26o 



PLANT RESPONSE 



seconds, and indicated its presence by a diminution of 

 pressure on the contact (fig. 107), resulting in sudden 

 diminution of current. After the passage of the excitatory 

 wave the tissue was restored to its original condition, as 

 shown by its once more exerting its normal pressure. This 

 process of response and recovery by variation of pressure 

 is exhibited by the galvanometer record. The recovery is 

 usually found to be attained in the course of a half to one 

 minute, according to the intensity of stimulus. As the result 

 of this experiment, we find the velocity in this particular 

 stem to be 8 mm. per second, which 

 is very near the determination already 

 made by other methods, with different 

 specimens of stem of Mimosa. 



Frequently as I have obtained this 

 response during transit by diminution 

 of pressure, its opposite, that is to say, 

 response by increase of pressure, is by 

 no means uncommon. I have already 

 explained how it is possible for excita- 

 tory contraction to give rise to two 

 such opposed effects, in consequence 

 either of different dispositions of the 

 contractile elements in the tissue, or 

 by the presence of anisotropy in the 

 organ. By means of the electrotactile 

 method, then, we are able to demon- 

 strate the passage of the excitatory wave, and also to measure 

 its velocity, in tissues which are not motile. 



The electromotive method. — I shall now describe the 

 second, or electromotive, method which I have used for the 

 detection of the excitatory wave during its passage through 

 a vegetable tissue. I have already explained that when the 

 plant-tissue is directly excited, the state of excitation is 

 invariably accompanied by an electromotive variation, the 

 excited point becoming galvanomctrically negative (p. 32). 

 Hence, when an excitatory wave is transmitted through the 



Fig. 107. Electrotactile 

 Response in Stem of 

 Mimosa 



Stimulus applied at the mo- 

 ment x , at a distance 

 of 4 cm. below the 

 detector. 



