152 DISEASK IN I'L.WTS. 



here favouring the parasite and not improving 

 the chances of the host, even if they do not 

 directly disfavour it. 



As another instance I will take the Larch- 

 disease, which is due to the ravages of a Peziza 

 {Dasyscj'pha Willkouunii) the hyphae of which 

 obtain access by wounds to the sieve-tubes and 

 cambium of the stem, and gradually kill them over 

 a larger and larger area and so ring the tree, with 

 the symptoms of canker described below. 



Now the Larch fungus is also to be found on 

 trees in their Alpine home, but there it does very 

 little damage and never becomes epidemic except 

 in certain sheltered regions near lakes and in other 

 damp situations. How then are we to explain the 

 extensive ravages of the Larch disease over the 

 whole of Europe during the latter half of this 

 century ? The extensive planting, providing large 

 supplies for the fungus, does not suffice to explain 

 it, because there are large areas of pure Larch in 

 the Alps which do not suffer. 



In its mountain home the Larch loses its leaves 

 in September and remains quiescent through the 

 intensely cold winter, until May. Then come the 

 short spring and rapid passage to summer, and the 

 Larch buds open with remarkable celerity when 

 they do begin i.e. when the roots are thoroughly 

 awakened to activity. Hence the tender period 

 of young foliage is reduced to a minimum, and 

 any agencies which can only injure the young 

 leaves and shoots in the tender stage must do 

 their work in a few days, or the opportunity is 



