THE GEO-PERCEPTIVE LAYER 327 



The principle of the method will be readily understood 

 from the following. As every side of a radial organ is 

 geotropically excitable, the geo-perceptive cells must be 

 disposed in a cylindrical layer at some unknown depth from 

 the surface, which, in a longitudinal section of the shoot, 

 would appear as two straight lines G and G' (fig. 196). In a 

 vertical position the geo-perceptive layer will be electrically 

 neutral, but rotation through 90° will initiate an excitatory 



Fig. 196. Diagrammatic representation of the geo-perceptive 

 layer in nnexcited vertical and in excited horizontal position 

 {see text). 



reaction. This sensitive tissue will then respond to the 

 stimulus and become the focus of excitation, the state 

 of excitation being detectable by accompanying galvano- 

 metric negativity, which would be most intense at the 

 perceptive layer itself. The excitation of the perceptive 

 layer will irradiate into the neighbouring cells in radial 

 directions with diminishing intensity, reaching the cortical 

 cells which effect the curvature. 



The distribution of the excitatory change, initiated at 

 the perceptive layer and irradiated in radial directions, 

 is represented by the depth of the shading, the darkest 

 shadow being on the perceptive layer itself (fig. 196). Were 



