7 
114 A TEXTBOOK OF GENERAL BOTANY 
In nature, when upright stems grow in the dark, they usually 
arise from underground structures such as bulbs or rhizomes, or 
are produced by germinating seeds. In such cases the relatively 
longer, slender structure of etiolated stems has a tendency to make 
the plants reach up into the light. The production of small leaves 
on etiolated stems also seems to be of advantage, as leaves in the 
dark cannot carry on photosynthesis and so are of no particular 
use to the plant. Moreover, large leaves would hinder the growth 
of stems through the ground, while their formation would re- 
quire material which could be used in elongating the stem. 
Leaves that normally arise directly from underground structures 
behave differently from ordinary aérial leaves when grown in the 
dark. Instead of being smaller than usual they become longer. 
This has the same effect as the elongated growth of upright stems 
in that there is a tendency to bring the leaves into the light. 
Self-pruning. The heavily shaded portions of branches fre- 
quently shed their leaves, while whole branches that are heavily 
shaded usually die and fall from the plant. It is for this reason 
that the leaves of trees are usually found near the circumference 
of the crown and not in the interior. 
The loss of shaded branches can be seen very plainly in the 
case of trees which grow close together and produce long trunks. 
As the lower branches become heavily shaded they die and fall 
from the tree, and leave the lower part of the trunk clear of 
branches. It is a common observation that trees growing close 
together are tall and slender in form and have long, clear trunks, 
while a tree growing in the open, where it is not shaded, usually 
has a spreading crown and a short trunk. These differences are 
due to the fact that if trees grow close together the lower 
portions are shaded and the lower branches die, which is not the 
case with trees which develop in the open. The dying and falling 
of shaded branches is called self-pruning. Foresters take advan- 
tage of the self-pruning of trees, and plant seedlings close together 
so that long, clear trunks will be produced. If it is desired that 
a tree growing in the open shall have a long, clear trunk, it fre- 
quently becomes necessary to remove the lower branches. 
