22 



ELECTRO-PHYSIOLOGY 



CHAP. 



is required, in which the positivity of the lower surface diminishes 

 slowly, before the second phase again appears distinctly. The 

 stronger the positivity of the lower surface ab initio, the less 

 can it be increased by excitation of the leaf, and conversely, 

 the plainer will be the primary opposite variation. 



In an unmodified leaf with outgoing leaf-current (Burdon- 

 Sanderson's "descending" current), the variation consequent on 

 excitation is again found, on leading off from opposite points of 

 the respective surfaces, to be diphasic. The first "entering" phase 

 (ascending in the leading-off circuit), which lasts about a second, 

 and in which the upper and previously positive leaf-surface 



PIG. 145. Photogram of the variations of an outgoing leaf-current, on exciting one lobe and 

 leading oft' from the other (cf. Fig. 143). 10 divisions of the time-marking correspond to 1 sec. 

 (Burdon-Sanderson.) 



suddenly becomes negative, is often preceded by a momentary 

 alteration in the opposite direction, as shown in Fig. 145. 

 Here again the opposite (outgoing or descending) " after-effect " 

 (second phase) only appears plainly in the first excitation, and 

 owing to its very slow decline is wanting in those that immediately 

 succeed it. 



The most important conclusion from these observations is 

 that the leaf of Dioncca is excitable in both the unmodified and 

 the modified condition, independent of the direction of the rest 

 current, save that the galvanic effects of excitation are reversed 

 pari passu with the reversal of the current of rest. 



It is evident that the " modifications " of the leaf-current 

 consequent on repeated excitation are only the after-effect of the 

 slowly declining second phase of the excitatory variation. For 

 exact measurements of time, as well as of the E.M.F. of the varia- 

 tion, Burdon-Sanderson employed a pendulum myograph arranged 

 as a rheotome, which in swinging past opened three keys in suc- 

 cession (Fig. 146). Opening Kl caused a "break" induction 



