64 



General Botany 



on all sides that the blades take their normal positions directly 

 out from the nodes. If an erect shoot be placed in an incHned 



position, it is easy to see that the 

 leaves are no longer well displayed to 

 the hght. As may be readily seen by 

 examining the branches of trees and 

 the stems of traihng plants, horizontal 

 or inclined stems become twisted dur- 

 ing development through the influence 

 of unequal illumination upon the rela- 

 tive growth of different sectors of the 

 stem. The twisting of the stems 

 brings the leaves into better-illumi- 

 nated positions, but it often obscures 

 the normal arrangement of the leaves. 

 The positions of leaves with ref- 

 erence to light. If leaves are moder- 

 ately sensitive to hght, their posi- 

 tions when mature are approximately 

 at right angles to the Hne along which 

 the greatest amount of light reaches them. Consequently the 

 leaves on most of our common trees, shrubs, and herbs have an 

 approximately horizontal position (Figs. 33, 34). The sugar 

 maple and the magnolia are examples of trees whose leaves 

 are displayed in this manner (Figs. 30, 35). In the cottonwood 



Fig. 33. Vertical branch of dog- 

 wood, showing the opposite ar- 

 rangement of the leaves. 



Fig. 34. Horizontal branch of dogwood. Compare with Figure $3- 



