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 contact 
with 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 the outer bark scales from parts of the 
latter. It is not clear whether by this treatment the living 
tissucs 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 
