116 IRRITABLE MOVEMENTS [CH. 



differences in the rate of local growth, and movements of 

 nutation generally, is that in these spontaneous movements 

 the turgescence of the local sets of cells does not result in 

 that fixation and consequent changes which we term 

 growth. We might perhaps say that the cells begin to 

 grow, but relax before the act of growth is consummated. 



It is, however, scarcely worth while to enter further 

 into the discussion of this complex matter of spontaneous 

 periodic movements, because we are entirely ignorant as 

 to how much or how little they can have to do with the 

 physiology proper of the leaf. They must be looked upon 

 as curious phenomena, more or less exuberant in nature, 

 the further study of which is interesting because it may 

 throw unexpected light on the nature of cell-life in general. 



The last remark may also be applied to another set of 

 curious movements of certain leaves, which are usually 

 termed irritable movements though no doubt all move- 

 ments are really irritable because they are usually only 

 induced by mechanical contacts or shakings, which act as 

 stimuli on the turgid cells of the fully-developed leaves 

 when in a suitably irritable, or sensitive condition. 



In certain species of Oxalis, Robinia, Mimosa and 

 other Leguminosae, the fully-extended leaves rapidly pass 

 into a position more or less similar to the nocturnal one 

 if violently shaken, or, in the case of some of the more 

 irritable species e.g. the Sensitive Plant (Mimosa pudica), 

 Oxalis sensitiva if a leaflet or pulvinus is merely touched 

 with the point of a needle, or lightly squeezed between 

 the fingers, &c. In Dioncea it suffices to touch a hair. 



Perhaps the most startling feature in these cases of 

 sensitive plants, is not so much that the contact or shaking 

 suddenly induces the local change of turgidity of the cells 

 of the pulvinus which causes the curvature of the leaf- 

 stalk or lamina, as that this change is propagated from 



