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



particular attitude assumed by the leaves is brought about by transmitted 

 " nervous impulse," which impinges on the motor organ, which is not simple 

 but highly complex ; that there are several distinct impulses which react 

 on the corresponding effectors grouped in the motor organ. 



For a full and satisfactory explanation of the phenomena, it will be 

 necessary to deal briefly with the characteristics of the motor organ and 

 the nervous impulse which actuates it. It will also be necessary to show 

 that the physiological reactions of the " sensitive " and " ordinary " plants 

 are essentially similar. As a type of the former I shall take Mimosa pudica, 

 and for the latter, Helianthus annum. I propose to deal with the subject 

 in the following order : — 



I. — General description of the dia-heliotropic phenomena. 



II. — Characteristics of the motor organ : 



1. Mechanical response due to differential excitability of the pulvinus of 



Mimosa and pulvinoid of Helianthus. 



2. Eesponse to stimulation of adaxial and abaxial halves of the motile 



organ. 



3. The mechanism of heliotropic curvature. 



4. The diurnal movement. 



5. Torsional response to lateral stimulation. 



III. — The nervous mechanism in plants : 



6. Keceptor, conductor, and effector. 



7. Localisation of nervous tissue in plants. 



IV. — The transmitted nervous impulse : 



8. Definite innervation. 



9. The directive action of propagated impulse in heliotropic leaf adjust- 



ment. 



I. — General Description of the Dia-helioteopic Phenomena. 



Before entering into the experimental investigation of the subject, it is 

 desirable to describe the dia-heliotropic phenomena, as typically exemplified by 

 Mimosa and Helianthus. A photograph of the former is reproduced in fig. 1, a, 

 in which the plant placed in a box had been exposed to the northern sky and 

 not to direct sunlight. It will be seen that the leaves which directly front the 

 light have been raised, and so placed that the sub-petioles, with their leaflets, 

 are at right angles to the strongest illumination. The side or lateral leaves 

 have, on the other hand, undergone appropriate torsions — the plane of the leaflets 

 being adjusted perpendicular to the light. It will be noticed that in executing 

 this, the petioles to the right and the left have undergone opposite torsions. 



