STIMULATION AND ITS RESULTS 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 
iway, 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 
