EXCITATORY POLAR EFFECTS OF CURRENTS 191 



other very distant from it (fig. 89). If the plant is not very 

 excitable the effect produced at the distant point will not 

 reach the motile organ, and we shall obtain the isolated effect 

 of a particular electrode. Again, if we wish to observe the 

 effects at both the electrodes simultaneously, we may employ 

 the Bi-polar method, in which both electrodes will be placed 

 at or near the motile organs. The most suitable means for 

 the application of electrical stimulus will be either a constant 

 electrical current from a voltaic battery, or the discharge from 

 a charged condenser. 



We have again to study the respective effects of feeble, 

 moderate, and excessively strong electromotive forces, 



In experimenting on polar excitation in animal tissues, 

 a nerve-and-muscle preparation is generally used, the ex- 

 citation of the nerve being studied by means of the indication 

 given by the terminal motile organ, the muscle. On the 

 other hand, experimenting on Biophytitm for instance, the 

 petiole acts as the conductor of stimulus, and is provided 

 with— not a single terminal motile organ, but — a number of 

 lateral motile organs, viz. the ' sensitive ' lateral leaflets. 

 The analogous case in animal tissue would be a hypothetical 

 nerve, provided with a hypothetical series of contractile 

 muscles attached to it laterally. The relative advantage 

 possessed by such a vegetable organ is, that the changes in 

 the excitabilities, throughout every portion of the excitable 

 conducting tissue, are visibly manifested. 



I experimented altogether on some hundreds of specimens. 

 Some of these were very sensitive ; others only moderately 

 so. The results under normal conditions were perfectly 

 consistent. As it would entail much mere repetition to relate 

 every one of these experiments, I shall here give only typical 

 instances in detail. While I was studying the effect of the 

 establishment or cessation of a constant current I made a 

 practice — whenever the leaves or leaflets recovered within a 

 moderate time from the effects of the stimulus of a current 

 flowing in one direction — of trying a second experiment on 

 the same plant, by reversing the direction of the current, so 



