346 CHAP. XXX. THE GEO-PERCEPTIVE LAYER 



These results afford another striking demonstration of 

 the fact that the layer which contains the starch-grains 

 becomes the focus of excitation when the organ is geo- 

 tropically stimulated by change from the vertical to the 

 horizontal position. 



Another significant fact was noticed in the case of 

 Calendula. Its geotropic excitability was very marked 

 at the beginning of its proper season, but disappeared later. 

 Microscopic sections showed that this insensitive condition 

 was associated with the disappearance of the starch-grains 

 from the two layers. Of these, the starch-grains in the layer 

 nearer the centre were the first to disappear. 



Summary 



The distribution of excitation induced in an organ under 

 the stimulus of gravity may be mapped out by means of 

 the exploring Electric Probe. 



The induced galvanometric negativity of the upper half 

 of an organ (indicative of excitation) shows variation 

 in different layers of the organ. The excitatory reaction 

 attains a maximum value at a definite layer, beyond which 

 there is a decline. 



The geo-perceptive layer is experimentally localised by 

 measuring the depth of intrusion of the probe at which 

 maximum deflection of galvanometric negativity occurs. 



The geo-perceptive layer thus determined is found to be 

 at or near the starch-sheath which contains a number of 

 large-sized starch-grains. 



In certain plants the distribution of geotropic excita- 

 bility exhibits two maxima ; that is, the focus of excitation 

 is not single but double. Microscopic sections showed that 

 the starch-sheath in these is double, and that the positions 

 of the two electric maxima coincide with those of the two 

 starch-sheaths. 



The activity of geo-perception undergoes seasonal varia- 

 tion. It declines with the growing subtonicity of the 



