LEAF EXPOSURE TO SUN AND AIR 145 
much yet to be learned about the meaning and importance 
of leaf-movements. 
153. Cause of Sleep-Movements. — The student may 
very naturally inquire whether the change to the noc- 
turnal 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. 
EXPERIMENT XXVI 
Remove a pot containing an oxalis from a sunny window to a 
dark closet, at about the same temperature, and note at intervals of 
five minutes the condition of its leaves for half an hour or more. 
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Fig, 113.— A Leaf of Acacia. 
A, as seen by day; B, the same leaf asleep at night. 
154. Structure of the Parts which cause Leaf-Motions. — 
In a great number of cases the daily movements of leaves 
are produced by special organs at the bases of the leaf- 
stalks. These cushion-like organs, called pulvini (Fig. 
114), are composed mainly of parenchymatous tissue 
