HEART-ROT 121 
operating which might account for the death of some of the 
roots before they became attacked by the fungus? Also 
what is the frequency of heart-rot in conifers planted on 
heaths ? Where the soil is peaty the aeration of the sub- 
soil must be very poor, and heart-rot might be expected. 
I have had no opportunity of investigating these points, 
and foresters’ notes on the subject would be of great value. 
Extreme caution is necessary when attempting to prophesy 
the effect of different types of subsoil on the frequency of 
heart-rot. In one part of Terringham Wood the subsoil is 
a hard strong clay, which must be far inferior in porosity 
to that in other parts of the wood. And yet in this portion 
several of the larch trees were sound. I thought that 
this must be a case in which the theory failed, until 1 
had one of the trees grubbed up and found that the 
roots were all superficial. The subsoil had proved too 
hard for them, and consequently the tree had been saved 
from infection. 
Two points of interest are involved in this theory of the 
method of infection. First, the roots which grow vertically 
downwards are not those which are most likely to come in 
contact with the diseased roots of other trees, so that 
Hartig’s contact theory can hardly apply. It seems much 
more probable that the roots become infected either directly 
by spores and conidia or through the soil being penetrated 
by mycelium. The other point is the depth of the soil at 
which infection takes place. When a root is killed because 
its lower extremities lack sufficient oxygen, it does not 
necessarily follow that only those lower extremities die. 
Unless the upper part of the root has sufficient vigour to 
send out branch roots into the upper layer of soil (and I have 
never noticed such branches in roots with primary rot), 
the upper part will cease to receive the full supply of water 
and nutriment, and will probably die. In one case I found 
‘a layer of resin across the wood of a vertical root with 
primary rot at a point less than a foot below the surface of 
the soil, in fact quite near the rootstock, and this probably 
represented the upper limit of death before infection. 
