6o PLANT RESPONSE 



leaflets. But if this petiole be subjected to any form ol 

 stimulus, it responds by a depression of the leaf as a whole. 

 On the cessation of stimulus there is the usual recovery. In 

 a subsequent chapter I shall describe in detail the responses 

 of the petiole of BiopJiytiiui to different forms of stimulation. 



Conducting properties of various tissues. — With 

 regard to the next point, this is to say, the transmission 

 of excitation to a distance, it will be shown in Chapter XX. 

 that the transmission of the excitatory state is brought about 

 by the propagation of protoplasmic changes from point to 

 point We may therefore expect that tissues in which there 

 is more or less uninterrupted protoplasmic continuity will be 

 those which will, other things being equal, show the greatest 

 power of transmitting stimulus. Even in such cases it will 

 be understood that the transmission becomes enfeebled with 

 distance. Now the tissues in which this continuity is most 

 uninterrupted are the fibro-vascular elements. Hence, stimu- 

 lus applied on the petiole, or its prolongation, will reach the 

 motile organ, with the greater intensity, the nearer the point 

 of application is to the motile region. We should expect, 

 on the other hand, that indifferent tissues, like leaf parenchyma 

 proper, would prove to be, owing to the more or less complete 

 cellular partitions, incapable of transmitting stimulus to a 

 distance. These considerations I have been able to verify 

 experimentally by means of electric response. I shall here, 

 however, describe experiments in which the same conclusions 

 are established by the method of mechanical response. 



Moderately strong electrical stimulus from an induction 

 coil was applied on the petiole of Artocarpus at points 

 increasingly far from the laminal pulvinoid ; these produced 

 motile responses which diminished rapidly with distance, as 

 was described in the last experiment. But when such 

 stimulus was applied on the lamina, at a point relatively near 

 the pulvinoid, there was no response. This shows that the 

 lamina is not to any extent the perceptive organ, and that 

 stimulus received on such an area does not cause movement 

 of the leaf as a whole. 



