44 
ALBAN STEWART 
a stem normally herbaceous. The vessels are smaller than in the normal 
stem and are reduced in number. With the exception of occasional broad 
ray-like masses of parenchyma, the rays are but little broader as a whole 
than they are in the normal stem. In tangential section the rays are short- 
ened vertically. As a whole there is less ray tissue per unit of area than in 
the normal stem (fig. 12). There are occasional broad ray-like masses of 
parenchyma (shown by the heavy lines in the ray, fig. 13), which call to 
mind very strikingly the similar condition that has been described in the 
rays of some of the lower angiosperms. The vessel segments are much 
shortened (compare figs, i and 12). The only features worthy of special 
mention found in the stem which had been twisted are the bundle ellipses, 
one of which is shown in figure 10. They are very similar to the bundle 
ellipses in the Protomyces gall (fig. 9). 
General Summary 
After having considered the structural changes brought about in the 
Ambrosia stem through the action of the different stimuli treated in this 
article, it seems desirable to give a brief comparison and summary of the 
results. One of the more marked features of pathologic plant tissue is the 
change in general direction of the cells, and in the cases being considered 
this is more marked in the truamatic wood. An interesting thing in this 
connection is the fact that the changes brought about by wounding are 
about the same as those which occur in traumatic tissue of the lower angio- 
sperms. An examination of the literature has revealed practically nothing 
concerning the structure of wound tissue in the Compositae. A great 
misplacement of xylem cells takes places, as a result of which they are often 
90° or more from their normal position. Gnarl-formations and similar 
structures result from such misplacements, the bundle ellipses being a 
marked feature in this respect. With few exceptions the cells in the Pro- 
tomyces gall occupy more nearly their normal positions and there is but 
little of the violent misplacement so common in traumatic wood. There 
are, however, occasional gnarl-like arrangements of cells, best illustrated 
by the bundle ellipses which are very similar in structure to those of trau- 
matic wood. The insect gall shows no marked misplacement of cells. 
Another characteristic feature is the production of parenchyma at the 
expense of the xylem, a feature which is more marked in the Protomyces 
gall than in any other. In extreme cases considerable portions of the xylem- 
may be converted into parenchyma. Broad ray-like masses of parenchyma 
are a constant feature in certain types of traumatic wood. Intermixed 
with the ray tissue there are often fibers singly or in groups. In the insect 
gall the production of additional parenchyma tissue is chiefly confined to the 
proliferation of the ray cells into the gall cavity. 
The rays show the greatest alteration in the Protomyces gall, where 
