334 



CHAP. XXX. THE GEO-PERCEPTIVE LAYER 



The following table gives the results obtained with 

 twelve different specimens of the petiole of Tropaeolum. 

 The specimens were unequally thick ; hence the sensitive 

 layer was found at a depth of 0-15 mm. in thin, and at 

 0-20 mm. in thick, specimens; the maximum electric 

 excitation was in all cases found to occur at the starch- 

 sheath. 



Table XXII. — ^Geo-Electric Reaction at Various Depths in 

 Different Specimens (Petiole of Tropaeolum). 



Decline of Geotropic Excitability at Distance 

 FROM THE Perceptive Layer 



The experimental locahsation of the perceptive layer 

 is greatly facihtated by the abrupt enhancement of exci- 

 tation at that layer. This wdll be fully reahsed from the 

 resultant curve obtained from data derived from twelve 

 different specimens. Taking the perceptive layer itself 

 as the point of reference, successive distances, say, of 

 0-05 mm. are measured to the left and to the right of the 

 point of reference. The abscissa to the left is towards 

 the centre of the petiole, that to the right towards the 

 surface. The mean values of the excitatory reactions at 

 the different points are the ordinates for the curve. It 



