. It looks, however, 
as though the wound stimulus were able in a way to overcome the gall 
stimulus and to imprint its effect on the tissue formed after wounding, or 
at least to modify to a certain extent the effects of the gall stimulus. 
