vi ELECTROMOTIVE ACTION IX YWJKTABLE CELLS 13 



Dutrochet also pointed out that the transmission of excitation 

 depended on the movement of fluid contained in the conducting 

 elements. This view was subsequently confirmed by the experi- 

 ments of Meyen (13), Sachs (14), and Pfeffer (15). Meyen 

 observed that a drop of fluid was exuded on cutting the stalk of 

 3fiiiiosc(, immediately before the excitatory movement of the leaf. 

 This drop of fluid, which starts out instantaneously if the leaf or 

 stalk of Mimosa is wounded, has been an important factor in 

 nearly all the proposed explanations of conductivity, and formed 

 the basis of the "physical" theory. Sachs (I.e. p. 482) concludes, 

 from the rapid exudation of a drop of water from the incised 

 wood, that the fluid in the fibre-vascular bundles stands at very 

 high pressure in a sensitive mimosa, the excitable parenchyma 

 cells of the lower half of the pulvinus being also -in the highest 

 degree turgescent. " The water thus tends on the one hand to 

 exude from the endosmotically over-filled cells of the pulvinus, 

 and on the other, to penetrate them, on account of its high 

 pressure in the woody bundle." In the unexcited plant these 

 pressures are at equilibrium. Incisiug the stalk disturbs the 

 balance, the water in the wood migrates towards the wound, 

 pressure diminishes, and the water filters out of the highly 

 turgescent excitable parenchyma of the pulvinus into the walls 

 of the cells. Here it follows the direction of diminishing tension, 

 and flows towards the woody bundle of the axial cord. The 

 excitatory movement appears along with the diminished turgor of 

 the lower portion of the pulvinus. 



From this point of view the true excitable cells are found in 

 the parenchyma of the lower side of the pulvinus only, where 

 any stimulus renders the plasma permeable to water, which then 

 filters through the cell-membrane into the intercellular spaces. 

 The relation between distant pidvini would thus be purely physical, 

 caused by the tension of a constant mass of water in the woody 

 parts of the plant. There is, however, another alternative, which 

 may seem a priori the more probable. Excitable cells may be 

 present in the vascular bundles also, and propagate the stimulus 

 from joint to joint. This theory finds substantial confirmation 

 in the discoveries of Tangl, Eussow, and Gardiner (Art. d. bot. 

 Inst. zu Wurzbury, iii. 1884) in reference to the connection of 

 adjacent cell-bodies by fine protoplasmic threads. Such a con- 

 nection was actually found to exist between the cells of the 



