328 CHAP. XXX. THE GEO-PERCEPTIVE LAYER 



excitation attended with a change from hght to shade, 

 there would be witnessed the spectacle of a deep shadow 

 (vanishing tow^ards the edges) spreading over the different 

 layers of cells during displacement of the organ from the 

 vertical to the horizontal ; the shadow would disappear on 

 the restoration of the organ to the vertical position. 



The Electric Probe 



Different intensities of excitation in different layers 

 are capable of detection by means of the Electric Probe, 

 insulated except at the tip, which is gradually forced into 

 a horizontal stem from the surface (fig. 197). Increasing 



Fig. 197. The Electric Probe. 



Figure to the left represents one electric contact made with a 

 sepal of Nymphaea, and the other with the peduncle by 

 means of the Probe ; the included galvanometer is repre- 

 sented by a circle. Figure to the right is an enlarged view 

 of the Probe. 



excitatory change of galvanometric negativity will be ex- 

 hibited as the probe approaches the perceptive layer at 

 which the electric excitation will be maximum. After 

 this, as the probe passes beyond the perceptive layer, the 

 electric indication of excitation undergoes dechne and final 

 abohtion. The characteristic excitation occurs only under 



