ECOLOGY OF LEAVES 109 
extend their surfaces more nearly in a horizontal direction 
during the cooler hours. 
The so-called “sleep” of plants has long been known, 
but this subject has been most carefully studied rather 
recently. The wood sorrel, 
or oxalis, the common bean, 
clovers, and the locust tree 
are some of the most fa- 
miliar of the plants whose 
leaves assume decidedly 
different positions at night 
from those which they 
occupy during the day. 
Sometimes the leaflets rise 
at night, and in many 
instances they droop, as 
in the red clover (Fig. 76) Fie. 75. Opposite Leaves of Deutzia, 
amd: the AeaGia: (Fig. iD. as arranged on a Vertical Branch. 
One useful purpose, at any rate, that is served by the leaf’s~ 
taking the nocturnal position is protection from frost. It 
has been proved experi- 
mentally that when part 
of the leaves on a plant 
are prevented from as- 
suming the folded posi- 
tion, while others are 
allowed to do so, and the 
Fie. 76. A Leaf of Red Clover. plant is then exposed 
At the left, leaf by day; at the right, durin ga frosty night, 
the same leaf asleep at night. the folded ones ma y 
escape while the others are killed. Since many plants in 
tropical climates fold their leaves at night, it is certain 
