160 



Sir J. C. Bose and Mr. S. C. Guha. 



presently find that the responsive reaction in all dorsiventral organs obeys 

 a definite law in regard to the relation between the direction of incident 

 stimulus and the resulting torsion. The torsion induced is either right- 

 handed or left-handed, clockwise or anti-clockwise. In describing the 

 direction of torsion, the position of the observer in regard to the plant 

 must be definite ; he should stand in front of the responding leaf and look 

 at the central stem. When the right flank of the pulvinus or the petiole 

 is struck by a horizontal beam of light r, coming from the right, the induced 

 torsion is right-handed. Light acting on the left flank induces a torsion 

 which is in the opposite or left-handed direction. On the cessation of 

 stimulus the leaf recovers its normal position (fig. 3). 



R A 





1 M 



L 





• 



V 



Fig. 3. — Eecord of torsional response of pulvinus of Mimosa by lateral light. Stimula- 

 tion of the right flank induced right-handed torsion, It, represented by up-curve : 

 stimulation of the left flank induced left-handed torsion, L. The two thick dots 

 represent the duration of stimulus. Successive dots are at intervals of 20 seconds. 



The response described above takes place when the pulvinus or the 

 petiole is exposed to lateral light, the leaflets of the lamina being 

 completely shielded from it. The differentially excitable organ thus 

 undergoes a twist, in consequence of which the less excitable upper half 

 of the pulvinus is made to face the stimulus. The leaflets or the lamina 

 attached to the petiole are thus carried passively, like so many flags, to face 

 the hypothetical source of light. It is obvious that the response is brought 

 about by a definite physiological reaction and not for the utilitarian purpose 

 of securing maximum illumination of the leaflets or the lamina. Teleo- 



