DISEASES CAUSED BY WOUND FUNGI. 31 



After the fungus has once obtained a start in the wood of the trunk 

 it may encroach upon the sapwood, and after a time reach the newer 

 sapwood immediately under the bark, thereby causing the death of 

 that portion of the tree, and, when the entire diameter of the trunk i- 

 affected, of the whole tree itself. This is especially true of the aspen 

 and the striped maple. As this fungus occurs so generally upon living 

 trees, it has been thought that it would cease growing after the host 

 tree had died. From a large number of observations made by the 

 junior writer it has been found that this is not really the case, for 

 many trees have been found entirely dead amk leafless upon which 

 the fungus was still growing vigorously. This is true not only, with 

 one or two of the host species, but, so far as could he determined, 

 with all of them. In some cases a vigorously growing new layer was 

 found on the underside of sporophores growing on trees and stubs 

 which certainly must have been dead for a year or more. Standing 

 stumps were also found bearing living sporophores several years after 

 the death of the diseased tree. This finding that the sporophores 

 will continue to develop on dead wood is believed to be of consider- 

 able importance, because the dead wood coming from a tree diseased 

 by this fungus in which the mycelium is present must be considered 

 a possible source for further infection of healthy trees. 



INFLUENCE OF ENVIRONMENT ON PREVALENCE OF WHITE HEART-ROT. 



The possible factors which may influence the distribution and 

 growth of Fomes igniarius are many and varied in character. Among 

 them the following deserve consideration: Climate, surrounding for- 

 ests, character of the soil, species of host, age of host, rapidity of 

 growth of host, rate of healing of branch w r ounds, and presence of 

 wounds on host. 



The climate has very little to do with the distribution ami viru- 

 lence of this disease. It occurs in the humid forests of the eastern 

 United States as well as in the very dry regions of New Mexico and 

 Colorado; and it seems to flourish as well in semitropical and tropical 

 countries as in the colder regions of the north. 



The character of the surrounding forest seems to have absolutely 

 no influence upon the frequency of the disease, except as it does or 

 does not contain some of the host species in abundance. The fungus 

 seems to occur with equal frequency in pure stands and in mixed 

 forests. In other words, the aspen is found diseased just as fre- 

 quently in a mixed stand with other trees as in a pure stand of 

 aspen. 



The character of the soil has little or nothing to do with the occur- 

 rence of Fomes igniarius. The species of the host, on the other hand, 

 has a very considerable influence on the frequency of the occuro 

 79152— Bui. 149—09 3 



