Growth and Movement. 



121 



to plants, and 

 therefore light 

 is the most fea- 

 sible guide in 

 determining the 

 position of leaves 

 and leaf-bearing 

 branches. Since, 

 however, the 

 source of great- 

 est illumination 

 hourly changes 

 as the sun ad- 

 vances from east 

 to west, it is plain 

 that if the posi- 

 tion of leaves is 

 fixed once for all, 

 the best possible 

 light relation is 

 not attained. 

 The behavior of 

 a rapidly grow- 

 ing sunflower 

 shows how 

 plants may take 

 account of the 

 shifting light. 

 The three photo- 

 graphs of Fig. 58 

 show the same 

 plant as it ap- 

 peared in the 



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