232 Transactions British Mycological Society. 
well defined by a rather deep crack and sometimes the forma- 
tion of a callus round the healthy regions is noticeable during 
the summer. PI. IX, fig. 1, represents a scar as it occurs in the 
middle of a branch although usually a scar of this sort is asso- 
ciated with a dead truss. Pl. IX, figs. 2, 3 and 4, are of a branch 
which became infected at the top, the disease progressing down- 
wards. One occasionally finds scars which have become com- 
pletely healed over by the growth of wound wood formed by 
a cambium at the edge of the scar (Pl. IX, fig. 5). Normally, 
however, this is prevented by the growth of the fungus. 
The dead tissue is usually found covered with innumerable 
small fructifications scattered over its surface. Under normal 
conditions only the openings of the fructifications can be seen, 
but after a damp foggy night the spores accumulate at the 
openings and can be recognised as small white points (Pl. IX, 
fig. 6). After a shower of rain the spores are washed away and 
are no longer visible to the naked eye. 
A rather unusual feature of this disease is the seasonal activity 
of the fungus. At only one period of the year do the scars 
increase, usually towards the end of summer, but this varies 
according to the season. In 1920, at Long Ashton, they started 
growth in October. In 1921, a much earlier ripening year than 
1920, they started in August; whilst in 1922—a late season— 
the very first signs of the extension of the scars were visible on 
July 19th, but it was not until September that the fungus was 
really active. After a comparatively short period during which 
time the scars extend rapidly, the fungus becomes quiescent 
and remains so until the following year when it once more 
bursts into new growth. The interesting point is that although 
the fungus is always present, and presumably ready to grow, 
yet it is only at one period during the annual cycle that it can 
do so. The cause of this is obscure, but one cannot help thinking 
that the physiological condition of the tree, or perhaps ‘of the 
fungus, or possibly of both, alters during the time when growth 
of the fungus is just taking place. The first sign of any activity 
on the part of the parasite is the appearance of faint cracks in 
the bark some little distance below a scar (PI. IX, fig. 4). These 
cracks, which at first may be entirely dissociated from any 
previous crack, become more distinct in a few days and finally 
link up to form a well-defined line of demarcation at the edge 
of the bark canker, between the healthy and what subsequently 
becomes diseased tissue. The tissue within the crack browns and 
dies off, producing, after some little delay, the usual fructifica- 
tions. The rapidity with which the fungus advances, once it 
has started into activity, is rather striking. With the canker 
produced by Nectria galligena and Monilia cinerea, the host 
