Leaves in Relation to Light 



39 



(page 310) furnish excellent examples of the whorled arrange- 

 ment. 



However, it is only on upright 

 stems which receive the light equally 

 on all sides, that the blades take 

 their normal positions directly out 

 from the nodes. If an erect shoot 

 be placed in an inclined position, it 

 is easy to see that the leaves are no 

 longer well displayed to the light. 

 As may be readily seen by examin- 

 ing the branches of trees and the 

 stems of trailing plants, horizontal 

 or inclined stems become twisted 

 during development because of un- 

 equal illumination (Fig. 26). The 

 twisting of the stems brings the 

 leaves into better positions to re- 

 ceive light, but it often obscures the 

 normal arrangement of the leaves. 



The positions of leaves with reference to light. If leaves 

 are moderately sensitive to light, they assume a position 



Fig. 25. Vertical branch of dog- 

 wood, showing the opposite ar- 

 rangement of the leaves. 



Fig. 26. Horizontal branch of dogwood. Compare with Figure 25. 



