HEART-ROT 111 
The effect of the addition of bacteria is clearly shown in 
fig. 45. It is thus proved that the unsterilized soil contains 
some factor which is very strongly inhibitive to the growth 
of the mycelium of Fomes annosus, and this factor appears 
to be bacterial. 
2. It is evident that chemical changes are induced by 
boiling humus soils containing roots and other plant remains 
which have not been completely disorganized. When twigs 
and wood blocks are autoclaved in test-tubes, a brown stain 
nearly always runs down on to the cotton-wool at the 
bottom of the test-tube. When infected with Fomes annosus 
the latter grows down on to the cotton-wool and flourishes 
on the stained portion. No doubt a similar liquid escapes 
from roots into the soil containing them when the whole is 
boiled, and this probably assists the growth of fungus 
mycelium. (This, however, cannot apply to such arable 
soils as f and g, which contained no organized remains.) 
There is also a physical change, for soil which is autoclaved 
becomes very evenly moistened, whereas soils which have 
been dry sterilized and subsequently wetted are very 
difficult to moisten evenly, and this may partially account 
for the poor growth of mycelium on dry-sterilized soils. 
From the foregoing experiments it is clear that the 
mycelium of Fomes annosus will grow on soils under certain 
conditions. One condition is the absence of a certain 
bacterium which is so frequently present in the soil as to 
inhibit the mycelial growth in all my cultures on unsterilized 
soils. These experiments were all performed in the summer, 
when bacterial activity is at its greatest. It is possible that 
in the spring, when bacterial growth is less active, more 
auspicious conditions for Fomes mycelium may obtain. 
Infection experiments. A series of infections was made 
on two-year-old potted plants of larch and spruce. 
These were designed to answer the following questions : 
1. Can unwounded roots become infected with the 
fungus ? 
2. Can infection take place through slight wounds ? 
3. Can infection take place through dead roots ? 
