GEOTKOPISM 495 



of gravity, it is necessary to eliminate the action of tlie latter 

 force and to observe the direction of growth under the new con- 

 ditions. This can be done by causing the plant to grow upon 

 an apparatus known as a Klinostat, which is a wheel rotating 

 upon a horizontal axis at a regular rate, a complete revolution 

 being made in about twenty minutes. The plant is placed in a 

 horizontal position on the revolving wheel, so that each side of 

 the axis comes alternately under the influence of the force. All 

 parts of it are so affected equally, and it is found that then no 

 curvature of the horizontal axis of the plant occurs in either 

 direction. Another experiment, due to Knight, pointing to the 

 same conclusion, is that of growing a plant upon a rapidly re- 

 volving wheel mounted on a vertical axis. When the wheel 

 revolves sufficiently rapidly, though the plant is exposed all the 

 time to the action of gravity, 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, and the roots grow outwards from the centre of 

 the wheel, while the stem grows inwards towards it. The force 

 acts much like that of gravity, 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. 



As in the case of heUotropism, 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 is some distance further back, where active growth is 

 taking place. 



As in the former case, the movement of geotropism or 

 apogeotropism is 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 grass is beaten down by 

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

 position is due to the renewal of growth on the under side of the 

 fiwoUen nodes, which is excited by the stimulus and proceeds 

 tiU the stem is again vertical. 



As in the case of heliotropism, the sensitiveness varies very 

 much in different plants. The movement is usually a modifica- 

 tion of circmnnutation. 



Contact with a foeeign Body. — Many instances of sensitive- 

 ness to this form of stimulus have been observed. When a leaf 

 of Mimosa pudica is handled, the leaflets all droop downwards 

 with great suddenness, and if the handling is very rough all 



