LEAF EXPOSURE TO SUN AND AIR. 99 
124. Experiment 24.— Remove a pot containing an oxalis from 
a sunny window to a dark closet, and note at intervals of five minutes 
the condition of its leaves for half an hour or less. 
Some plants have the power of directing the leaves edge- 
wise towards the sun during the hottest parts of the day, 
allowing them to extend their surfaces more nearly in a 
horizontal direction during the cooler hours. 
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Fic. 85.— A Leaf of Acacia. 
A, as seen by day; B, the same leaf asleep at night. 
125. Vertically Placed Leaves. — Very many leaves, like 
those of the iris, Fig. 34, always keep their principal surfaces 
nearly vertical, thus receiving the morning and evening sun 
upon their faces, and the noonday sun (which is so intense as 
to injure them when received full on the surface) upon their 
edges. This adjustment is most perfect in the compass-plant 
of the prairies of the Mississippi basin. Its leaves stand very 
