pulvinus of mimosa 25i 



Experimental Arrangements 



I will first describe a typical experiment on torsional 

 response under the action of light. It has been shown, in 

 the case of the pulvinus of Mimosa, that light of moderate 

 intensity and of short duration applied on the upper half 

 induces a slow up-movement, while the stimulus of hght 

 apphed below induces a more rapid down-movement. The 

 difference is due to the fact that the lower half of the pul- 

 vinus is relatively the more excitable. Vertical light thus 

 induces a movement in a vertical plane. But an interesting 

 variation of the response occurs under the lateral action of 

 light. A stimulus will be termed lateral when it acts on 

 either the right or left flank of a dorsiventral organ. 



The present series of experiments was carried out with 

 the leaf of Mimosa, and in order to eliminate the effect of 

 w^eight, and also to obtain a record of pure torsion, the 

 following device was employed : The petiole was held by 

 a hooked support made of a thin rod of glass, the points of 

 support being the concavity of a smooth surface. Friction 

 and the effect of weight are thus practically ehminated ; 

 the hooked support prevented up or down movement and 

 yet allowed perfect freedom for torsional response. This 

 latter is magnified by a piece of stout aluminium wire fixed 

 at right angles to the petiole (fig. 144). The end of the 

 aluminium wire is attached to the short arm of a recording 

 lever ; there is thus compound magnification of the torsional 

 movement. The Oscillating Recorder gave successive dots 

 at intervals which could be varied from 20 seconds to 

 2 minutes. Time-relations of the response can thus be 

 obtained from the dotted record. 



The experimental device just described makes possible 

 the study of the effect of various stimuli apphed on the 

 flank of the pulvinus including the junction of the upper 

 and lower halves of the organ. The observer standing in 

 front of the leaf is supposed to look at the stem. Torsional 

 response will then appear as a movement either with or 



