THE LARCH CANKER 35 
the first year ; the formation of the phloem outside aB has 
kept the mycelium away from the cambium, though the 
mycelium has spread, especially in a tangential direction. 
Fig. 12, c, shows the position in the following spring. The 
section CD of the cambium has been killed by the inward 
growth of the fungus, which has also come sufficiently near 
the segments EC, DF to affect them in the same way as AB 
had been affected a year earlier. By the next spring the 
canker has grown to the stage seen in fig. 12,D; cop has 
made no wood, EC, DF have made abnormal wood similar 
to aB, and during the winter the mycelium has killed the 
further stretches of cambium Gc and pH. Fig. 12, £, shows 
the canker after developing one more year on the same 
system. It is thus seen how the amphitheatre-like canker 
is formed, and also how the abnormal wood is related to it. 
The step-like configuration of the sides of the amphi- 
theatre clearly shows that the fungus makes greater inroads 
on the host in the winter than in the summer months. This 
can be accounted for most simply by the winter cessation 
of growth on the part of the tree, and it is quite unnecessary 
to assume with Hartig that the fungus grows more actively 
in winter than in summer. Hartig suggested that in the 
summer the tree tissues contain a larger percentage of air 
than during the winter (since in summer the soil and air 
are drier and the transpiration pull is greater), and that 
the reduction in water content inhibited the mycelial 
development of the fungus. But Miinch (see p. 26) has 
shown that a high air-content stimulates, rather than retards, 
the growth of the fungus, so that Hartig’s explanation 
is not in keeping with the experimental evidence. Further, 
Miinch’s discovery that the lower limit of temperature for 
fungal growth is above freezing-point precludes the possi- 
bility of fungal development during an Alpine winter. It 
is sufficient for our purpose to observe that during the 
winter the dormant cambium is more exposed to attack 
by the hyphal secretions, whereas during the summer it is 
protected by a constantly thickening layer of new and 
active phloem. 
D2 
