24 ELECTRO-PHYSIOLOGY CHAP. 



the under surface is positive to the upper, appears not only in 

 consequence of quickly repeated mechanical or electrical excita- 

 tions, but also as the after-effect of the prolonged passage of a 

 constant current. If such a current is led through a leaf by 

 means of non-polarisable electrodes at right angles to its surface, 

 the electrodes, as in certain polarisation experiments in muscle, 

 serving simultaneously to lead off to the galvanometer, there is 

 always, if the galvanometer circuit is closed immediately after 

 opening the exciting circuit, an ingoing after-current in the leaf, 

 directed from above downwards, whatever the direction of the 

 polarising current. An exciting current homodromous with the 

 after-current is, however, much more effective, other conditions 

 being uniform. 



Burdon - Sanderson used a specially constructed rheotome 

 for these experiments, which made only three revolutions 

 in the minute, and thus closed the polarising current once in 

 2 sec. for ^ to ^j sec. ; after an interval of ^ sec. the 

 galvanometer circuit was closed for ^ sec. and the effect was 

 noted. 



" If the polarising current is comparatively weak, the after- 

 effect gradually diminishes, and disappears in a few seconds. 

 But if somewhat stronger currents are employed, the after-effect 

 will only partially disappear, leaving a permanent alteration 

 {modification) in the electromotive response of the leaf." With 

 repeated closure of the polarising current at regular intervals of 

 about 20 sec., the modified state is very quickly developed, and 

 reaches considerable proportions. " In one leaf, e.g., the lower 

 surface was negative to the upper (P.D. = 140 degrees of com- 

 pensator) before the passage of the current; four excitations 

 reduced the P.D. to zero, after which the lower surface subse- 

 quently became positive to the upper, and each excitation by the 

 current increased the effect, until it reached 320 degrees of the 

 compensator." 



As in muscle, secondary electromotive manifestations appear 

 as galvanic indications of the action of the polarising current, in- 

 dependent of visible signs of activity, so here Burdon-Sanderson 

 observed a modification, with currents so weak that their 

 " make " produced no trace of excitatory reaction after closure. 

 The " modification " then remained local, and was not transmitted 

 further, so that a lobe or part of the same can be affected 



