270 RESEARCHES ON IRRITABILITY OF PLANTS 



with Li and L2. The mode of procedure was to apply a 

 current which was gradually increased till successive types 

 of responsive reactions were observed in one or other of 

 the two leaves : — 



It is apparent that while a maximum current of 8 micro- 

 amperes induced responses of Type I. in the very young 

 leaf Li, a very much feebler current, of 5*6 micro-amperes 

 was sufficient in the case of L2 to induce Type II. A 

 similar relative exaltation of effect in slightly older 

 specimens occurs in subsequent types also. Thus while 

 127 micro-amperes gives rise in Li to Type II., half that 

 intensity is enough to induce Type III. in L2. And lastly, 

 while 20 micro-amperes gives Type III. in Li, the same 

 current gives Type IV. in L2. 



Having thus verified our inference with regard to the 

 relative excitabilities of two young leaves, we may expect 

 contrasted results with older leaves, where the excitability 

 will be on the wane. Table V. shows the comparative 

 effects of increasing currents on leaves of the same plant, 

 L3 and L4, which are old and very old respectively. For 

 greater facility of comparison the effects on Li and L2 are 

 repeated. 



Table V. — Polar Effects of Currents on Leaves of 

 Different Age 



From this experiment it will be seen, (1) that the general 

 excitability, having reached a maximum in L2, has under- 

 gone a progressive decline with age ; (2) that while the 

 excitatory efficiency of Km has undergone but slight decline, 



