VARIATION OF POLAR REACTION 271 



that of Ab is very marked. This is seen in the fact that while 

 Ab excitation was exhibited by the youngest leaf Li under 

 127 micro-amperes, and by L2 under 5-6 micro-amperes, no 

 such excitation was exhibited by L3 and L4 under a current 

 as strong as 19 micro-amperes. The decline in excitatory 

 efficiency of Am, on the other hand, was but slight ; in Li 

 anode-make excitation was induced by 20 micro-amperes, 

 in L2 by 6*3 micro-amperes, in L3 by 20 micro-amperes, and 

 in L4 by 23 micro-amperes. It will be noticed that in the 

 two leaves L3 and L4 there was no excitation of type Km Ab. 



Apparent Vanishing of Type II, Km Ab 



We will now consider the possibility of the appearance 

 of new types of polar reaction by the progressive diminution 

 of Ab excitation. A reference to Table V. will show that by 



Fig. 128.— Abrupt transition from Type I. to Type III., with the vanishing 



of Type II. 



its greater loss of excitatory efficiency Ab has been reduced 

 to the level of the excitatory reaction of Am. Normally 

 speaking, the excitation induced by anode-break Ab takes 

 place earlier than that caused by anode-make Am. And this 

 is the reason why Km Ab, or Type II., precedes Km Am Ab, 



