Chap. III.] DOWNWARD GROWTH OF THE ROOT. 
That light, not gravity, is the main conductor 
of the growth of the heads of plants is probable 
from the fact that, where trees stand close to¬ 
gether, their chief growth is upwards, and their 
side-branches die; and as long as their stems 
are thus in the shade, they show no disposition 
to shoot out sideways again. But the moment 
such an over-thick wood is over-thinned, the 
stems burst out sideways to the light which is 
admitted. And he who is most wedded to the 
extraordinary paradox, that the leader owes 
its vertical direction to gravity, will, I think, 
scarcely assert that the same cause produces the 
horizontal growth of the branch. Paradoxical 
as it may sound, if a side-branch of a tree de¬ 
scends from a height till it touches the ground, 
its growth all the time it is descending is ra¬ 
ther upward than downward ; that is, the new 
growth, or shoot, at the end of such a bough is 
generally slightly curved upwards by the action 
of light on the cellular structure of its upper 
side. Gravity, indeed, draws the whole branch 
down bodily, for light has no power to act 
through the dead bark; but light will so draw 
the new end up against gravity, that, when the 
branch comes to the ground, it will rest on a 
curved elbow, not on its end. This .fight be- 
