NATURE OF PLANTS 



25 



tion in the arrangement of the leaves is simply a device to bring 

 the leaves into the light and prevent the shading of one leaf by 

 another. In opposite two ranked leaves each succeeding leaf 

 is over the one below it but the cutting off of the light is pre- 



v 





Fig. 13. 



Fig. 1 



Fig. 13. 

 Fig. 14. 



Branch of Forsythia, leaves decussate, in four rows. 

 Branch of poplar with leaves spirally arranged in five rows. 



vented by lengthening the internodes and thus separating the 

 leaves. The decussate arrangement of the leaves is a more eco- 

 nomical arrangement because more leaves can be developed on 

 a given length of stem without danger of shading. The spiral 

 distribution of leaves is a still better device since, each succeeding 

 leaf is placed a little above and to one side of the next lower leaf. 

 By this means the maximum of leaf surface can be exposed to 

 the light in a given length of stem. No one can look at the leaves 

 of the maples, dogwoods, ailanthus, creeping and climbing vines. 

 etc., and not be impressed with the fact that the leaves are 

 arranged so as to catch the most favorable light without shading 

 or interfering with one another. Furthermore the arrangement 



