Chap. IV.] 
MISCELLANEOUS. 
153 
have been smothered. In this case, the whole 
tree may considered as a huge cutting which has 
(mirabile dictu /) struck . But, generally, the new 
tier of roots is not strong enough to supply the 
exhaustion of the old head; and if death does not 
result, the head dies in and rots the stem from 
above, while the old roots do the same from 
below. Roots are obliged to keep the surface, 
because the food of plants lies there, though 
many think that this food is imbibed from the 
atmosphere; if so, trees would not be injured 
by having their roots covered. 
I believe Sir Humphrey Davy first remarked, 
on the assumption that the upward and downward 
growth of plants is vertical , that woods and crops 
growing on the side of a hill would derive no 
greater advantage from the additional space than 
if they grew on the horizontal surface of its base. 
But it must be recollected, that, as the plants on 
the side of a hill rise tier above tier, with the 
same light and aeration from above they have a 
greater side light and aeration. They are, in 
fact, placed head above head, like people in a 
race-stand, where, but for this arrangement, the 
spectators would have good opportunity for look¬ 
ing upward at the roof, but none for looking side¬ 
ways at the race. But the merit of this prin- 
That a hill af¬ 
fords no more 
space for growth 
than its base 
would, an error. 
