144 FOUNDATIONS OF BOTANY 
152. Daily Movements of Leaves. — Many compound 
leaves have the power of changing the position of their 
leaflets to accommodate themselves to varying conditions 
of light and temperature. Some plants have the power 
of directing the leaves or leaflets edgewise towards the 
sun during the hottest parts of the day, allowing them to 
extend their surfaces 
more nearly in a hori- 
zontal direction during 
the cooler hours. 
The so-called “sleep” 
of plants has long been 
known, but this subject 
Fic. 112.— A Leaf of Red Clover. 
At the left, leaf by day ; at the right, the same has been most carefully 
was fe ania: rigoczse studied rather recently. © 
The wood sorrel, or oxalis, the common bean, clovers, 
and the locust tree are some of the most familiar of 
the plants whose leaves assume decidedly different posi- 
tions at night from those which they occupy during the 
day. Sometimes the leaflets tise at night, and in many 
instances they droop, as in the red clover (Fig. 112) and 
the acacia (Fig. 113). 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 experimentally 
that when part of the leaves on a plant are prevented from 
assuming the folded position, while others are allowed to 
do so, and the plant is then exposed during a frosty night, 
the folded ones may escape while the others are killed. 
Since many plants in tropical climates fold their leaves 
at night, it is certain that this movement has other pur- 
poses than protection from frost, and probably there is 
