The Dia-Heliotropic Attitude of Leaves. 



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extreme case where one half, say the right, is alone stimulated, either by 

 light or by electric shock. The two effectors for torsional movement, the 

 right and the left, are the responding tissues in the right and left flanks of the 

 petiole. These are actuated by the nervous impulses transmitted along the 

 two conducting strands. When the right half of the lamina is stimulated 

 the transmission of excitation along the conducting strand on the right is 

 detected (Experiment 5) by an electric change of galvanometric negativity, 

 and the corresponding mechanical response of the right effector is, as shown 

 before, by a right-handed torsion. We may next inquire the nature of the 

 transmitted impulse along the left flank of the petiole concomitant with the 



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Tig. 12. — Torsional response due to transmitted excitation in Helianthus : (a), right- 

 handed torsion due to electric stimulation of the nerve-ending in the right half of 

 the lamina ; (b), right-handed and left-handed torsions due to transmitted excitations 

 caused by alternate illumination of the right and left half of the lamina. Light 

 was stopped after the thick dot. 



excitation of the right half of the lamina. It is obvious that a similar 

 excitatory impulse on the left flank (the electric indication of which is 

 galvanometric negativity) would oppose and thus neutralise the particular 

 directive movement. Hence for ensuring a steady directive motion, in 

 response to stimulation of the right half of the lamina, all excitatory impulse 

 to the left flank of the petiole should be inhibited. Further, the directive 

 movement induced by the stimulation of the right half of the lamina would 

 be actively helped if the motor reaction of the left flank of the petiole be of 

 an opposite character to that in the right flank. We found that the right- 

 banded torsion is induced by a differential contraction of the right flank 

 and for concordant effect the reaction of the left flank should be opposite, 

 i.e., a differential expansion. The nervous impulse which actuates the right 



b 



