STIMULATION AND ITS KESULTS 387 



apheliotropism is usually much slower than that of helio- 

 tropism. 



The bending is not caused by a direct interference of 

 the light with the part actually growing, but by a modifica- 

 tion of growth caused by the stimulated protoplasm, the 

 nature and extent of the alteration being conditioned by 

 the intensity of the stimulation. It would seem at first 

 as if the retarding effect of light upon growth might explain 

 the bending of the organ towards the light-source, the 

 non-illuminated side continuing to grow and the illuminated 

 one being prevented from doing so. This explanation is 

 directly contradicted by the phenomenon of apheliotropism. 

 It is moreover proved to be an insufficient explanation by 

 the fact that the part which is sensitive to the stimulus is 

 not the part which actually bends. Darwin showed this by 

 preventing the access of the light to a small region about 

 one-tenth of an inch in length close to the tip of the seedling, 

 when he found that the heliotropic curvature did not take 

 place, although the normally bending part was illuminated. 

 Further, when the region normally curving under the 

 influence of the stimulation is mechanically hindered from 

 bending, the curvature takes place at a part a little lower 

 down, which normally remains straight. 



The curvature is produced by an acceleration of the rate 

 of growth on the convex side, together with diminished 

 growth on that which becomes concave. When the lateral 

 light is fairly intense the resulting movement takes place 

 uninterruptedly ; when it is only weak the position is 

 assumed by a series of zigzag movements, indicating that the 

 new movement is an exaggeration of the ordinary cir- 

 cumrmtatiori of the part. When the final position is reached 

 the organ is found to circumnutate about the new direction 

 of the axis. 



The degree of the intensity of the lateral light is a very 

 important factor in the stimulation. While, speaking 

 generally, stems bend towards a lateral light and roots 

 away from it, a great increase in the intensity will cause 



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