82 General Care of Trees 
with rubble or broken stone before putting down earth and 
sod, keeping the space around the bole open and in addition 
introducing through the filled ground aérating pipes which 
connect the air chamber with the outer air. 
In any case, trees so treated should be at the same time 
severely pruned, since some of the feeding roots are bound 
to die and it is necessary to establish an equilibrium between 
root system and crown system. Otherwise nature will do 
the pruning and perhaps not in the manner desired. 
Different species vary in their sensitiveness to the with- 
drawal of air and, as we have seen before, trees which-have 
grown up in swampy conditions make provision in other 
ways to meet their requirements for air, and may therefore 
be covered to a considerable extent with impunity. In 
general it will be found that the trees naturally growing in the 
swamp will stand filling best. 
The depth and character of the soil used for cover will 
naturally also have to be considered, since with a loose soil 
and a moderate depth the respiration may not be materially 
interfered with, while a thinner layer of clay would be 
detrimental. 
In most cases, however, the safer way is either to avoid 
the grading or to replace with new trees. 
Whenever it becomes necessary in grading, making of 
streets, laying of conduits, etc., to cut into and destroy part 
of the root system, a corresponding amount of the crown 
system should be removed by proper pruning, keeping in 
mind the fact that the side on which the roots are destroyed 
is the one that needs to be most severely pruned. 
The aération and watering of street trees is a most difficult 
question, especially where macadam or other solid pavement 
is used. It is, therefore, necessary in the first place to choose 
species which are frugal in that respect. Here again, species 
