Leaves in Relation to Light 



63 



leaves that the node bears, the leaf arrangement is designated 



as alternate, opposite, or whorled. 



In the alternate arrangement each node 



bears one leaf. This is also called the spiral 



arrangement, because a line drawn through 



successive leaf bases forms a spiral about the 



stem. Sometimes, as in the corn plant, the 



spiral passes half around the stem in going 



from one node to the next. In other plants, 



hke the sedges, the spiral passes but a third 



around the stem between nodes. In several of 



our common fruit trees, as, for example, the 



apple and the peach, the spiral between 



nodes passes two fifths around the stem. 



These variations of the spiral arrangement 



of leaves on stems are called the two-ranked 



(Fig. 37), three-ranked (Fig. 31), and five- 

 ranked arrangements (Fig. 29). 



In the opposite 

 arrangement two 

 leaves occur at each 

 node (Fig. ^^). The leaves at succes- 

 sive nodes, however, are at right 

 angles to each other, giving four ranks 

 of leaves. The maple, ash, dogwood, 

 and lilac furnish examples of the op- 

 posite arrangement. In the whorled 

 arrangement the leaves are in a circle 

 about the node (Fig. 32). The Indian 

 cucumber root (Medeola) and the 

 wood Hly furnish excellent examples 

 of the whorled arrangement. 



However, it is only on upright 

 stems which receive the light equally 



Fig. 31. A sedge {Duli- 

 chiuni), showing three- 

 ranked arrangement of 

 the leaves. 



Fig. 32. Indian cucumber root, 

 showing the whorled arrangement 

 of the leaves. 



