160 ARMILLARIA MELLEA, 



required in devising experiments to elucidate the problems 

 connected with it which require solution. But there are 

 some general considerations which may lead to the adoption 

 of a reasonable view, in keeping with the present state of 

 our knowledge. 



No conidial form of reproduction occurs anywhere in the 

 life-history of Armillaria, so that the fungus is confined to 

 two methods of attack by spores and by rhizomorphs. In 

 my opinion the role of these two organs is quite distinct, 

 as will be shown by the following analysis. In the discussion 

 the fungus will be regarded as an enemy whose sole object 

 is to attack living trees. Actually, this is not quite true to 

 fact, as no doubt the fungus is just as happy living sapro- 

 phytically on dead stumps as when sapping the life of 

 growing trees ; but if it confined its attention to a saprophytic 

 existence, we should have no quarrel with it, and it is only 

 in its career of depredation that we, as pathologists, are 

 interested in it. 



Infection by spores has never, to my knowledge, been 

 experimentally demonstrated, but it is safe to formulate 

 the following postulates with respect to spore-infection : 



(i) Spores are incapable of infecting a tree through 



healthy bark of root or stem, 

 (ii) Infection is not known to take place through stems 



or roots which are so young as not to have 



developed a bark, 

 (iii) Consequently infection can only be effected through 



wounds or dead roots or dead stems. 



Further, dead branches are not a suitable medium for 

 infection by a root-attacking fungus like Armillaria, pro- 

 bably because the surface is not kept sufficiently damp for 

 the germination of the spores. And dead roots can only 

 very occasionally become exposed to the air-borne spores. 

 Thus it is fairly safe to assume that spores can only attack 

 a living tree through wounds in the rootstock just at the 

 surface of the soil. Owing to mice and other rodents such 

 wounds are not uncommon, but, as they soon become 



