114 HEART-ROT 



rub them off on to other roots which they pass. As roots 

 are often broken by rabbits while making their burrows, it 

 appears likely that such spores may occasionally come in 

 contact with wounds by which they can infect the trees. 

 Hartig attached little importance to this mode of infection, 

 as he found a very low percentage of germination in the 

 spores which he examined, but Brefeld found subsequently 

 that, when ripe, the spores germinate almost invariably. 



(ii) Infection by means of healthy roots coming into confact 

 ivith diseased roots. This might happen when a healthy 

 root grows so as to meet a diseased root, or two roots might 

 be touching and the disease might spread down one and 

 cross to the other. In order to discuss this theory it is 

 essential to know whether a root can become infected whilst 

 still undamaged. Unfortunately, Hartig's experiments leave 

 this question unsolved. To demonstrate the possibility of 

 such infection he placed diseased roots in contact with living 

 roots after removing tJie outer bark scales from parts of the 

 latter. It is not clear whether by this treatment the living 

 tissues of the roots were exposed, i. e. whether the roots 

 were wounded or not. In my own experiments, infection 

 did not take place unless the roots were definitely wounded, 

 and it may be regarded as extremely unlikely that living 

 roots can be infected except through wounds. If this is 

 so, this theory of infection demands not only that a diseased 

 root should be in contact with a living root, but that it 

 should actually touch a wounded portion of a living root. 

 This must greatly reduce the frequency of opportunities 

 for such infection, and it no longer appears reasonable to 

 account for the infection of groups of trees, which un- 

 doubtedly takes place, by this means. The following theory, 

 though difficult to demonstrate in the woods, seems to have 

 a wider application. 



(iii) Infection through dead roots. My infection experi- 

 ments have shown that trees can become infected when 

 attached dead roots are brought into contact with diseased 

 roots or soil infected with the fungus. These experiments 

 may, however, be criticized on the ground that natural 



