2 ;8 DISEASES OF TREES 



sand, the processes of diffusion proceed but slowly. Although.it 

 may not be for some decades, this interference with the air- 

 circulation may ultimately induce asphyxiation of the deeper 

 roots, by preventing their obtaining a sufficient supply of 

 oxygen. 



Root- rot never occurs in dicotyledonous trees, and only with 

 extreme rarity in pines that are mixed with dicotyledons. 

 Possibly this may be explained by the fact that during half of 

 the year the soil is subjected to the minimum of umbrageous 

 shelter, and consequently there is more air-circulation than in a 

 wood composed entirely of conifers. 



This brings us to the immediate consideration of the best 

 means of prevention. These must always be directed towards 

 securing better aeration of the soil. The circumstances of any 

 particular case must determine which of the following courses 

 is to be taken : cultivating mixed woods of dicotyledons 

 and conifers, or, should this be impracticable, the pine may 

 be replaced by the shallow-rooted spruce ; the removal of 

 excessively large accumulations of leaves in hollows ; or the 

 abstraction from the soil of stagnant water by drainage. 



The death of the deeper roots on trees that have been 

 too deeply planted may to a certain extent be described as a 

 variety of root-rot. The heavier the soil, so much the more 

 dangerous is it to plant deeply. It is best that such a tree 

 should succumb at once, but in most cases it lingers through 

 several decades without being able to produce new roots to 

 replace those that have rotted off. Only a few trees, such as 

 willows, poplars, and especially shrubs, develop a plentiful supply 

 of adventitious roots immediately beneath the surface of the 

 ground, by means of which a new root system is formed, as in 

 the case of cuttings which are absolutely destitute of roots. 



Similar conditions are induced when the roots of older trees 

 are covered with a thick layer of earth, as often occurs during 

 the operations attending road-making, mining, &c. In such cases 

 less damage is done if the air can get at the roots from the side, 

 as usually happens when trees grow on sloping ground, but if 

 the entrance of air to the roots is rendered a matter of great 

 difficulty the trees either die off entirely, or at all events their 

 growth is seriously impaired. I found close beneath the surface 



