RESPONSE OF TIPS OF SHOOT AND ROOT 523 



The verification of these theoretical inferences- will be 

 given in full in the following chapter. 



Summary 



From electrical investigation we find that the responsive 

 peculiarities of the tip of the root are not characteristic of 

 that organ alone, but of the tip of the shoot also. 



The different effect between stimulation applied to the tip 

 and to the growing region of an organ lies in the fact that 

 from the former — it not being a good conductor of excitation 

 — only the indirect effect, that is to say the positive turgidity- 

 variation, is transmitted to the responding organ. In the 

 latter case, however, direct excitation gives rise to negative 

 turgidity- variation. 



Moderate unilateral stimulation of the tip, therefore, gives 

 rise on the same side of the responsive region to increased 

 turgidity, which, translated into mechanical response, means 

 a negative curvature or movement away from stimulus. 



Stronger and long-continued stimulation of the tip causes 

 transmission of the direct effect. This will gradually neu- 

 tralise or even reverse the first effect, the positive turgid ity- 

 variation giving place to negative. These events translated 

 into terms of mechanical response mean a change from 

 negative to positive response, or movement towards stimulus. 



Direct unilateral stimulation of the responsive region 

 causes negative turgidity-variation of the proximal and 

 positive turgidity-variation of the distal. The mechanical 

 expression of these will be a movement towards stimulus. 



By strong and long-continued action the stimulus ceases 

 to be unilateral and becomes internally diffused. The excita- 

 tion thus reaches the distal side. The difference of turgidity- 

 variation on the proximal and distal is thus gradually 

 abolished, or even reversed. The corresponding mechanical 

 response will be a neutralisation or reversal into negative, 

 that is to say a movement away from the stimulus. 



