3^ 



PLANT RESPONSE 



(-<*) 



course, in the opposite direction, that is from B to A. 1 The 

 excited point A is thus electro-positive, but in physiological 

 text-books it has been ambiguously termed negative. In 

 order therefore to keep touch with the older terminology and 

 yet avoid the implied error, I shall refer to the excited point 

 as ' galvanometrically negative.' 



If the B end of the specimen be now excited, the direction 

 of the responsive current will be reversed. With greater 

 intensity of stimulus the electrical response will be found 

 correspondingly increased. The record of such responses is 

 (/>) obtained on a revolving drum, 



by following the deflection of 

 a spot of light reflected from 

 the galvanometer mirror in a 

 manner precisely similar to 

 that employed with the Optic 

 Lever (fig. 24). 



Method of transmitted 



stimulation. — In the simple 



:trical Response in Plant case just described, the tissue 



by Method of Block was stimulated directly and 



(a) The plant is clamped at c, between , 



a and is. locally at the end A, say by 



(/,) Responses obtained by alternately tors i ona l vibration. There is, 

 stimulating the two ends. Stimula- 

 tion of a produces upward response, however, another method of 

 and that of b downward. transmitted stimulation, by 



which a stimulus,— say by application of cut, or hot wire, 

 or electrical shock, — is given at a point X some distance 

 away, say to the right. The excitation now travels with a 

 velocity characteristic of the specimen, and when it reaches 

 the proximal electrode produces galvanometric negativity 

 of that point. The interval of time which elapses between 

 the application of stimulus and response will therefore depend 

 on the velocity of transmission and the distance of the point 



1 The failure to understand this point clenrly has been the source of many 

 grave errors in some physiological text books. From the fact that the current in 

 the external circuit is seen l<> flow in the direction of A, it has been erroneously 

 supposed that that point is negative, or zinc like. See Bose, Response in the 

 Living and Non- Living. 



Current of response when 



A is stimulated-* 

 Current of response when 



B is stimulated <— 



F'IG. 23. 



