THE ELECTRO-MOTIVE RESPONSE OF PLANTS 



electrical currents, of light and of gravity, will be taken 

 up in special chapters devoted to their consideration, while 

 here I shall demonstrate the exci- 

 tatory effects of the other forms 

 of stimulus enumerated. 



We have already observed the 

 responsive effect which results 

 from the sudden application of 

 heat, by means of a hot wire. 

 The effects of various forms of 

 mechanical stimulation may now 

 be subjected to demonstration, 

 and first we have to observe the 

 effect of the stimulus resulting from 

 sudden tension. The specimen is 

 clamped securely in the middle 



(fig. 1 6), SO that when a Vertical pull FIG. 16. Excitation by Sudden 



is given to the upper half, that Plant secure^da^ped. When 

 half alone is subjected to a sud- 

 denly increased tension, the lower 

 being left entirely unaffected. 



Under these circumstances, there is an electrical response, 

 A becoming galvanometrically negative. A is next subjected 

 to mechanical compression, and for this purpose the piece of 

 moistened cloth surrounding the specimen, and making the 

 electrical connection at A, is placed between the two grooved 

 halves of a cork. The enclosed plant tissue at A may now 

 be made to undergo sudden compression, by squeezing the 

 pieces of cork together. This gives rise to the same electrical 

 response as before. 



This fact, that both tension and compression will give rise 

 to similar excitatory responses of galvanometric negativity, 

 may receive independent demonstration by first making an 

 electric connection at A with the upper side of the speci- 

 men (fig. 17). When the tissue at A is now suddenly bent 

 down, this upper side becomes convex : that is to say, it is 

 subjected to tension. This gives rise to the excitatory 



suddenly pulled, tension in- 

 duces galvanometric nega- 

 tivity of A. 



