264 RESEARCHES ON IRRITABILITY OF PLANTS 



of the reversal is therefore a presumptive evidence for the 

 occurrence of secondary poles. 



It may be asked next whether the effects seen in Mimosa 

 as Types III. and IV., opposed as these are to Pfluger's 

 Laws, could be explained away by any of the physical 

 conditions of the experiment. In answer to this it may 

 be pointed out that in the experiments described, which 

 were carried out according to the bi-polar method, the 

 electrical connections were made directly on the two 

 contractile pulvini themselves, the thread forming a 

 complete loop round each organ, which was thus equally 

 and throughout its circumference anode or kathode, as 

 the case might be. The organ in this case, moreover, 

 was isolated and free, like the insulated ureter. Hence 

 there was an absence of all those conditions which might 

 favour the formation of secondary poles. Had there 

 been any such possibility we should have expected to 

 see the reversal of normal effects, more or less, from the 

 beginning, and the reversal should have occurred at the 

 two bi-polar contacts simultaneously. 



Instead of this, we found on the contrary that in the 

 first two stages, with feeble and moderate currents, the 

 reactions of the organ were absolutely normal. It was only 

 after this, with the same specimen and with identical 

 connections, that by merely increasing the current we 

 obtained the effect characteristic of the third type — 

 namely simultaneous excitation of kathode and anode at 

 make, and excitation of anode at break. Had there been 

 any induction of secondary pole, the result would have been 

 excitation at anode at make and at kathode at break. 



From these considerations, in addition to others already 

 given, it appears that the normal effect of strong currents 

 on Mimosa and other sensitive plants is to cause excitation 

 at both kathode and anode at make, and anode only at break. 

 These results, and those of Type IV. which immediately 

 succeed, are definite and different from either of the two 

 types that had previously occurred. These characteristic 



