16 ELECTRO-PHYSIOLOGY CHAP. 



the traction corresponding with a single impact is sufficient, an 

 excitatory movement will be discharged, and the cells contract- 

 ing from loss of water stimulate all the other excitable cells of 

 the joint by means of the pull they exert " (Haberlandt, I.e. 

 p. 53). It is still harder to explain the mechanism by which 

 a stimulus is propagated from the relaxed parenchyma of the 

 curving pulvinus to the excitable parenchyma of an adjacent 

 joint, after a single mechanical stimulus, or with chemical or 

 thermic excitation. In this case the pressure associated with 

 the relaxation of the excitable half of the joint, and resulting 

 curvature, could alone effect a possible disturbance of hydrostatic 

 equilibrium in the conducting system, adequate for the trans- 

 mission of a stimulus. And when Haberlandt compares the re- 

 sulting movement of the sap " to that within an india-rubber tube 

 containing water at a given hydrostatic pressure, in which increase 

 of pressure at any point is propagated in the form of an undulatory 

 wave from one end of the tube to the other," the anatomical 

 relations of the conducting cells hardly seem to justify such a 

 presumption. The experiments on the conductivity of Mimosa 

 would have to be scrupulously repeated before forming any final 

 judgment, and the galvanic effects of excitation might prove a 

 convenient instrument for further investigation. 



However this may be, it is in other cases certain that con- 

 ductivity depends upon excitation of the plasma of the connected 

 cells : and this must be true of Dioncea. 



As in Mimosa, the visible excitatory movements are effected 

 by migration of water, and the normal position of the non- 

 excited leaf is the result of equilibrium between two forces, 

 one of which tends to close the leaf, the other to open it. The 

 cells of the upper surface of the resting (open) leaf are highly 

 turgescent, like those of the under side of the pulvinus in 

 Mimosa. If, as was observed by Munk, we picture the cushion 

 of the primary leaf-stalk of Mimosa as flattened out superficially, 

 with the characteristic veins in place of the wood-mass, we have 

 physiologically a lobe of Dioncea, save that the excitable side is 

 turned downwards ; while if two such altered pulvini are imagined, 

 so connected at a right angle that the excitable parenchyma of 

 the swellings is uninterrupted at the point of junction, the entire 

 leaf is practically before us. 



The upper layer of cells exerts pressure upon the lower, so 



