72 



sap and growth is extremely liable to 

 fermentation and decay. 



It is a dangerous experiment to cover 

 up the roots of trees. Their chief duty 

 appears to be to absorb moisture in the 

 soil ; but atmospheric aeration is neces- 

 sary to them, and under the eternal agency 

 of physical causes, acting probably on the 

 peculiar structure of their cellular organi- 

 sation, the roots of each tree otow at the 

 level best adapted to thenijand to the offices 

 which they have to perform. This should 

 not be interfered with. If dressing is laid 

 on the roots, it should not be deep, or of 

 a nature impermeable to air. Trees which 

 have had their roots deeply covered up 

 languish and die, unless they throw out a 

 new set of roots above the old ones which 

 have been smothered ; but, generally, the 

 new tier of roots is not strong enough to 

 supply the exhaustion of the old head. 



Liebig tells us that the presence of 

 oxygen. — consequently of atmospheric 



