RESPONSE TO STIMULUS OF LIGHT 



399 



in the snail's heart, under a sufficiently high degree of 

 internal hydrostatic pressure. 



As regards conduction in general, we know that a 

 strong stimulus is transmitted to a greater distance than a 

 weak. In the next record (fig. 235) this may be seen in an 

 interesting manner. At first a moderate stimulus was em- 

 ployed, and this gave rise to (a) a maximum positive varia- 

 tion of the distal point B. The stimulus was then increased 

 and we observe that (b) the 

 excitatory effect, now reach- 

 ing B, causes a reversal of 

 the curve, owing to induced 

 galvanometric negativity. If 

 at the beginning we had used 

 a stimulus of fairly strong 

 intensity the first effect would 

 have been a positivity of B, 

 due to the indirect effect of 

 stimulus ; and, secondly, the 

 excitatory effect would have 

 reached the point gradually, 

 neutralising and afterwards 

 reversing the first. I shall 

 now describe the correspond- 

 ing effects, both mechanical 

 and electrical, which are in- 

 duced by stimulus of light. 

 We take a Mimosa plant, and 

 subject only the upper half of 

 one of its primary pulvini to the action of sunlight. The 

 effects thus induced are (i) the local contraction of the excited 

 upper half, and the expansion of the lower half by the hydro- 

 positive effect ; (2) the gradual percolation of true excitation 

 to the lower half, and consequent initiation of excitatory con- 

 traction there ; and (3) the continued action of excitation 

 and increasing contraction on the more excitable lower half. 

 All these effects are exhibited in the mechanical response 



FIG. 235. Record of Different Specimen 

 under same Experimental Arrange- 

 ment when Stimulus is first Moderate 

 and then Increased 



(a) Positive response, due to hydro- 

 positive effect ; this is converted to 

 negative in (6) due to transmission of 

 excitatory effect under stronger stimu- 

 lation. 



