UPPER AND LOWER HALVES 



335 



will be seen how abruptly it rises to the maximum 

 at the perceptive layer and falls beyond it inwards 

 and outwards (fig. 199). 



Fig. 199. Curve showing distribution of geotropic excitability. 

 Maximum excitation occurs at geo-perceptive layer o. 

 Excitatory reaction rapidly declined inwards and outwards. 



The geo-perceptive layer may thus be experimentally 

 localised by measuring the depth of intrusion of the probe 

 at which maximum deflection of galvanometric negativity 

 is found to occur. 



Opposite Reactions in Upper and Lower Halves 



The experiments with Tropaeolum, Nymphaea, and 

 Bryophyllum brought out the striking fact that under the 

 stimulus of gravity the excitatory electric reaction in the 

 lower half is of opposite sign to that in the upper half, 

 a positive instead of a negative electric variation, the maxi- 

 mum positivity occurring at the lower starch-sheath. Since 

 galvanometric negativity is associated with contraction 



