SPECIAL SENSES 11 



But Sachs showed in 1873 that the root of a 

 bean fixed horizontally over mercury could 

 penetrate the mercury in assuming a vertical 

 position. As mercury is about thirteen times 

 as heavy as the tissues of a young root, it is 

 evident that far more force was expended in 

 penetrating the mercury than could have been 

 derived from the physical action of gravity, 

 that is, from the simple weight of the root. 

 The experiment has since been tried in another 

 and more obvious way by harnessing a root 

 tip lying horizontal to a weight suspended 

 over a pulley, the weight being raised as the 

 root bends downward in response to gravity. 

 From these experiments we must conclude 

 that gravity does not act physically but phys- 

 iologically to induce the curvature, that is, it 

 acts as a stimulus. It is a small spark that 

 fires the gun. The spark will fire a pistol or 

 a cannon, the result depending solely upon the 

 amount and arrangement of the explosive 

 material. So in theroot,if thereis the proper 

 mechanism and storage of force, gravity will 

 release this force and cause the bending, the 

 amount of work done being enormously out 

 of proportion to the initial expenditure of 

 energy. 



But when the bending takes place, will it be 

 upward or downward? If it were a purely 

 mechanical device, it is evident that by know- 



