126 PLANT PHYSIOLOGY. 



and fill it with mercury ; pin the seed, previously germinated in sphag- 

 num until the root is about 3 to 4 cm. long, firmly to the wood so that 

 the root-tip lies upon the surface of the mercury ; cover the whole 

 with moist sphagnum and a bell-jar, and examine after a few hours. It 

 is easy also by use of an inverted corked test-tube, and an inner smaller 

 tube holding the seed, to make the growing root lift vertically the 

 weight of the seed, and the tube in addition, proving in another way the 

 tendency of the root to grow vertically downwards in preference to any 

 other direction, even though a lateral growth would offer much less 

 resistance. 



From the preceding experiments it will readily be inferred 

 that, while gravitation is all-important in determining the 

 existence and the direction of geotropic growth curvatures, 

 nevertheless it does not produce those effects by acting upon 

 the plant as weight. It obviously acts simply as a stimulus to 

 set in motion the geotropic growth processes and as a guide to 

 direction. If this be true, it should be possible to produce 

 upon the plant similar effects if the force of gravitation could be 

 replaced by another force of similar intensity and as constant 

 in direction. This can be accomplished by use of centrifugal 

 force. 



What is the effect upon the growth of geotropic parts if 

 the force of gravitation is replaced by centrifugal force ? 



Experimentation by individual students upon this subject is 

 hardly practicable, but you should become acquainted with it 

 through observation of the demonstration experiment and the 

 literature. 



An excellent centrifugal apparatus driven by an electric battery is 

 described by Arthur in the Botanical Gazette, 22, 463, and is supplied 

 with his apparatus. Another, very similar but driven by a hot-air 

 motor, is described by Hansen in Flora, 84, 352. 



A simple form driven by a water-motor is described by Stevens (Bo- 

 tanical Gazette 20, 89), and another simple form is figured by MacDougal. 

 p. 56. * Probably one of the centrifuges used in Bacteriology, driven by 

 the incandescent-light current, could readily be adapted to this use if 

 supplied with a form of governor. A water-motor driven from a con- 

 stant-pressure source would be best. 



Your experiments have shown that stems and roots are 



