ELECTROTACTII.E DETECTION OF EXCITATORY PULSE 255 



Chemical method of detecting excitatory expulsion of 

 cell-sap. — My own experiment differs from this in several 

 particulars. Since there is a general impression that certain 

 specialised tissues in the pulvinus are alone excitable, it was 

 my object to show that cells which do not exhibit any motility 

 will also give rise to the expulsion of water by excitatory 

 contraction, and I desired further to utilise this effect for the 

 determination of the velocity of transmission of excitation in 

 that tissue. The essentials to this purpose were : some means 

 of detection of the excitatory expulsion at the exact moment 

 of its occurrence ; some means of marking accurately the 

 moment of application of stimulus ; and, lastly, the use of a 

 fairly long tract of tissue, in order that the interval between 

 application of stimulus at one end, and the manifestation of 

 its reaction at the other, might be of a duration capable of 

 exact measurement. For this purpose I took petioles of 

 Mimosa and the non-motile stems of the same plant, and 

 placed their cut ends in very dilute solution of sodium chloride. 

 So dilute a solution of salt does not, as 1 find, appreciably 

 affect the excitability of the tissue. Selecting one of the 

 specimens, say a petiole, I adjusted the electrothermic 

 stimulator at a distance of, say, 4 cm. from its lower or cut 

 end, the specimen being held vertical by means of a clamp. 

 The vessel of salt solution in which it had hitherto been 

 placed was now removed. The end of the petiole was 

 carefully rinsed, to remove all traces of salt from the outside, 

 and a small beaker of very dilute silver nitrate solution was 

 substituted. At this point it became necessary to finish the 

 experiment rapidly, as silver nitrate solution is likely after a 

 time to affect the excitability of the tissue. Momentary 

 thermal stimulus was given by brief closure of the electric 

 circuit. The excitation then travelled through the intervening 

 4 cm. of tissue, with a velocity characteristic of the plant. 

 When it reached the cut end, the excitatory contraction 

 produced an expulsion of cell-sap containing the salt 

 solution previously absorbed. This expulsion was instantly 

 made visible by the formation of a dense white precipitate 



