DIRECTIVE ACTION OF STIMULUS 253 



right flank, indicated by dotted arrow, the responsive tor- 

 sion was clockwise ; the responsive reaction thus made the 

 upper and less excitable half of the pulvinus face the stimulus. 

 Fig. 145 gives the record of the torsional response ; cessation 

 of stimulation is followed by recovery. 



Fig. 145. Fig. 146. 



Fig. 145. Record of torsional response of pulvinus of Mimosa 

 pudica. Clockwise response to stimulation by light applied 

 on the right flank (up-curve). 



Fig. 146. Record of anti-clockwise torsional response to light 

 appHed on the left flank (down-curve). Duration of apphca- 

 tion between two thick dots. Successive dots at intervals 

 of 10 seconds. 



Directive Action of Stimulus 



Experiment 136. — If now the direction of stimulation 

 be changed so that the light strikes the left flank instead 

 of the right, the torsional response will be anti-clockwise 

 (fig. 146). Here also the responsive movement is such that 

 it is the less excitable upper half of the organ that is made to 

 face the stimulus. It may therefore be concluded that the 

 direction of torsion, clockwise or anti-clockwise, depends on 



