BEHAVIOUR OF PLANTS TOWARD LIGHT 143 
position at night, and on the following morning they 
are brought by the influence of light into the day posi- 
tion again. This drooping position 
of leaves at night has been termed 
the “sleep of plants,” but it is not in 
any sense a sleep. In the “ compass” 
plant the leaves stand vertical and 
point north and south. 
The night position of leaves is due 
to unequal growth. When the leaves 
are young and in the bud, they closely 
overlap one another. ‘This is due to 
the fact that growth takes place more | 
rapidly on the under surface of the 
young leaf. This causes the leaf to 
curve upward and in, over the end 
of the stem. But as the leaves be- 
come older, growth takes place more 
rapidly on the upper surface. This 
causes them to curve outward and 
later downward so that they occupy 
Iq. 174. Young sunflower 
: os : F 
a drooping position. This can be “ plant tumed toward light 
from window. 

demonstrated by covering for several 
days a bean plant, or by covering an oxalis plant for a 
day, so that it will be entirely in the dark. 
To illustrate it here a sunflower plant grown in a pot 
was used, the plant being four or five weeks old. It 

