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Sir J. C. Bose and Mr. S. C. Guha. 



appropriate movements and torsions. In the afternoon the plant bent over 

 to the west, all the previous adjustments and torsions being completely 

 reversed. The plant continued to exhibit these alternate swings day after 

 day till the movement ceased with age. 



II. — Characteristics of the Motor Organ. 



I have shown elsewhere (2) that there is no essential difference between 

 the response of " sensitive " and " ordinary " plants. I shall now show that 

 all the characteristics of the response of the leaf of Mimosa are also found in 

 the leaf of Helianthus. These will be specially demonstrated as regards 

 normal response and recovery, the response of adaxial and abaxial halves of 

 the organ to stimulus, the effect of direct and indirect stimulus in inducing 

 heliotropic curvature, the daily periodic movements of the leaves, and the 

 torsional response to lateral stimulation. 



1. Mechanical Response due to Differential Excitability of the Pulvinus of 



Mimosa and Petiole of Helianthus. 



In Mimosa, owing to differential excitability of the upper and lower halves 

 of the pulvinus, a diffuse stimulus, such as that of an electric shock, causes a 

 responsive fall from which there is a recovery on the cessation of stimulus. 

 It has been thought that the upper half of the pulvinus is inexcitable. I have 

 shown (3) that this is not the case, since local stimulation by light induces a 

 contraction and resulting up-movement of the leaf. The upper part of the 

 pulvinus is about eighty times less excitable than the lower half. 



Experiment 1. — In Helianthus, the entire petiole acts as a motor organ, of 

 which the upper half is relatively less excitable. Diffuse stimulation by 

 electric shock induces a responsive fall, followed by a recovery on the cessation 

 of stimulus. The response-records thus obtained are very similar to those 

 obtained with the leaf of Mimosa. In Helianthus the reaction is relatively 

 sluggish and the contraction is not so great as in Mimosa. The difference 

 between the two responsive reactions is one of degree and not of kind. 



2. Response to Stimulation of Adaxial and Abaxial Halves of the Organ. 



As stated before, the upper half of the pulvinus of Mimosa responds to 

 application of light by local contraction ; the leaf is thus erected and the 

 movement towards light may be described as positive heliotropism. The 

 leaflets attached to the sub-petioles are thus made to face the light. Under 

 strong and long continued sunlight the excitation is transmitted across the 

 pulvinus, and causes at first a neutralisation, and finally a reversed or 



