304 COMPARATIVE ELECTRO-PHYSIOLOGY 



current, which exhibit a complete cycle in the course of 

 thirteen minutes. Thus, as a consequence of the after-effect 

 of stimulus, a cyclic variation of relative conditions is 

 induced, as between any two anisotropic surfaces, such as 

 those of skin or pulvinus. This cyclic variation of relative 

 conditions is indicated by the concomitant variations induced 

 in the resting-current, shown in the shiftings of the base line. 

 I have been able, further, to demonstrate the interesting 

 fact that such phasic variations are capable of exhibition 

 even through mechanical response. I have already ex- 

 plained that autonomous pulsations, such as those of the 

 lateral leaflets of Desmodium gyrans, may be regarded as 

 the after-effect of stimuli previously absorbed and held 

 latent by the tissue. In taking the record of a series 

 of such pulsations, I have often found phasic variations 

 to occur, similar to those obtained with long-continued 

 response of skin or pulvinus. If, for example, the lower 

 half of the pulvinus of the lateral leaflet of Desmodium 

 undergoes an increase of turgidity above the average, that 

 half will become more convex, and the base-line of the 

 record will be correspondingly tilted. The converse will 

 take place under the opposite change. Thus the phasic 

 variations shown in the record (fig. 185) clearly indicate 

 that the relative turgidities of the two surfaces of an 

 anisotropic organ may undergo a periodic change. The 

 corresponding electrical expression of this we have seen in 

 the variation of the current of rest. This variation may 

 sometimes be so great as actually to reverse the normal 

 current of rest. Thus, while under normal standard con- 

 ditions the resting-current in the pulvinus of Mimosa is from 

 the upper half to the lower, across the organ, this normal 

 direction may sometimes be found to be reversed. 



It may now be asked, What is it, in the case of the skin, 

 which determines the respective directions of the resting- 

 current and the current of response? We have seen that 

 the current of rest in the frog's skin, from outer to inner, is 

 generally attributed to the possession of glands by the outer, 



