382 VEGETABLE PHYSIOLOGY 
When the plant is placed in a horizontal position on 
the revolving plate, every face of its axis comes succes- 
sively under the influence of gravity, so that all parts of 
it are affected equally and similarly. No curvature of the 
horizontal axis of the plant then occurs in any direction. 
Another experiment, due to Knight, pointing to the 
same conclusion, is that of growing a plant upon a rapidly 
revolving wheel mounted on a vertical axis. When the 
speed of the revolution is sufficiently great, though the 
plant is exposed all the time to the action of gravitation, 
the centrifugal force of the apparatus 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 demon- 
strated 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 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 
new position is due to the renewal of growth on the under- 
sides of the swollen nodes, which is excited by the stimulus 
and proceeds till the stem is again vertical. 
