2l6 COMPARATIVE ELECTRO-PHYSIOLOGY 



The total duration of the down-movement is again much less 

 than that of the up. 



We have seen that a single excitation has a single con- 

 comitant electrical pulsation. We have also seen the multiple 

 electrical responses corresponding to multiple excitations. 

 It remains, then, to find out whether autonomous pulsations 

 have any electrical concomitant, and if so, of what nature. 

 I shall here, therefore, describe experiments for the recording 

 of the electrical pulsation of the Desmodium leaflet. For this 

 purpose I selected specimens in which the movement of the 

 leaflet was not spasmodic, but gradual and continuous, and 

 where the up and down movements were approximately in a 

 straight line. 



In order to obtain those responsive electro-motive changes 

 which might accompany the automatic movements of the 

 leaflets, it was necessary to make one of the electric contacts 

 at a point on the tissue which was free from excitation, the 

 second contact being made at a place where the excitatory 

 reaction was at its maximum. I have shown elsewhere l that 

 the seat of autonomous excitation in Desmodium is neither 

 central nor peripheral, but localised at the slender pulvinated 

 joints to which the leaflets are attached, the latter thus serving 

 merely as indicating flags. Acting on these considerations, I 

 made one contact with the slender pulvinule of a lateral 

 leaflet, the other being made with the common petiole. 

 These electrical connections were made securely by means of 

 cotton threads moistened with normal saline solution, and 

 attached to non-polarisable electrodes. The electro-motive 

 variations induced in the plant now gave rise to correspond- 

 ing deflections in the galvanometer in circuit. On taking 

 records of these electrical responses, I was surprised to find 

 that, corresponding with each complete mechanical vibration, 

 there was a double electrical pulsation a large principal 

 followed by a smaller subsidiary wave. In a given case, 

 where the period of the complete mechanical vibration was 

 about 3-5 minutes, the period of the principal of these two 



1 Bose, Plant Response, p. 299. 



