92 
UPWARD GROWTH OF THE HEAD, AND [Part II. 
either towards or from the light. Thus ivy, or 
the tendrils of climbing plants, are turned from 
the light towards any opaque body, while the 
generality of plants are turned from the opaque 
body toward the light. The whole of a plant 
kept in a room will grow sideways towards the 
window. The same plant placed where the light 
comes from above will immediately turn straight 
up. But though this principle is not so striking 
out of doors, it is perpetual and universal. 
As the light falls equally on the leader of a 
tree, it goes up straight. In proportion as the 
leader intercepts the light from above, the 
branches grow towards the light at the sides; 
and if one outstrips its neighbours, the light 
from above turns its end up. If the lower tier 
of branches outgrows the tier above it, in turn¬ 
ing up, it will take its neighbour with it. That 
tier will turn the tier above; and so in succes¬ 
sion all the branches will grow upward. This 
growth is often seen in beech-trees. 
It is this principle which prevents the boughs 
of a tree from growing against one another. 
In proportion to their opacity they grow one 
from the other. 
Notwithstanding the original downward ver¬ 
tical determination of the root, and upward 
