130 COMPARATIVE ELECTRO-PHYSIOLOGY 



An essential condition of this investigation is first to obtain 

 a uniform series of responses. Having once done this, those 

 subsequent changes in the response which are due to the appli- 

 cation of a given reagent can be demonstrated in an unmis- 

 takable manner. I have already explained in Chapter III. 

 that this may be done by either of two different methods : 

 namely, those of direct and of transmitted stimulation. In the 

 first of these we employ vibrational stimulus, using the Method 



Before f After 



FIG. 89. Photographic Record of Effect of Chloroform on Responses of Carrot 

 Stimuli of 25 torsional vibration at intervals of one minute. 



of Block. In the second, the stimulus of thermal shocks is 

 used, the excitation of the proximal contact being due to 

 transmitted stimulation. 



We shall investigate the effect of chemical reagents by 

 both these methods. And first I shall give results obtained 

 by the employment of the Method of Block, the tissue being 

 subject to direct stimulation. In cases where the effect of 

 gaseous reagents, like chloroform, is to be studied, the 

 vapour is blown into the plant-chamber (see fig. 21). In 



