CHAPTER IV 



ON CONDITIONS FAVOURABLE TO THE CONSPICUOUS 

 EXHIBITION OF MECHANICAL RESPONSE 



Differences of degree of motile sensibility in sensitive plants so called — Response 

 of anisotropic organ b'-ought about by differential contraction— Production ot 

 response by artificial variation of turgidity — Variation and counter-variation ot 

 turgescence, causing two opposite responsive movements — Differences between 

 hydrostatic and true excitatory effects— Distinction of plants as ordinary and 

 sensitive, arbitrary - Sensitive plants may be excited, yet give no mechanical 

 response— Certain conditions necessary to exhibition of differential response — 

 Balanced action as result of diffuse stimulus on radial organ- Slight 

 differential contraction of pulvinus magnified by long peliolar index. 



By means of the electric mode of investigation detailed 

 in the preceding chapter, I have shown that all vegetable 

 organs, whether of ordinary or of sensitive plants, are excit- 

 able. Hence the supposed absence of motile indications, in 

 ordinary plants, is not to be considered as due to want of 

 sensitiveness, but to the lack of proper conditions for mechanical 

 movement. What these conditions are will be described 

 presently. But before entering upon such considerations, I 

 first wish to demonstrate that even in the matter of the 

 pulvinar motility itself there is no abrupt division, but a very 

 gradual transition, from those plants in which it is scarcely 

 perceptible, to others which exhibit it in a marked degree. 



Range of sensitiveness in sensitive plants. -Three 

 typical instances of plants possessing extremely sensitive, 

 moderately sensitive, and almost insensitive leaflets, arc : 

 (i) Mimosa ptidica, {2) Biophvtnm sensitivmn, ?ind (3) Phil- 

 anthus urinaria. In PJiilanthus the small leaves borne on 

 pulvini are arranged on two sides of the long twig or petiole, 

 in a manner somewhat resembling the arrangement of leaflets 

 in Biopliytiini. The plant appears on casual inspection to be 



