352 RESEARCHES ON IRRITABILITY OF PLANTS 



employment of the Direct and the Differential Methods. 

 The automatic records afford measurement of time as 

 short as '05 second. The highest velocity of transmission 

 of excitation that has been found in the petiole of Mimosa 

 is 30 mm. per second. In a sub-tonic tissue the velocity 

 of transmission of excitation is enhanced under increased 

 intensity of stimulus. The tissue became a better conductor 

 of excitation in consequence of previous stimulation. 

 Fatigue depresses the rate of conduction of excitation. 

 Velocity of transmission becomes markedly enhanced at a 

 higher temperature (p. 149). 



The transmission of excitation takes place in both 

 directions ; but the velocity is not necessarily the same in 

 the two directions. In Biophytum the velocity in the 

 centrifugal direction is quicker than in the centripetal. 



Excitatory Character of the Transmitted Impulse 



' The enhancement of velocity under favourable condition 

 of rising temperature proves the physiological character of 

 the transmitted effect. In support of this there are various 

 confirmatory proofs, some of which may be regarded as 

 crucial. 



Transmission of excitatory electric impulse. — The excita- 

 tory change in the plant is accompanied by a concomitant 

 electric change of galvanometric negativity. Electrical 

 investigation shows that this excitatory electric impulse is 

 transmitted to a distance through certain plant organs. 



Excitatory impulse in absence of mechanical disturbance. — 

 The hydro-mechanical theory presupposes the occurrence 

 of a strong mechanical disturbance to give rise to the 

 transmitted impulse. But initiation of excitatory impulse 

 is found to take place under the polar action of an electrical 

 current in the absence of any mechanical disturbance. 

 This is realised when we find that an excitatory impulse is 

 initiated and transmitted by the action of a current which 

 is so feeble as not to be perceived by the very sensitive 



