406 LIFE MOVEMENTS IN PLANTS 



light index was attached to the leaflet for magnifying the 

 subsequent torsional movement. A strong beam of parallel 

 light from an arc lamp was thrown down on the pulvi- 

 nule from above ; this fell at the junction of the more 

 excitable outer with the less excitable inner half of 

 the organ, the plane of separation of the two unequally- 

 excitable halves being, as previously explained, vertical. 

 I have shown that under lateral stimulation, a differentially 

 excitable organ undergoes torsion by which the less excit- 

 able half is made to face the stimulus. Since it is the 

 inner half of the organ that is the less excitable, the 

 attached leaflet becomes twisted so as to expose its (former 

 infolded) surface upwards, at right angles to the incident 

 light. 



As a confirmatory test, strong light was made to strike 

 the pulvinule from below with the result that the leaflets 

 exhibited an opposite torsion by which their surfaces faced 

 downwards, so as to be at right angles to light that struck 

 them from below. 



Under normal conditions sunlight comes from above ; 

 stimulation thus takes place at the junction of the two 

 differentially excitable halves of the organ, the plane of 

 separation of which is vertical. The torsion induced makes 

 the less excitable inner half turn in such a way that the 

 inner surfaces of the leaflets are placed perpendicular to 

 the incident light. 



ADVANTAGES OP THE METHOD OF TORSIONAL RESPONSE. 



The torsional response not only affords a uew method 

 of enquiry on the reaction of various stimuli, but it also 

 possesses certain advantages. For instance in studying the 

 response of the leaf of Mimosa under light, the records were 



