PATHOLOGIC CONDITIONS IN AMBROSIA TRIFIDA 
45 
they are much broader, and the intervening strips of fibers and other 
elements are narrower than in normal tissue. There is, furthermore, but 
little vertical shortening of the rays. In traumatic wood, on the other 
hand, there is a marked vertical shortening of the rays, a feature common 
to the traumatic wood of most species of angiosperms. The insect gall 
shows but little alteration in ray structure. 
The number and size of the vessels is also greatly influenced by the 
different stimuli in question. The reduction in number is very marked in 
both traumatic wood and Protomyces gall, but in the insect gall there is 
very little change in this respect. There is a noticeable shortening of the 
vessel-segments in all the types of pathologic tissue under consideration. 
As regards the effect of the different stimuli acting together, the insect 
is able to exert an influence on the growth in all parts of the stem which are 
in close proximity to the insect. The fungus, on the other hand, is able 
only to influence growth where the fungus is actually present in the tissue. 
The insect is therefore able to exert an influence much farther away from 
the source of the stimulus than is the fungus. When both parasites occupy 
a portion of the stem at the same time, the insect is able to influence the 
growth on all sides of the stem from the start, while the stimulus from the 
fungus is limited owing to the relatively slow spread of the fungus through 
the tissues. In places where the application of the two stimuli evidently 
began at about the same time the resulting tissue is the same in structure 
as that which occurs in the Protomyces gall, the stimulus from the insect 
being apparently inactive under such conditions. In portions free from 
the fungus the changes are the same as are usually found in the insect gall. 
If the fungus extends into such tissue, however, there is an abrupt change 
in the tissue formed afterwards, this tissue having the same structure as in 
the Protomyces gall. From these facts we may conclude that the stimulus 
from the fungus is much the stronger of the two and is able to overcome or 
neutralize the stimulus from the insect. 
Observations on wound and fungus stimuli acting together rest on too 
few observations to warrant any definite conclusions. It is likely, however, 
that the- wound stimulus is more powerful than the gall stimulus; this at 
least is suggested by certain modifications in the wounded gall examined. 
This conclusion is further substantiated by the fact that in general the gall 
stimulus is not effective for any great distance away from the source of the 
stimulus, while in the case of the wound stimulus tissue modifications take 
place at considerable distances from the wound. 
The most marked similarities between the structure of the fungus gall 
and that of traumatic wood are in the presence of bundle ellipses and in 
the reduction in the number and size of the vessels. Similarities between 
the insect gall and traumatic wood consist in a greater radial growth of 
xylem, resulting in a more distinctly woody growth than takes place nor- 
mally. 
