LOCALISATION OP GEO-PERCEPTIVK LAYER 4.S1 



the focus of irritutiou ; the state of excitation is, as we have 

 seen, detected by induced galvanoinetric negativity, and the 

 electric change would be most intense at the perceptive 

 layer itself. As the power of transverse conduction is feeble, 

 the excitation of the perceptive layer will irradiate into the 

 neighbouring cells in radial directions with intensity dimini- 

 shing with distance. Hence the intensity of responsive 

 electric change will decline in both directions outwards and 

 inwards. 



The distribution of the excitatory change, initiated at 

 the perceptive layer and irradiated in radial directions is 

 represented by the depth of shading, the darkest shadow 

 being on the perceptive layer. Had excitation been attended 

 with change of light into shade, we would have witnessed 

 the spectacle of a deep shadow (vanishing towards the 

 edges) spreading over the (.lifferent layers of cells during 

 displacement of the organ from vertical to horizontal ; the 

 shadow would have disappeared on the restoration of the 

 organ to the vertical position. 



Different shades of excitation in different layers is, how- 

 ever, capable of discrimination by means of an insulated 

 electric probe, which is gradually pushed into the organ 

 from outside. It will at first encounter increasing excitatory 

 change during its approach to the perceptive layer where 

 the irritation will be at its maximum. The indicating 

 galvanometer in connection with the probe will thus indi- 

 cate increasing galvanometric negativity, which will reach a 

 maximum value at the moment of contact of the probe with 

 the perceptive layer. 



It will be understood that the surface electric reaction 

 under geotropic stimulus, which we hitherto obtained, would 

 be relatively feeble compared to the response obtained 

 with direct contact with the maximally excited perceptive 

 layer. When the probe passes beyond the perceptive layer 



