114 HEART-EOT 



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

 are often broken by rabbits while mailing their burrows, it 

 appears Ukely that such spores may occasionally come in 

 contact with wounds by which they can infect the trees. 

 Hartig attached Uttle importance to this mode of infection, 

 as lie found a very low percentage of germination in the 

 spores whicli lie examined, l>ut Brefeld found subsequently 

 tliat, when ripe, the spores germinate^ almost invariably. 



(ii) Infection hy means of healthy roots coming into contact 

 inith diseased roots. Tliis might lia]ipen when a healthy 

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

 be touching and the disease might spread down one and 

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

 essential to know whether a root can become infected whilst 

 still undamaged. Unfortunately, Hartig's experiments leave 

 tliis question unsolved. To demonstrate tlu^ possil)iHty of 

 such infection he placed diseased roots in contact with living 

 I'oots after removing the outer hark scales from paiis of the 

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

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

 were wounded or not. Tn 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 r/. n^ounded portion of a living root. 

 This must greatly reduce the frequency of <)j)j)oitunities 

 for such infection, and it no longer appears reasonable to 

 account for the infection of groups of trees. M'hich un- 

 doubtedly takes place, by this means. The folhnving 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 d(^ad roots are brought into contact with diseased 

 I'oots or soil infected with the fungus. These expeiiments 

 may, howevei', be criticized on the ground that natural 



