20 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



spring saves the larches from complete or excessive defoliation, 

 which, especially when often repeated, results in the crippling 

 and death of the trees. The Larch Aphis also, Ghermes laricis, 

 damages the foliage of the larch to no small extent, though not 

 nearly so much as the moth. The disease which is induced by 

 P. Willkommii differs entirely from the crippling which larches 

 experience as a result of the attack of the moth, aphis, etc. 

 This parasite is indigenous to high Alpine regions, where it 



produces the same disease that 

 has resulted in the destruction 

 of innumerable woods in Ger- 

 many, Denmark, and Scotland. 

 In its native habitat, however, 

 it is only under special condi- 

 tions of environment that it 

 destroys whole woods. In 

 order correctly to appreciate 

 this point, we must first review 

 the course of development of 

 the parasite. 



The spores — which originate 

 in cup-shaped fructifications to 

 be afterwards described — soon 

 germinate in the presence of 

 sufficient moisture, with effect 

 not on an uninjured tree, how- 

 ever, but only on a wound. 

 Such wounds are very often due 

 to hailstones, or to the dwarf- 

 shoots being devoured in spring 

 — as was mentioned above — or 

 they are formed in the upper 

 angle of the base of a branch 

 (Fig. I, b) owing to its depres- 

 sion under accumulations of snow or hoar-frost. From such 

 wounds the vigorous, copiously ramifying, septate mycelium 

 spreads in the soft bast, partly between and partly in the cells 

 advancing in the sieve-tubes, and killing and browning the tissues. 

 The mycelium also grows into the wood, and even penetrates as 

 far as the medulla. 



That portion of the corticle tissues which has been killed 



Fig. 1. — A canker-spot that has 

 been recently formed in the upper 

 portion of the stem of an eight- 

 year-old larch from the Tyrol. 

 Infection has occurred above the 

 branch, b, where a crack has been 

 formed in the tissues, ow^ng to the 

 branch having been depressed 

 under a load of snow. Numerous 

 immature ascophores, c, have 

 already formed on the dead 

 cortex. 



