LEAF EXPOSURE TO SUN AND AIR 147 
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 
B 
Fic. 115. — Leaves standing nearly Vertical in Compass-Plant (Silphium laciniatum). 
A, view from east or west ; B, from north or south. 
_ their edges just about north and south (Fig. 115), so that 
the rays of the midsummer sun will, during every bright 
