2 y6 DISEASES OF TREES 



superficial layers of the soil, partly by the processes of diffusion, 

 and partly by the entrance of water containing dissolved oxygen. 

 The greater the daily and annual variations of temperature in the 

 upper layers of the soil, and the greater the depth at which they 

 operate, so much the more thorough is the interchange of gases, 

 or, as it is sometimes called, the " respiration " of the soil. As 

 is well known, the temperature of the soil depends, in a great 

 measure, on its specific heat. The lower the specific heat, so 

 much the more quickly is the soil heated or cooled. Water and 

 humus possess a high specific heat, and the more of these sub- 

 stances a soil contains the greater will be the quantity of heat 

 required in order to raise its temperature. A forest soil that is 

 unprotected by umbrage, that is easily dried owing to exposure, 

 and that has lost the greater part of its humus, is much more 

 easily warmed than a soil that is protected by a dense wood, 

 is constantly moist, and contains abundance of humus. 



It is further evident that a forest soil which is exposed to 

 direct insolation is much more easily warmed, though it also cools 

 much more easily owing to radiation of heat, than one where 

 the crowns of the trees and a covering of leaves and humus 

 afford a double protection. 



So far as the diffusion of air in the soil is concerned, we know 

 that it only occurs to a considerable extent in porous soil 

 which is not over-wet. In the case of dense, firm wet soil the 

 mixing of gases proceeds with extreme slowness. It may 

 happen, under certain circumstances, that the interchange of 

 gases in the soil is so limited as to induce asphyxia and decay 

 in the roots of plants. I have applied the term " Root-Rot " to 

 cases where the roots die by asphyxia, in contradistinction to 

 infectious root-diseases. 



ROOT-ROT 1 



This disease is specially destructive in the young Scotch pine 

 woods of the north of Germany. It seldom appears before the 

 twentieth year, usually not till the thirtieth, and is characterized 

 by the trees appearing unhealthy for a short time and then 

 falling over while still perfectly green, after snow or a strong 

 gale has supplied an external impulse. The tap-root will be 

 1 Op. cit.j pp. 74 et seq. 



