CHAPTER V 



MECHANICAL RESPONSE IN ORDINARY LEAVES 



Pulvinoid and pulvinus -Demonstration of mechanical response in ordinary 

 leaves — Response of Ar'.ocarpiis similar to that of Biophytum — Response 

 to stimulus, even in old tissues, by expulsion of water — Localisation of 

 motile organ in ordinary leaves — Conducting properties of various tissues 

 — Lamina is not the perceptive organ — Response in ordinary leaves, though 

 sluggish, yet comparable in extent to that of Mimosa — Peculiar phenomenon 

 of fatigue-reversal seen in Mimosa observed also in ordinary plants — Periodic 

 reversals. 



We have seen, in the course of the last chapter, that even a 

 sensitive plant will fail, under certain conditions, to give any 

 mechanical indication of its state of excitation. We shall 

 now proceed to determine whether, on the other hand, 

 motile response may not be detected in the case of ordinary 

 plants. 



It has already been said that the popular division of 

 plants into sensitive and ordinary is purely arbitrar)\ It 

 is the erroneous impression consequent on the use of these 

 terms which is responsible for the fact that inquirers have 

 accepted without hesitation the assumption on which the 

 classification rests. Had such not been the case, it must 

 long ago have been discovered that the leaves of even 

 ordinary plants respond to stimulus by mechanical move- 

 ments, in precisely the same manner as do those of sensitive 

 plants. 



We have seen that the condition necessary for the pro- 

 duction of lateral responsive movement is, that the organ be 

 anisotropic ; that is to say, there must be a differentiation 

 as between the upper and lower halves. The petioles of 



