GRADES AND LEVELS 59 
while less is often the occasion of a severe 
set-back or general decline from which they 
recover very slowly and perhaps never com- 
pletely. This is because roots grow at the 
depth which insures them the right degree of 
moisture, of warmth, and of air, and any 
change in this depth seriously affects all three. 
The removal of even a small amount of earth 
allows the sun to bake them as well as to draw 
away the precious moisture from them. And 
of course it increases the air supply as well — 
dangerously. 
On the other hand, the addition of earth 
shuts this off, smothering them; and it upsets 
too, the moisture and warmth equilibrium 
which is so carefully and nicely adjusted. So 
if trees are to be preserved they must be al- 
lowed their places undisturbed. No change of 
more than two inches either way should be 
made in the ground around them within the 
space covered by the full spread of their 
branches, which is the space occupied by the 
spread of their roots — and even this is not 
desirable. 
I have spoken almost entirely of grading 
and grade treatment on irregular ground, be- 
cause ground of this character naturally de- 
mands more attention than ground which is 
