PATHOLOCV OF THE .JACK 1'LXK. 
brain — cerebroid. Those blisterlike swellings are orange-yellow at 
first; after the rupture of the peridinm and the dispersal of the 
golden yellow seciospores they become whitish. The gall formation 
causes great injury to the trunk and branches (fig. 1). The infection 
usually begins by means of some injury to the bark or cambial layer. 1 
The gall swellings gradually increase from year to year from the 
growth of a perennial mycelium, so that they finally encompass the 
entire branch, resulting eventually, if the galls are near the trunk, in 
its death below and 
above the hypertro- 
phy. Whether or 
not the entire branch 
dies depends upon 
the presence of lat- 
eral, knifed branches 
below the gall. 
In dry sandy areas 
Peridermium cere- 
brum confines itself 
more generally to the 
branches, occurring 
rarely on the trunk 
but frequently in the 
axils of the branches. 
This latter condition 
usually results in a 
combination trunk 
and branch gall, 
which in numerous 
instances produces 
greater damage than 
either of the other 
two types of galls. 
The branch and 
trunk are girdled by 
abnormal wood 
tissue and are thus 
weakened (fig. 2). This results usually in either the branch or the 
tree being blown down by the wind. Personal observations show 
that borers and wood-rotting fungi entering at the burl often hasten 
the decline of the tree. 
From a careful examination of young twigs showing very recent 
infections at leaf scales, leaf traces, and at the bases of young pistillate 
Fig. 2.— Cross sections of the main trunk of a jack pine heavily in- 
fected with Peridermium cerebrum. Note the progressive girdling 
by the resinous burl tissues in the upper figure and its effects on 
the increment of the trunk below, as shown in the lower figure. 
1 Wounds made by sapsuckers, ovipositors of bark-stinging insects, rodents, and ice and snow break:- 
are common means of entrance for Peridermium cerebrum. 
