The Dia-Heliotropic Attitude of Leaves. 



177 



which continues till 9 P.M. After this the leaves erect themselves, till the 

 maximum erection is attained at 6 a.m., which is the thermal dawn. The 

 movement of the leaves is then reversed and there is a continuous fall till 

 the thermal noon at 2 p.m. 



A very important motile reaction in the adjustment of leaves is the torsional 

 response to lateral stimulus. The following is the law which determines the 

 directive movement : An anisotropic organ when laterally stimulated by any 

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

 the stimulus. In a dorsi-ventral organ the upper side is, generally speaking, 

 the less excitable side, and the response of such an organ to lateral stimulus 

 may be expressed in the following simple 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. 



The effects described above take place by direct stimulation of light. They 

 also take place under transmitted excitation. 



The motor organ may be regarded as consisting of four effectors; the 

 response of the right effector is by a right-handed torsion, and of the left 

 •effector by a left-handed torsion. The upper and lower effectors respond by 

 rectilinear up-and-down movements. 



The nervous tissue in plants was localised by means of the Electric Probe 

 which was made to pass by successive steps through the petiole. The 

 maximum transmitted excitation was localised at the phloem portion of the 

 fibro-vascular bundle. Hence the phloem functions as the nerve of the plant. 



Excitation at the receptive region is propagated along a definite conducting 

 channel, which is traced from the receptive area in the lamina to the corre- 

 sponding effector in the motor region. 



In a petiole of Mimosa, provided with two sub-petioles carrying rows of 

 leaflets, stimulation of the right row of leaflets by light gives rise to an 

 excitatory impulse which reaches the right effector and induces a right-handed 

 torsion. Stimulation of the left row of leaflets induces the opposite, or left- 

 handed torsion. When both the sub-petioles are illuminated equilibrium is 

 only possible when the entire leaf surface (consisting of the two rows of 

 leaflets) is perpendicular to the incident light. The dia-heliotropic attitude 

 of leaves is thus brought about by distinct nervous impulses initiated at the 

 perceptive region actuating the different effectors. 



In Mimosa with four sub-petioles, illumination of the second sub-petiole 

 induces an up-movement ; that of the third sub-petiole a down-movement. 

 The leaf is thus adjusted in space by the co-ordinated action of four reflexes. 



Eesults similar to the above were also obtained with Helianthus. 



