STIMULATION AND ITS RESULTS 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 bemg 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 mtense 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- 
cumnutation 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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