2QO COMPARATIVE ELECTRO-PHYSIOLOGY 



responsive effects in the different preparations are different 

 and that, even in the same preparation, they may be reversed 

 under unknown changes of circumstances. 



It appeared to me, as already said, that much light might 

 be thrown on the questions thus raised by means of an 

 investigation carried out on plants. The most perfect 

 method of experiment here would consist in observing the 

 separate responsive effects on upper and lower surfaces of 

 the preparation. Waller employed single induction shocks 

 for this purpose, observing the after-effect. But in this 

 case, the action of polarisation was not excluded. It 

 would thus be more satisfactory, in order to eliminate 

 this unknown element, to employ either a non-electrical 

 mode of stimulus or an electrical form which would leave 

 no resultant polarisation effect. The latter condition, as 

 we have seen, was fulfilled by the employment of rapidly 

 alternating currents, whose alternating components were 

 absolutely equal. 



As regards the application of a non-electrical form of 

 stimulus, both thermal and mechanical forms may theoreti- 

 cally be employed. Engelmann and others used heated 

 metals in the proximity of one of the electrodes, for the 

 production of thermal stimulus. This, however, has the 

 disadvantage of thermo-electrical variation, due to unequal 

 heating of the two contacts. Besides this, there is also the 

 effect of a rising temperature, which, as we have seen, is 

 opposite to that of sudden variation, the latter alone consti- 

 tuting the excitatory effect. I have already explained 

 how these difficulties may be overcome by using thermal 

 shocks in which a sudden thermal variation is made to act 

 on both contacts at once. The resultant response thus 

 obtained was shown to be determined by the differential 

 excitability of the two contacts under examination. As 

 regards the mechanical mode of stimulation, previous 

 observers have employed pressure or friction. Such stimulus, 

 however, is at best merely qualitative. If it be applied at 

 the contact itself, objections may be taken to the effect, as 



