MECHANICAL RESPONSE IN ORDINARY LEAVES 59 



the motile area itself. Next comes that of the possibility 

 of excitation reaching the motile organ from a distance, 

 through certain specific tissues, and the determination of the 

 nature of such tissues. By using the experimental methods 

 which I have described, it is possible to determine these 

 important points. But, before doing this, it will be well to 

 enter upon the theoretical considerations in connection with 

 the subject. 



As regards the first point, it has been shown that any- 

 flexible anisotropic organ is capable of lateral response owing 

 to differential action. In the leaf, when young, the power of 

 movement extends throughout the length of the petiole and 

 its prolongation, the midrib. But when older, as has been 

 said, the motile power becomes localised at the pulvinoids or 

 pulvini. This conclusion is verified by experiment. The 

 young petiole is found motile throughout its length, but when 

 older the existence of certain specialised areas is made evident 

 by the fact of the strong response obtained, when stimulus is 

 applied on such areas, and its rapid diminution on application 

 at gradually increasing distances. For example, in the case 

 of Artocarpus leaf, when this distance from the pulvinoid is 

 increased to I cm. the normal response to moderate stimulus 

 disappears altogether. In this connection it is well to bear 

 in mind the fact, which will be fully demonstrated later, that 

 the distance to which the effect of stimulus is transmitted 

 depends not only on the conductivity of the tissue, but also 

 on the intensity of the stimulus. A moderate intensity 

 of stimulus is only effective in producing motile indications 

 when the application is on, or very near, the pulvinoid. We 

 thus see, as regards motile organs, that there is a strict 

 continuity between those cases in which the property of 

 motility is diffused through a large area, and others in which 

 it is contracted to one or more definite points, this last form 

 culminating in the maximum flexibility of pulvini proper. 



The petiole of the leaf of Biophytum itself gives an excellent 

 example of a diffuse pulvinoid. It is not provided with any 

 specialised pulvinus, such as that of Mimosa, or of its own 



