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 ; CD 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 GO and DH. Fig. 12, E, 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 Munch (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. 



D 2 



