CHAPTER FOUR 



LEAVES IN RELATION TO LIGHT 



The leaf, as we have seen, must receive light in order to 

 produce food. Leaves are variously arranged on stems, and 

 stems have all sorts of positions. 

 Many of these leaf arrangements and 

 stem positions are not advantageous 

 for the display of leaves to the light. 

 The leaf, however, and especially the 

 petiole, is so influenced by Hght dur- 

 ing its development, that the leaf 

 when mature has the best possible 

 position with respect to light. The 

 raised leaves of the pumpkin, the fig. 21. Vertical branch of 



mosaics of leaves formed on the sides ^^SnoVm. Note the alternate 



. arrangement of the leaves. 



of buildmgs by the Boston ivy, and 



the successive tiers of leaves on a beech tree illustrate 



different arrangements by which large numbers of leaves 



are efficiently displayed to the light. 



The arrangement of leaves on stems. Leaves develop 



from somewhat thickened places on the stems, called the 



nodes. Each node 

 may bear one, two, 

 or several leaves. 

 According to the 

 number of leaves 

 that the node 

 bears, the leaf ar- 

 rangement is desig- 



FiG. 2 2. Horizontal branch of magnoha. Compare leaf *" ' 



positions with those of Figure 2 1 . OppOSUe, OX whorled. 



37 



