CALIFORNIA, COLLEGE 

 of PHARMACY 



STIMULATION AND ITS KESULTS 391 



Another experiment, due to Knight, pointing to the 

 same conclusion, is that of growing a plant, preferably a 

 seedling, upon a rapidly revolving wheel mounted on a 

 vertical axis. When the speed of the revolution is suffi- 

 ciently great, though the plant is exposed all the time to 

 the action of gravitation, the centrifugal force of the appa- 

 ratus is so much greater than the force of gravity that the 

 plant does not respond to the latter. Instead, it responds 

 to the stimulus of the rapid rotation or centrifugal force, and 

 the root grows outwards from the centre of the wheel while the 

 stem grows inwards towards it. The force acts much like 

 that of gravitation, and the plant responds to it in a similar 

 way, the root growing in the direction of the force and the 

 stem in one opposite to it. If the rotation is conducted at 

 less speed, so that the centrifugal force is about equal to 

 that of gravitation, the position assumed by the axis of the 

 plant is that of a resultant between the two forces, in which 

 it makes an angle of about 45 with the vertical. 



As in the case of heliotropic curvature, the part which 

 receives, or is sensitive to, the stimulus is not the part which 

 curves. In the case of a root it has been demonstrated by 

 Darwin, and more recently by Pfeffer, that the sensitive part 

 is the tip ; while the curvature takes place at a point further 

 back, where active growth is taking place. The curvature 

 is caused by a similar modification of the growth on the 

 two sides of the curving organ. 



The action of a seedling under the stimulus of gravity 

 seems to indicate that the plant possesses an appreciation 

 of direction. In whatever position the seedling is placed, so 

 long as it is free to grow without interference, its root will 

 grow vertically downwards, executing whatever curvature 

 is necessary for it to attain that direction. 



The movements of geotropism and apogeotropism are 

 not confined to growing organs. When the haulm of a 

 grass is placed horizontally on the ground, as is the case 

 when a patch of wheat or other cereal is beaten down by 

 wind or storm, after a time it again becomes erect. The 



