HEART-ROT 113 



and in these it was found that the hyphae had penetrated to 

 the stem. 



Thus question 1 can be answered in the negative, ques- 

 tions 2, 3, and 4 in the affirmative. Infection does not take 

 place through un wounded roots, at any rate under the 

 circumstances of the experiments (too much confidence 

 must not be placed in a negative result), but can take place 

 through a living wounded root or through a dead root. 



Mode of attack in nature. The habit of the fungus leaves 

 no doubt as to the part of the tree which is first attacked. 

 It is always the roots which first become infected, and in 

 every case that has been recorded the disease has spread 

 from the roots to the stem, and not vice versa. Infection 

 then, must be subterranean, or immediately on the surface 

 of the soil. 



Hartig (1878), who at that time was unaware of the 

 existence of conidia, recognized two possible means of 

 infection. One was by the spores of the fungus reaching 

 the roots of healthy trees, and the other by living roots 

 coming into contact with roots which contained the fungus. 

 These two methods will be discussed in turn, and as they 

 do not seem adequate to account for all the infections which 

 occur in nature, a third method will be suggested. 



(i) Infection by means of spores. The fructifications of 

 the fungus are usually subterranean, or if they grow above 

 ground they bear their spores so near to the soil that wind 

 can play but a small part in the dispersal of the spores. 

 Often fructifications which begin to develop subaerially 

 become covered by fallen leaves and shoots of ivy, but this 

 seems rather to encourage than retard their development. 

 Thus it is presumed that spores are carried by other means 

 than air currents, and, as Hartig suggested, rabbits, voles, 

 and other burrowing animals are probably the chief agency 

 in dispersal. The fructifications require some open space 

 for development, and burrows, made by animals under the 

 trees, afford the kind of station in which they flourish. So 

 there is reason to think that such animals catch the spores 

 in their fur or pick them up on their feet, and they may 



1888 



