120 HEART-ROT - 
books. In this way a large volume of soil is partially dried 
by each root, and during a drought water is drawn from all 
the subsoil which is penetrated by the root system of the 
trees. The rigidity of the roots prevents a wholesale sub- 
sidence of the soil, and consequently air must be drawn in 
from the outside. When rain comes again, the water slowly 
percolates through the interstices of the soil and drives out 
the air, and the same cycle begins anew. This process, 
carried on through a number of years, has an important 
loosening effect on the soil, and the living conditions of the 
roots improve as the forest becomes older. This explains 
the fact which is well known to foresters that the subsoil 
in woods is more porous than in open ground. 
Sufficient evidence has now been advanced on which to 
base a theory accounting for the frequency of heart-rot in 
the first rotation of larch on land that has previously been 
cultivated. The argument for this theory may be given 
under three headings : 
1. The subsoil in arable or pasture land is very poorly 
aerated, and the tree roots which grow more or less vertically 
downwards into this soil not infrequently die from lack of 
oxygen. 
2. It is shown by experiments described on pp. 111 and 
113 that larch trees readily become infected through dead 
roots which are still attached to living trees. 
3. The pioneer roots which penetrate the subsoil, and 
later die from lack of aeration, form suitable infection 
points for heart-rotting fungi. Such dead roots are found 
especially in the first rotation of trees on new forest land. 
This theory may be accepted as a working hypothesis. 
Much experience will be needed to prove or disprove it, 
and the following points immediately present themselves 
for elucidation. First, it would be interesting to know 
whether plantations on arable or meadow land, in which 
the subsoil was for some reason well aerated before plant- 
ing, are less liable to heart-rot than new plantations on soil 
which is otherwise conditioned. Next, in old woods in 
which heart-rot has occurred are there any other causes 
