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. Ill 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 



