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. 



