154 Johnson's experiment. [ch. vii 



surface of the water. The geotropic curvature is absent 

 in the case of the submerged specimens. 



(198) Johnson's experi'ment\ 



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 radicleV' i-e- to show that 

 the root does not bend by mere plasticity. A method 

 of performing the experiment is shown in Pfeffer's 

 Physiologie, vol. II. p. 320, fig. 36. 



A similar expefiment 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 

 base of the root should be kept damp by a strip of filter- 

 paper hanging over the seed like a rider and dipping into 

 the water below. 



(199) Pinot's experiment. 



This is essentially the same as the last, but the 



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

 " From the title of Johnson's paper. 



' Ann. Sc. Nat., series 1, T. xvii. 1829 (Bibliography, p. 94). See 

 also Hofmeister in Pringsheim's Jahrbiicher, Vol. iii. p. 105. 



