388 



COMPARATIVE ELECTRO-PHYSIOLOGY 



stimulus, may now be expected to reassert itself, thus con- 

 verting expulsion into renewed suction. 



In any case, whatever the explanation, I find that the 

 result of this mode of stimulus is, first, a movement of 

 expulsion, followed, under continued intense stimulus, by 

 renewal of the upward suction. These various effects 

 are seen in the following photographic record, where 

 the up-curve represents the normal unbalanced suction 



(fig. 232). Continuous 



stimulation was now applied 

 at the point marked with a 

 vertical line. It will be seen 

 that normal suction is here 

 diminished, and afterwards 

 reversed into expulsion. 

 This expulsive movement 

 continues, as I already knew 

 from previous experiments, 

 for a considerable length of 

 time, before the second re- 

 versal to suction is brought 

 of about by fatigue of the lower 

 zone. In order to expedite 

 the reversal, so that the 



FIG. 232. Photographic Record 

 Response to Continuous Sub-terminal 

 Stimulation 



Response was taken without balance 

 Continuous stimulation applied from 

 moment represented by vertical line. 

 This induced diminution, arrest, and 

 reversal of response to expulsion. 

 Stronger stimulation applied at second 

 vertical line. This induced a second 

 reversal to suction. Thin white line 

 shows duration of application of 

 stimulus of moderate intensity ; and 

 subsequent thick line, of greater 

 intensity. 



curve might remain within 

 the plate, I applied a still 

 stronger stimulation, at the 

 point marked by the second 

 vertical line. This was done 

 by increasing the voltage 

 which worked the primary 

 of the induction coil from 

 seen how this reversed the 



six to eight volts. It will be 



expulsion, converting it into renewed suction. 



We have seen, as already stated in the electrical response 

 of roots, that while less excitable old roots will generally 

 give positive response, highly excitable young roots give 



