56 COMPARATIVE ELECTRO-PHYSIOLOGY 



circumstances, stimulus will induce a positive turgidity- 

 variation. If we now apply moderate stimulus on the 

 stem of Mimosa, at a certain distance below the pulvinus, an 

 excitatory expulsion of water will occur at the point directly 

 stimulated. Such an expulsion of water will then cause, it 

 is clear, a hydrostatic disturbance of increased pressure. 

 And this hydro-mechanical disturbance will be transmitted 

 with relatively great velocity. Now such an increase of 

 pressure, as we have seen, causes an increase of turgidity at 



the pulvinus, in consequence of 

 which the leaf ought to be erected. 

 And although this hydrostatic 

 disturbance is transmitted very 

 quickly, yet a certain time is con- 

 sumed in the process of forcing 

 water into the pulvinus, by which 

 to bring about the erection of the 

 leaf. After the passage of the 

 hydrostatic wave, there follows 

 FIG. 42. Experimental Arrange- the wave of true excitation, passing 



ment. for obtaining Records f rom ce jl to ce U anc j inducing the 

 on Smoked Drum of Responses 



given to Direct and in- characteristic reaction, of negative 



*mmofa imulation ' by Leaf f turgidity - variation. And when 



Thermal stimulator at s produces this excitatory wave reaches the 



direct stimulation and conse- pu lvinus, the previous erectile 



quent fall of leaf. Moderate L f 



stimulation, at a distant point, movement should give place to 



of erectTorT * ^^ ***** excitatory depression, or fall of 



the leaf. In fig. 42 are seen the 



arrangements for an experiment on Mimosa by which these 

 inductions may be verified. Moderate thermal stimulus is 

 applied at S, at a certain distance below the indicating leaf. 

 This latter is attached by a thread to a writing-lever, which 

 traces the response-record on a smoked revolving drum. 

 When the stimulus is applied at a point S very near the 

 pulvinus, the response takes place by a negative turgidity- 

 variation, with a concomitant fall of the leaf, seen in fig. 43 

 (a) as an up-curve. When a moderate stimulus is applied 



