X.] "CANKER": THE LARCH DISEASE. 241 



characterize the larch-disease in its early stages, can 

 be artificially produced by exposing the stem to 

 temperatures below zero, so as to freeze the water 

 in the cells. 



Given the above conditions for producing frost- 

 wounds, then, and the presence of spores of Peziza 

 Willkommii, there is no difficulty in explaining the 

 well-known phenomena of the larch-disease. 



But Hartig has brought to light some other facts 

 of great importance in considering this admittedly 

 complex question. We have already stated that 

 the Peziza does occur at the margins of the wounds 

 in trees growing in the Alps where the larch is 

 native. In these higher regions, however, the air 

 is usually relatively dry during periods of active 

 growth, and the young fructifications of the fungus 

 are particularly sensitive to drought ; consequently 

 even when many scattered trees are affected, the 

 cups developed at the edges of the wounds are apt 

 either to dry up altogether, or to produce relatively 

 few spores, and these spores have fewer chances of 

 germinating. In fact, the fungus enjoys at best a 

 sporadic existence, chiefly at the bases of trees where 

 the herbage ensures a certain degree of dampness. 



When the larch was brought down to the plains 

 and valleys, however, and planted in all directions 



