Chap. III.] DOWNWARD GROWTH OF THE ROOTS. 
93 
vertical determination of the stem, the annual 
vertical growth in elongation of a tree, either 
upward or downward, is nothing in comparison 
with the growth of it, which forms angles with 
a vertical line: that is, out of all the numerous 
points of elongation of the head of a tree, there 
is but one which can go vertically upward in 
continuation of the line of the stem; and out of 
all the numerous points of elongation of the 
root, there is but one which can go vertically 
downward in continuation of the line of the 
stem; and I believe that one ceases to do so 
very soon. 
If the leader of a tree is killed, the light 
falling equally from above on many buds, a 
multiplicity of leaders may be developed; though 
if one grows more vigorously than the others, 
by overshadowing them from above with its own 
side-growth, it will force them to grow sideways 
to the light, and the tree will again become 
single-leadered. Without pruning, a tree may 
become round-headed from the merest accident; 
for instance, an insect, or a bird, or the wind 
destroying the top bud. A tree on the side 
of a steep hill may be seen to grow from the 
opaque side above it, and, after it has reached a 
certain height, to curve back again. 
