MECHANICAL RESPONSE IN ORDINARY LEAVES 55 



Later, we find the pulvinoid localised at one or two points 

 only, such as the stem and laminal junctions, the motile 

 function persisting here for some considerable period. And 

 finally, in the markedly motile pulvini of the so-called 

 ' sensitive ' plants, we have the property of motility manifested 

 throughout a still greater length of time. It will thus be 

 seen that we can scarcely draw any sharp line of demarcation 

 between pulvini proper and the pulvinoids of ordinary 

 plants. And when the leaves arc old, both alike cease to be 

 motile. 



Mechanical response of Artocarpus. — I shall now 

 describe the method by which I have obtained records of 

 these mechanical responses in ordinary leaves. For this 

 purpose I took a pot-grown specimen of Artocarpus integri- 

 folia, or Jack-fruit plant, the leaves of which are stiff and, as 

 far as the eye can judge, singularly inappropriate for the 

 exhibition of motile effects, and selected for my investigation 

 the third leaf from the top of a stem, this being neither too 

 young nor too old. I have said that in an anisotropic petiole 

 the response ought to take place by the induced concavity 

 of the more excitable half, whether upper or lower. In this 

 case it was impossible to know from inspection which was 

 the more excitable. But I have described, in the last chapter, 

 an electrical method by which differences of excitability, arising 

 from molecular or anatomical differentiation, can be distin- 

 guished. It was there explained that the electrical current 

 of response flows from the relatively more to the relatively 

 less excitable. And on repeating the electric experiment, 

 in the present case, I found the responsive current to flow 

 from below upwards. This proved that in Artocarpus the 

 lower surface of the petiole was, as in the case of Mimosa, 

 the more excitable of the two. Hence, if we should obtain 

 the mechanical response from this apparently non-sensitive 

 leaf, we might expect that it would be downwards. In order 

 to produce stimulation, I used the electro-thermic stimulator, 

 in the manner already described. The stimulus was first 

 applied at a point on the petiole 3 mm. from the laminal 



