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98 ELEMENTS OF BOTANY. 
Where the stem on which the leaves are borne stands nei- 
ther horizontally nor vertically, but at some oblique angle to 
the earth’s surface, the leaf arrangement is more or less 
irregular, as in Fig. 83, which represents the leafage of a 
castor-oil plant growing in an inclined position, because it 
was shaded on one side. 
123. 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. 
The so-called “sleep” of 
plants has long been 
known, but this subject 
has been most carefully 
FIG. 84.— A Leaf of White Clover. studied rather recently. 
A, leaf by day ; B, the same leaf asleep at é 
night. 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 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 white clover, Fig. 84, and the acacia, Fig. 85. 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. The student may very naturally inquire whether the 
change to the nocturnal position is brought about by the 
change from light to darkness or whether it depends rather 
upon the time of day. It will be interesting to try an 
experiment in regard to this. 
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