1922] HYDE—GALL ON POPULUS IQI 
It is apparent from these figures that the diameter of the normal 
stem is composed of 10 per cent bark and go per cent wood, while 
in the diseased twig 25 per cent is bark and only 75 per cent is wood. 
The section through the gall has increased 110 per cent, the wood 
55 per cent, and the bark 600 per cent over the same tissues in the 
stem just below the gall. 
Oricin.—The hypertrophy first makes its appearance on the 
twig in the form of a slight swelling, at or about the time of renewed 
cambial activity in the spring, which for the region of western Mon- 
tana is about the first of May. This original swelling is brought 
about by the increase in number and also the increase in size of the 
cells of the recent phloem and xylem as secondary growth takes 
place. It would appear evident, therefore, that this increase in size 
and number of the cells is brought about by the stimulating effect 
produced on the cambium by the presence of the fungus. To say 
that this is due to enzymatic action is only a conjecture, but this is 
the most plausible explanation. 
In the majority of knots sectioned the distortion reached to 
the pith, at least in some portion of the stem, which indicates that 
infection took place in the infancy of the twig, as xylem once formed 
in the region surrounding the pith ordinarily is not subsequently 
distorted. This, coupled with the fact that the hypertrophy is 
almost always formed where the twig branches, indicates that the 
pathogene gains entrance to the host in the region of, and during 
the formative period of the lateral buds. 
The galls evidently arise as the result of the original infection. 
Frequently when a young lateral twig becomes infected its growth 
is stunted distad to the infection, and as a result numerous short 
lateral twigs are present that simulate spurs. Although these spurs 
may be only a few millimeters in length, they usually show several 
years growth, as evidenced by the number of terminal bud scars 
present. These spurs usually protrude from the branch at a dis- 
tinct right angle, while the normal twig protrudes at an angle of 
approximately 30° (figs. 5-7). This increased angle is brought 
about mechanically, the gall forming in the axil exerting a pressure 
that forces the spur downward. 
XyteM.—In looking at a transverse section of diseased wood such 
as is shown in fig. 2, one is impressed by the enormous broadening 
