214 RESEARCHES ON IRRITABILITY OF PLANTS 



results K'b and Ab. It will be noticed that in the second 

 half of this cycle of the operation we subject the experiment 

 to the further test of corroboration by reversal. 



The make-effect may be observed simultaneously at both 

 anode and kathode at the beginning of the experiment. 

 The break-effect also may be observed immediately after- 

 wards, provided there has been no excitation during make. 

 But if such excitation has occurred, the current may be 

 maintained till the leaf or leaflet affected has re-erected 

 itself, with accompanying recovery of excitability. The 

 time required for this process is from lo to 15 minutes 

 in the leaf of Mimosa and about 3 minutes in the case of 

 the leaflet of Biophytum. It is the sudden variation of the 

 current, and not its continuance, that causes conspicuous 

 excitation. The current may now be interrupted and the 

 break-effect observed. 



If it is desired to study the effect of break, apart from 

 a previous over-continued maintenance of current, con- 

 ceivably inducive of fatigue, we may proceed as follows : 

 By means of the potentiometer-slide already described the 

 current is initiated and brought to a maximum, so gradually 

 as to cause no excitation. The break can now be effected 

 suddenly, in order to note any excitatory effect that may 

 be characteristic. It will be seen that by adopting this 

 method the necessity for maintaining the current, while 

 waiting for the recovery of excitability, is avoided. In 

 order to avoid unnecessary repetition I may state here that 

 I have employed both methods in studying the break- 

 effect, and that as a matter of fact, in ordinary circum- 

 stances and within limits, there is but little modification of 

 normal results induced by the previous maintenance of 

 current, the intensity of which at its maximum is of the 

 order of millionths of an ampere. 



The distinctive effects of Types I. and II. forming the 

 subject of the present chapter, are brought about the former 

 by relatively feeble and the latter by moderate currents. 

 With a given specimen, increasing the e.m.f. from zero, 



