400 LIFE MOVEMENTS IN PLANTS 



will presently be shown to depend on the direction of incident 

 stimulus. In figure 149, anti-clockwise torsion is recorded 

 as an up-curve ; clockwise rotation is recorded as a down- 

 curve. 



ACTION OF STIMULUS OF LIGHT. 



Experiment 148. — The pulvinus of the leaf was stimu- 

 lated by a horizontal beam of light thrown in a lateral 

 direction ; the areas contiguous to line of junction of the 

 upper and lower halves of the anisotropic organ thus under- 

 went differential excitation. When light struck on the left 

 flank, the responsive torsion was anti-clockwise ; the respon- 

 sive reaction thus made the upper and the less excitable 

 half of the pulvinus face the stimulus. Figure 149 gives a 



Fiw. 149. — Record of torsional response of pulvinus of Mimosa pudica. 



record of the torsional movement when light struck the 

 left flank of the organ ; on the cessation of stimulus the 

 response is followed by recovery. 



DIRECTIVE ACTION OF STIMULUS. 



Experiment 149. — If now the direction of stimulus be 

 changed so that light strikes on the right flank instead 



