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. 32), 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 fuU on the 



.,r*^ft'v^ft«fe>. C'^^J:-®^ — -». -^ 





Fig. 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. 



