162 



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



responding pulvinus or petiole are directly exposed to light, the leaflets and 

 lamina being protected from it. 



The heliotropic adjustment of leaves often takes place, as we have seen, 

 when the motor organ is in the shade, or is artificially kept so. There must, 

 therefore, be transmitted impulses by which the distant motor apparatus is 

 so actuated that the leaflets or the lamina are placed at right angles to the 

 light. The transmitted impulse, if single or diffuse, cannot evidently exert 

 the necessary directive action. I shall presently show that the transmitted 

 impulse is of a nervous character, that the impulses are more than one, and 

 distinct from each other, and that they travel by different channels from the 

 lamina which perceives light to the distant motor region where movement is 

 effected in response to transmitted excitation. 



Fig. 4. — Torsional response of petiole of Helianthus in response to (a), electric stimulus, 

 and (b), to sti mulus of light. E and L are the opposite responses, due to stimulation 

 of the right and left flanks. Successive dots are at intervals of 20 seconds. The 

 prolonged latent period under light is not shown in the record. The portion of 

 record exhibiting recovery is also omitted. 



The Complex Character of the Motor Organ. — As regards the motor organ 

 itself, I have stated that it is not simple but very complex. This will be 

 understood from the following experiments on the pulvinus of Mimosa. Let 

 us imagine the pulvinus to be diagonally divided into four quadrants. When 

 the upper quadrant is subjected to light acting from above the responsive 

 movement of the leaf is upwards ; stimulation of the lower quadrant by light 

 acting from below induces a down movement. The stimulation of the right 

 quadrant (right flank) by a horizontal beam of light induces, as we have seen, 

 a right-handed torsion ; the left quadrant responds by a left-handed torsion. 

 The four quadrants may, therefore, be regarded as four independent effectors, 



