CHAPTER XII 



EXCITATORY CHARACTER OF TRANSMITTED IMPULSE IN 



PLANTS 



The hydro-mechanical theory— Inconclusive character of the anaesthetic 

 experiment of Pfeffer and scalding experiment of Haberlandt — 

 Kiihne's experiment showing transmission of excitation under intense 

 stimulation in a rigored nerve — Error introduced by employment 

 of excessive intensities of stimulus — Discriminative polar effect of 

 current in excitation — Block of transmission of excitation by local 

 application of cold — Restoration of normal conductivity by tetanising 

 shock in tissue paralysed by cold — Electrotonic arrest of excitatory 

 impulse — Action of various poisons in inducing block of conduction. 



In the previous chapter I referred to the prevailing belief 

 that the transmitted impulse in the plant was hydro- 

 mechanical, unlike the excitatory impulse in the animal 

 nerve. This view has been largely based on two well-known 

 experiments of Pfeffer and Haberlandt. In the former of 

 these, the effect of strong stimulus was found to travel over 

 chloroformed parts of the petiole. Pfeffer assumed that the 

 conductivity of this portion must have been abolished, since 

 chloroform is known to abolish motile excitability. In the 

 experiment of Haberlandt an intervening tissue was killed 

 by scalding ; in spite of this, stimulus was found to be 

 transmitted across the scalded area. 



From these two experiments it was infened that the 

 impulse which was transmitted could not have been of a 

 true excitatory nature. It was held that, instead of this, 

 a strong stimulus had given rise to a variation whether 

 of increase or diminution of hydrostatic pressure. This 

 variation of pressure, it was assumed, had been hydro- 

 mechanically transmitted, and on reaching the distant 



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