90 KNIGHT'S EXPERIMENT. [CH. vi 



are forms of geotropism, the root being positively, the 

 stem negatively, geotropic. A seedling bean placed on its 

 side gives evident proof of different kinds of sensitiveness 

 or irritability in its root and shoot, for under the influence 

 of one and the same force, viz. gravity, the root grows 

 towards, the stem away from, the centre of the earth. 

 The force of gravity is a stimulus to which different 

 parts of the plant react in different manners. Gravity 

 is as it were a sign-post by which the plant is enabled 

 to direct its growth in the most profitable manner. 

 The most striking proof that gravity thus plays the 

 part of stimulus, is supplied by the famous experiment of 

 Andrew Knight published in 1806. With the help of his 

 gardener he fitted up a small water-wheel which, being 

 driven by the stream in his garden, rotated rapidly and 

 exposed beans germinating on the circumference to strong 

 centrifugal force. If a flexible or ductile object is fixed 

 to a rotating wheel, it will bend until the free end points 

 radially outwards: in the same way when a bucket of 

 water is whirled violently round by a rope tied to the 

 handle, the water remains in the bucket even when it is 

 upside down, instead of flowing out in obedience to 

 gravity, as it would if the bucket were still. These 

 well-known results make one see that centrifugal force 

 replaces gravitation, and that it affects the object whirled 

 round like an imitation gravity acting in the direction 

 of the radius. Therefore if a stationary bean tends to 

 grow in the line of gravity, a bean whirled round on a 

 water-wheel must grow in the line of the imitation 

 gravity, that is in the line of the radius of the wheel. 



