CHARACTER OF TRANSMITTED IMPULSE 155 



pulvinus had inflicted on it a blow which had proved as 

 effective as if a mechanical stimulus had been applied 

 locally. It is thus held that in Mimosa there is a mere 

 transmission of stimulus but no transmission of excitation. 



I shall, however, be able to show that the two experiments 

 referred to are not as conclusive as has been supposed. But 

 before doing this it is as well to examine the basis of the 

 hydro-mechanical theory. According to the Dutrochet- 

 Pfeffer theory, migration of water is the sole cause of pro- 

 pagation of stimulus, transmission being due to movement 

 of water in the vascular bundles. When a wound is made 

 in the stem causing an incision of a vascular bundle, fluid 

 exudes from the wound on account of which there is a 

 diminution of the hydrostatic equilibrium in the bundle. 

 There may, again, be a propagation of stimulus caused by 

 exciting the pulvinus, when a certain quantity of fluid 

 given out by the excited parenchyma is supposed to pass 

 into the vascular bundle. 



According to Haberlandt, flaccidity ensues in the sensi- 

 tive parenchyma on direct stimulation of an articulation. 

 Owing to the deformation of the cells a pressure is induced 

 in the conducting-tissues which is propagated along them 

 and which, reaching a new pulvinus, stimulates it as if by a 

 blow from without. Haberlandt compares the transmission 

 of pressure in the plant with that in an indiarubber tube 

 filled with water, 1 



But transmission through long, and more or less closed, 

 capillary tubes is not the same as that through uninterrupted 



^ ' 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 stimu- 

 lus or with chemical or thermic excitation. . . And when Haberlandt 

 compares the resulting 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 to the other," the anatomical relations of the con- 

 ducting-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.' — Biedermann : Electro-physiology, 

 vol. ii. p. 16 (Macmillan). . . 



