CH. vii] Johnson's experiment. 167 



geotropic curvature should occur, the hydrogen is replaced 

 by air, when the root will bend downwards. 



(197) Free oxygen necessary. 



The same result may be more simply obtained by 

 keeping seedling beans completely submerged while 

 control specimens are in damp air, or just touching the 

 surface of the water. The geotropic curvature is absent 

 in the case of the submerged specimens. 



(198) Johnsons experiment^. 



The interest of this experiment (in which a root does 

 external work during geotropic curvature) is now some- 

 what historical. Its original object was to demonstrate 

 " the unsatisfactory nature of the theories proposed to 

 account for the descent of the radicle V' i.e- to show that 

 the root does not bend by mere plasticity. A method 

 of performing the experiment is shown in Pfeffer's 

 Plujsiologie, Vol. ii. p. 320, fig. 36. 



A similar experiment may be more simply arranged 

 in which the resistance is given by a spring. A pin is 

 driven vertically into the inside of the lid of a jar, and 

 from the lower end of the pin a thin copper wire projects 

 horizontally ; at the end of the wire a microscopic cover- 

 glass is cemented so that it lies horizontally. A bean is 

 now pinned to the lid so that its root projects horizontally 

 and rests on the glass-cover ; as the root curves down it 

 overcomes the elasticity of the wire. The cotyledons and 



1 Edinh. New Phil. Journal, 1829, p. 312. 

 " From the title of Johnson's paper. 



