112 COMPARATIVE ELECTRO- PHYSIOLOGY 



both sides are subjected to similar stimulus simultaneously ; 

 and the responsive current is now found to flow across the 

 tissue, from the lower to the upper side. The lower is thus, 

 as we expected, the more excitable. Since we can by means 

 of vibration apply measured stimuli, it will be seen that we 

 have here a quantitative method of investigation. Moreover, 

 as the stimulus is applied directly, it is applicable not only 

 to conducting but also to non-conducting tissues. 



If we next take a radial stem or petiole of Cucurbita, and 

 slit it longitudinally, we obtain, in either of the halves, a 

 specimen having an inner and an outer surface. As one of 

 these has been exposed to light and the other protected 

 from it, we should expect to find, on examination, that there 

 has been an induction of physiological anisotropy. As such 

 a specimen is not very well adapted for vibrational stimula- 

 tion, we may use that of pressure. Two moistened rags, in 

 connection with non-polarisable electrodes, pass through two 

 pieces of cork, adjusted on the two surfaces outer and 

 inner at diametrically opposite points. When the inter- 

 posed tissue is now subjected to sudden pressure its two 

 surfaces are excited simultaneously, and the responsive 

 current is found to flow from the inner concave to the 

 outer convex surface, proving that the former was the more 

 excitable. 



We might again use the chemical form of stimulation, 

 and the results obtained by this method will be described in 

 the course of the next chapter. But these forms of stimulus 

 by pressure, or by chemical means are not capable of 

 exact measurement. For quantitative observations, then, it 

 is necessary to employ some other form of stimulus, and the 

 electrical offers us in this respect many advantages. There 

 are, however, in this case many possible disturbing influences 

 to be considered, all of which must be carefully eliminated 

 before the method can be used without misgiving. How this 

 may be done will be shown in a future chapter. For the 

 present I shall describe another method of stimulation which 

 I have been able to bring to great perfection, by which 



