The Dia-Heliotropic Attitude of Leaves. 



161 



logical considerations, often adduced, offer no real explanation of the 

 phenomena ; such arguments are, moreover, highly misleading, for similar 

 responsive torsion is induced, not merely by light, but by modes of stimu- 

 lation so diverse as electrical, thermal, geotropic, and chemical. 



Eesponse of all anisotropic organs to lateral stimulus is included in the 

 following generalisation : — 



An anisotropic organ, when laterally excited by any stimulus, undergoes 

 torsion by which the less excitable side is made to face the stimulus. 



In a dorsiventral organ, it is the upper side which is, generally speaking, 

 the less excitable. Hence the above generalisation may be expressed in 

 the following simpler terms : Lateral stimulation of a dorsiventral organ 

 induces a torsion which is right-handed, when the right flank is stimulated. 

 Left-handed torsion is induced by the stimulation of the left flank. 



Torsional response of petiole of Helianthus. — The above generalisation finds 

 independent support from the response of the petiole of Helianthus to 

 various stimuli applied laterally. 



Experiment 2. — Two fine pins are thrust about 1 cm. apart on the right 

 flank of the petiole of Helianthus, to serve as electrodes for application of 

 induction shocks from a secondary coil; a similar pair of electrodes are 

 attached to the left flank. On application of ,a feeble tetanising shock to 

 the right flank, the petiole exhibited a right-handed torsion ; stimulation of 

 the left flank induced a left-handed torsion. Electric stimulation quickly 

 stirs up the internal tissues, hence the latent period is short, and the 

 responsive reaction is rapid (fig. 4, a). I next took a different specimen, 

 and applied the stimulus of light to the right and the left flanks alternately. 

 This gave rise to right- and left-handed torsions as under electric stimulus, 

 the only difference being in the slower reaction and prolonged latent period 

 (which was 15 minutes) (fig. 4, b). It must be remembered that in the case 

 of light the excitation is gradually transmitted from the outer surface to the 

 inner tissues. As regards the direct action of light, the results given above 

 show that the responsive reactions of sensitive and ordinary plants are not 

 different, but essentially similar. With reference to the heliotropic adjust- 

 ment of leaves, we found that, when light strikes symmetrically in front, 

 the leaf bends towards it. The growing stem itself is excitable, and its 

 induced curvature is a contributory factor in placing the surface of the 

 lamina at right angles to the light. Leaves struck laterally by light undergo 

 torsion which is definite, being determined by the direction of the incident 

 light. The torsion thus induced places the leaflets of the lamina at right 

 angles to the light. These effects are produced, as stated before, when the 



