256 PLANT RESPONSE 



of silver chloride. This was sometimes seen to be projected 

 as a white vortex ring. The interval between the application 

 of stimulus and this projection was found to be three seconds. 



It is thus clear that the excitatory wave is not one of 

 hydrostatic disturbance, for such a disturbance would be 

 transmitted with very much greater velocity. The velocity 

 of transmission in this petiole is seen to have been 13 mm. 

 per second, or practically the same as that obtained by a 

 separate experiment on the fall of the leaf with similar 

 specimens. 



In a similar manner I determined the velocity of trans- 

 mission through the stem. The stimulus was applied at a 

 distance of 5 cm. from the cut end, and the chemical pre- 

 cipitation was observed, after an interval of five seconds. The 

 velocity in the stem is thus seen to be 10 mm. per second. 

 A repetition of this experiment with another piece of stem 

 from the same plant gave similar results. 



It is thus seen that cells through which excitation is 

 proceeding undergo excitatory contraction, in consequence of 

 which there is an expulsion of water forwards, in the direction 

 of propagation. And this effect is produced, not in pulvinated 

 organs alone, but in others which are not usually regarded as 

 motile. This result is also arrived at independently by the 

 method of electrical response. 



The electrotactile method.— I shall next give an account 

 of the much more delicate methods which I have succeeded 

 in devising for the detection of the excitatory impulse during 

 transit, the first of which is the electrotactile method. It is 

 easy to understand that while the wave of excitation is passing 

 through any given section of tissue, it must produce there 

 certain form-changes, infinitesimal though they may be. Had 

 our sense of touch been more delicate, we might perhaps have 

 been able to perceive this pulse. It occurred to me, however, 

 that it might be possible, if it existed, to obtain its indication 

 by means of an electrical method of detection, the sensitiveness 

 of which could be exalted at will. 



As regards the pulse of form-changes we can see that two 



