VELOCITY OF TRANSMISSION OF EXCITATION 451 



There is, however, a more direct way of detecting the 

 excitatory wave during its passage through a vegetable 

 tissue. In this the Electro-motive Method the galvano- 

 meter takes the place of the motile leaflet. It has been 

 shown that when the plant tissue is directly excited, the 

 state of excitation is invariably accompanied by an electro- 

 motive variation, the excited point becoming galvano- 

 metrically negative. Hence any excitatory wave which is 

 transmitted through the tissue will always have an electro- 

 motive wave as its strict concomitant. The moment, there- 

 fore, at which excitation reaches any given point, may 

 always be determined by observing the arrival at that point 

 of the excitatory electrical disturbance of galvanometric 

 negativity. In order to prove that the arrival of excitation 

 at the given point is attended by this specific electrical 

 response, we may perform an experiment on a plant such 

 as Biophyttim, which is provided with motile leaflets. One 

 of the indicating leaflets is attached to the optic lever, its 

 base being connected with one of the electrodes of the 

 galvanometer, while the second is attached to a distant point 

 on the leaf. The two spots of light, one from the optic 

 lever indicating the mechanical response, and the other from 

 the galvanometer, indicating the electrical, are so adjusted 

 as to lie one above the other, on the same revolving-drum. 

 On now applying a stimulus, say thermal, at a distant point, 

 it will be found, after the lapse of a definite interval, that 

 both spots of light are deflected at the same time, showing 

 that both alike give an outward indication of that state of 

 molecular disturbance which is synonymous with excitation. 

 These manifestations, of both kinds, would therefore take 

 place at an identical moment, if only the inertia of the 

 two indicators were absolutely the same. But, just as the 

 same impulse would be indicated at slightly different times, 

 if one indicating-lever were light, and the other heavy, so 

 here also there may be a slight difference as regards time 

 between the appearance of the mechanical and electrical 



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