272 RESEARCHES ON IRRITABILITY OF PLANTS 



or Type III. But now when Am is equal to Ab the latter 

 cannot precede, and the two occur at the same minimal 

 intensity of current. Hence the sequence of excitation 

 with increasing current is I., Km ; II., absent ; and III., 

 Km Am Ab. A series of records are shown in fig. 128, in 

 which Type I., Km., passes abruptly to Type III., Km Am Ab, 

 as the acting e.m.f. is slightly increased from 5 to 6 volts. 

 It may be stated here that late in the season, and with 

 somewhat old leaves, I was frequently puzzled by this 

 vanishing of Type II. ; but the quantitative explanation 

 which has just been given will adequately account for this. 

 We have seen that under certain conditions the 

 Ab excitation declines at a greater rate than that of 

 Am, and have considered the resulting effect when the 

 two become equal. In certain circumstances it is quite 

 conceivable that this relative decline of susceptibility to 

 excitation by anode-break might proceed further. It 

 may then happen that Am becomes more effective than 



Table VI. — Modifications of Normal Types by Decline of Ab in 

 Excitatory Efficiency 



Ab. The sequences of excitatory effects would then be 

 I., Km ; II'., Km Am ; III., Km Am Ab. It will here 

 be noticed that to avoid adding to the number of types, 

 I have designated Km Am- — which takes the place of 

 Km Ab — as II'. or transitional II. In the accompanying table 

 these theoretical modifications of type, due to the decline 

 in excitatory efficiency of Ab, will be seen displayed in a 

 convenient form. Under normal conditions the excitatory 



