THE ELECTRO-MOTIVE RESPONSE OF PLANTS 17 



sensitive, yet the excitability of the lower is, generally 

 speaking, greater, and diffuse stimulation therefore causes 

 greater contraction of that half. Hence the resultant fall is 

 due to the differential contraction of the two sides of the 

 organ. The excitatory reaction of the organ, then, consists 

 of a contraction ; expulsion of water with consequent diminu- 



FIG. 12. Arrangement for observing Simultaneous Mechanical and 

 Electrical Responses 



Leaf stimulated by electro-thermic stimulator. Mechanical record obtained 

 by excursion of spot of light reflected from Optic Lever, which falls on 

 the right of drum. Electric record obtained by excursion of galvano- 

 meter spot of light adjusted to fall on the left side of the drum. 



:ion of turgidity, or negative turgidity variation ; and fall of 

 the leaf. 



In order next to determine the electrical concomitant of 

 this reaction, we make suitable electrical connections by 

 non-polarisable electrodes, with a galvanometer. One of 

 these is made with the pulvinus, whose excitation is to be 

 observed, and the other with a distant indifferent point. In 

 this way we can obtain the excitatory effect at the pulvinus, 

 uncomplicated by that at the distal point. A spot of light, 

 reflected from the galvanometer mirror, is thrown on the 



