vi ELECTKOMOTIVE ACTION IN VEGETABLE CELLS 23 



shock (O'l sec. after liberating the pendulum) ; opening K2 

 bridged the galvanometer circuit, which was broken, finally, by 

 opening K3. The distance between Kl and K2, as also between 

 K2 and K3, is variable. We shall return later to the results of 

 these experiments ; here it is sufficient to note that Burdon- 

 Sanderson, with the 

 help of the compensat- 

 ing method, determined 

 the E.M.F. of the first 

 phase at about 0'08 

 Dan., while that of the 

 second did not exceed 

 0'82 Dan. 



If one or other half 

 of the leaf is excited 

 by break induction 

 shocks (the electrodes 



, . ,, ,. -, FIG. 146. 



being usually applied, 



as in Fig. 143, to opposite and approximately central points of the 

 two leaf- surfaces), the coils of the induction apparatus must be 

 pushed tolerably close (about 10 cm.) before an effect is produced. 

 The direction of the induced current is by no means immaterial, since 

 the effect appears much sooner when the current flows from upper 

 to under surface than in the opposite case. The same applies to 

 battery currents also. If a current, of moderate strength, adequate 

 for excitation, is sent transversely through one half of the leaf 

 in the direction of upper to lower (ingoing), it will be found on 

 leading off from the other half that an excitatory variation of the 

 leaf-current occurs, as a rule, at closure only. Stronger currents 

 (1 Dan. to 2 Groves), on the contrary, excite on opening also, 

 and with a long closure (30 sec.) produce as the visible sign of 

 persistent excitation a whole series of oscillations of the leaf-current, 

 which occur at irregular intervals during the passage of the current. 



Summation of stimuli may also be demonstrated in the 

 Dioncea leaf, if stimuli (break induction shocks) are used of such 

 low intensity that a single shock is inadequate to produce an 

 effect, the interval between the stimuli being less than 0'4 sec. 

 At 0'5 sec. the result becomes uncertain. This applies both to 

 mechanical and to galvanic effects of excitation. 



The " modified " state of the leaf in which, as we have stated, 



