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 



A B 



FIG. 115. Leaves standing nearly Vertical in Compass-Plant (Silphium laciniatum). 

 A, view from east or west ; , 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 



