CHAPTER III 



THE APPLICATION OF QUANTITATIVE STIMULUS AND 

 RELATION BETWEEN STIMULUS AND RESPONSE 



Conditions of obtaining uniform response Torsional vibration as a form of 

 stimulus Method of block Effective intensity of stimulus dependent on 

 period of vibration Additive action of feeble stimuli Response recorder 

 Uniform electric responses List of suitable specimens Effect of season on 

 excitability Stimulation by thermal shocks Thermal stimulator Second 

 method of confining excitation to one contact Increasing response to increas- 

 ing stimulus Effect of fatigue Tetanus. 



A QUALITATIVE demonstration has been given in the last 

 chapter of the induction of galvanometric negativity in plant 

 tissues, in response to the excitation caused by various forms 

 of stimulus. This galvanometric response is thus a sign or 

 indication of the state of excitation ; and under normal con- 

 ditions it will be of uniform extent, provided only that the 

 stimuli are also uniform. Assuming this ideal condition to 

 be secured, it is clear that the physiological modifications 

 induced by various agents will be manifested by a corre- 

 sponding modification of response. The conditions essential 

 to such application of stimulus are, then, (i) that it should be 

 capable of uniform repetition ; (2) that it should be capable 

 of increase or decrease by definite amounts ; and (3) that it 

 should be of such a nature as to cause no injury, by which 

 the excitability of the tissue might be changed in some 

 unknown degree. These conditions, on which the success of 

 the electro-physiological investigation depends, are very 

 difficult to meet. Chemical stimulation, for example, cannot 

 be uniformly repeated. Electrical stimulation, again, which 

 has the advantage of being easy to render quantitative, is 

 open to the objection that by escape of current it may induce 



