NATURE OF PLANTS 



29 



any other organ does not remain in a fixed position during its 

 growth and development but owing to the fact that first one side 

 and then an adjoining side of the organ is growing faster than 

 any other part it comes about that the organ is bent from side to 

 side by the more rapidly growing cells and the apex of the organ 

 is often caused to travel through a rather irregular circle. These 

 growing organs are sensitive to light, gravity, moisture and other 

 stimuli. As a result of these movements the organ is brought 

 into relation with various intensities of light, heat, etc. In cer- 

 tain positions the stimuli are not favorable and they cause it to 

 grow away from this position while in other positions the stimuli 

 act in the most favorable way and the organ is stimulated to its 

 best growth. So during the development of the organ it is 

 brought into varied relations with the forces which affect it and 

 as a result of this experience the leaf or other organ is directed 

 by the stimuli and at maturity finally comes to rest in a fixed 

 position that is the most advantageous. The leaf ordinarily sc 



FIG. 20. 



FIG. 20. Shoot of wild lettuce, Lactuca, with leaves turned edgewise to 

 the light. 



FIG. 21. Position of young leaves of hickory: A, scales of bud curving 

 back showing the tightly folded leaves. B, later stage, leaves unfolding 

 but still erect. 



