14 
ALBAN STEWART 
is shown in text-figure ih. This gall is probably seven years old, 
as there are that many faintly marked rings of growth in it which 
correspond with the number of rings in the normal part on the stem 
just below it. Cross sections of other galls, which were examined 
microscopically, also show a correspondence in a number of rings 
both in the gall and in the stem close by. One side of the stem where 
the gall is located (see text-figure ih) has not as yet become changed 
at the center, showing that the fungus may spread very slowly in a 
peripheral direction. The two sides of the stem in this figure show a 
difference in time of infection, as the wood on the right side near the 
center is abnormal nearly to the pith, while on the left side there are 
nearly two rings of normal wood next the pith. The fungus evidently 
spreads quite as slowly vertically as it does horizontally. The lighter 
portion in the center of the gall (text-figure ih) is composed almost 
entirely of normal wood. These zones become broader towards the 
upper and lower sides so that the gall encloses two more or less cone-like 
masses of normal wood the apices of which meet near the center. 
Furthermore it can be seen in this figure that the bark^ is not appre- 
ciably thicker than the bark of the normal stem above and below. The 
outer portions of this are shed rapidly by the formation of periderm 
so that nothing remains but the inner phloem and rays. The phloem 
portion is composed mostly of large cells containing resin and other 
substances. According to Hartmann (9, p. 32) the bark may become 
more than three times as thick as normal on branches of the white 
fir infected with Peridermium elatinum; according to de Bary (4, 
p. 258) however, it may be as much as ten times as thick as normal. 
The first striking abnormality to appear in going from the normal 
wood to that of the gill is a sudden bending outward of the tracheids. 
The pitting in the walls of these differs rather strikingly from normal 
in places. According to Penhallow (13, p. 321) the bordered pits in 
the radial walls of the tracheids in this species of pine are arranged 
in one row, uniseriate, sometimes in pairs or distinctly two rowed. 
Specimens of normal wood examined show that the biseriate arrange- 
ment rarely occurs. In the wood of the gall the size and arrangement 
of the pits may differ greatly from normal. The usual arrangement 
is uniseriate, but two rows of pits often occur and rarely three rows. 
Where the arrangement is biseriate the pits are usually opposite, 
but instances are common where there is a strong suggestion of alter- 
1 See Barnes, C. R. "What is Bark?" Bot. Gaz. 22: 237. 1896. 
