382 VEGETABLE PHYSIOLOGY 



In using the apparatus care must be taken that the centre of 

 gravity of the plant and flower-pot shall coincide with the 

 spindle, or the clock will have varying amounts of work to 

 do in different parts of the rotation. 



For ordinary purposes it is well to arrange the apparatus 

 so that a complete revolution of the spindle may be made 

 once in twenty minutes. When the plant is placed in a 

 horizontal position on the revolving plate, every face of its 

 axis comes successively under the influence of gravity, so 

 that all parts of it are affected equally and similarly. It 

 is then found that no curvature of the horizontal axis of 

 the plant 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 Darwi,n, 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. 



