110 ESSENTIALS OF BOTANY 
that this movement has other purposes than protection 
from frost, and probably there is much yet to be learned 
about the uses of leaf movements. 
128. Vertically Placed Leaves.—Very many leaves, like 
those of the iris (Fig. 82), always keep their principal sur- 
faces nearly vertical, thus receiving the morning and even- 
ing sun upon their faces, and the noonday sun (which is 
so intense as to injure them when, received full on the 
Fic. 77. A Leaf of Acacia. 
A, as seen by day; B, the same leaf asleep at night. 
surface) upon their edges. This adjustment is most per- 
fect in the compass-plant of the prairies of the Mississippi 
basin. Its leaves stand very nearly upright, many with 
their edges just about north and south (Fig. 78), so that 
the rays of the midsummer sun will, during every bright 
day, strike the leaf-surfaces nearly at right angles during 
a considerable portion of the forenoon and afternoon, 
while at midday only the edge of each leaf is exposed 
to the sun. 
