662 



PLANT RESPONSE 



enhanced. All these conclusions will be found exactly 

 verified in the records given in fig. 270. In the first part of 

 the left-hand figure we see the normal positive torsional 

 response of J//;//(?j-(3: under the lateral action of light. The 

 excitability of the lower half of the pulvinus was then 



reduced by the local appli- 

 cation of chloroform, at the 

 moment represented by the 

 arrow from below (t). It 

 will be seen that the response 

 now undergoes reversal, 

 owing to the fact that it 

 is the upper side that is 

 the relatively more excit- 

 able. In the figure to the 

 right, again, is shown the 

 effect on the torsional re- 

 sponse of the leaf, of increas- 

 ing the already existing 

 difference as between the ex- 

 citabilities of the two halves, 

 when that of the upper is 

 reduced by the local applica- 

 tion of chloroform. It will 

 be seen that this application as marked by the arrow from 

 above (|) caused an enhancement of torsional response, as 

 seen in the greater steepness of the curve. 



Laws of torsional response. — From these experiments 

 we arrive at the following laws : 



1. An anisotropic organ, when laterally excited, under- 

 goes torsion, by which the less excitable side is made to face 

 the stimulus. 



2. Stimulus remaining constant, the intensity of torsional 

 response increases with the differential excitability. But when 

 the original difference is in any way reduced or reversed, 

 the torsional response undergoes concomitant diminution or 

 reversal. 



Fig. 270. Records showing Modilica- 

 tion of Torsional Response under 

 Induced Variations of Differential 

 Excitabilities in Pulvinus of Mimosa 



In left-hand figure the normal torsion is 

 seen to be reversed by application of 

 chloroform to the lower half (f), 

 reversing the differential excitability 

 of the organ. In the right-hand 

 record the normal responsive torsion 

 is enhanced by application of chloro- 

 form to the upper half ( j ), increasing 

 the natural differential excitability of 

 the organ. 



