CHAPTER XVIII 



ON CONDUCTIVITY AND EXCITABILITY 



Receptive excitability, conductivity, and motile excitability — Molecular model — 

 Modification of motile excitability : {a) by anaesthetics— (b) by cold — (c) by 

 fatigue— Variation of conductivity : (a) by cold — (b) by rise of temperature — 

 (r) by fatigue — {(/) by anaesthetics — Variation of receptive excitability by ether 

 — Conductivity versus excitability — Abolition of motile excitability without 

 abolition of conductivity — Hydro-mechanical theory of transmission of stimulus 

 untenable. 



HITHERTO we have been concerned mainly with the pecu- 

 liarities of the responding organ, by which the state of 

 excitation is outwardly manifested. Very often, the respond- 

 ing organ is not directly stimulated, but a distant point is 

 acted on by stimulus, and the state of excitation is trans- 

 mitted through the intervening distance, by the conducting 

 power of the tissue. In the actual life of a plant it is 

 frequently the case that the stimulus impinging on a recep- 

 tive area is transmitted along conducting channels, and is 

 manifested, on reaching some responsive organ. The whole 

 cycle of events is something like a telegraphic circuit, in 

 which the message taken at a transmitting station is sent to a 

 distance along conducting wires, and produces a signal at the 

 distant or responding station. 



In our experiments, for example, on Biophytum, as given 

 in the previous chapter, the stimulus was applied at the pe- 

 tiole, and was conducted along certain channels. On reaching 

 the specialised motile organ — the pulvinus — this transmitted 

 stimulus caused a responsive depression of the leaflet. The 

 petiole, during conduction of stimulus, was excited, but there 

 was no conspicuous external evidence of this, because, first, 

 the contractility of the tissue was relatively feeble, and 



