64 



PRACTICAL BOTANY 



Fig. 51. Nearly vertical leaves 

 of the olive 



themselves to flat surfaces, as of stones or bark, by developing 

 disks which act as suckers. These may stick so fast to the sup- 

 porting surface 

 that the tendril 

 can be broken 

 without tearing 

 them away. 



58. Leaf posi- 

 tions avoiding ex- 

 cessive illumina- 

 tion. While the 

 leaves of plants 

 growing in the 

 shade often suf- 

 fer from lack of 

 sunlight and are usually so arranged as to utilize most fully 

 what light there is (Fig- 50), it is possible for leaves in exposed 

 situations to have too much light. It seems certain that the most 

 powerful sunlight may injure the chloroplasts and therefore 

 cripple the power of the leaf to do its work of photosynthesis. 

 Compass plants, such as the common prairie species (>S77- 

 phimn) and the prickly lettuce, have leaves somewhat erect, 

 with edges directed nearly 



north and south, thus se- 

 curing good illumination 

 during the cooler morning 

 and evening hours, but pre- 

 senting the leaves nearly 

 edgewise to the sun at noon. 

 Many other plants maintam 

 some or all of their leaves 

 in a nearly vertical posi- 

 tion, but with the edges 

 not directed north and south. In the olive (Fig. SI") many 

 leaves point nearly upward, while in the commonest species 

 of ExcaJi/pfus the leaves hang vertically downward. 



Fig. 52. A leaf of red clover 



At the left, leaf by day ; at the right, the 

 same leaf at night. Natural size 



