;^98 LIFE MOVEMENTS IN PLANTS 



The expei-imeuts which I shall presently describe will, 

 it is hoped, throw light on the obscure phenomenon. I shall 

 be able to show : 



(1) that the torsional response is not dependent on the 



combination of two curvatures, 



(2) that it is also independent of the effect of weight, 



(3) that it may be induced not merely by stimulus of 



lignt but by all forms of stimulation, 



(4) that the direction of the tor.sional response depends 



on the direction of the incident stimulus and the 

 differential excitability of the organ, acd 



(5) that there is a definite law which determines the 



torsional movement. 



EXPERIMENTAL ARRANGEMENTS. 



I shall first describe a typical experiment on the responsive 

 torsion under the action of light. We have seen that in the 

 pnlvinus of Mimosa, light of moderate intensity and of short 

 duration applied on the upper half iuduces a slow up-move- 

 ment, while the stimulus of light applied below induces a more 

 rapid down-movement. The difference is due to the fact that 

 the lower half of the pulvinus is relatively the more excitable. 

 Vertical light thus induces a movement in a vertical plane. 

 But an interesting variation is induced in the response under 

 the action of lateral light. A stimulus will be called lateral 

 when it acts on either the right or left flank of a dorsiventi'al 

 organ. We shall presently find that a dorsiventral organ 

 responds to lateral stimulus by torsion. 



The present series of experiments were carried out with the 

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

 and also for obtaining record of pure torsion, I employed the 

 following device. The petiole was enclosed in a hooked 

 support made of thin rod of glass, the petiole resting on the 

 concavity of the smooth surface. Friction and the 



