MEMBRANE CHANGES DURING STIMULATION 373 



at the base of each leaflet and petiole. This turgor, as 

 in other herbaceous tissues, is maintained by the osmotic 

 pressure of the cell-contents; this pressure, acting 

 against the semi-permeable plasma membranes, causes 

 the entrance of water from the intercellular s])aces and 

 distends the cells until the pressure is equilibrated by 

 the elastic tension of the stretched ci-lhilose cell walls. 

 Evidently the continued maintenance ol this condition 

 depends on the preservation of semi-permeability. 

 On stimulation there is a sudden loss of turgor, accom- 

 panied by exit of water and dissolved substances from 

 the cells; the stretched cell walls of the puhini contract, 

 the leaves fall, and the leaflets fold together. Aj^parently 

 stimulation renders the plasma membrane suddenly 

 permeable to the osmotically active intracellular sub- 

 stances which maintain turgor. This elTect is reversible, 

 and under normal conditions turgor is gradually regained. 

 The leaves of the Venus' flytrap and the sensitive con- 

 tractile stamens of the CynarecB show a behavior essen- 

 tially similar to that of Mimosa. Temporary loss of 

 semi-permeabihty due to mechanical stimulation seems 

 to be a not uncommon phenomenon in plant cells; 

 Pfeffer cites the '' stimulatory plasmolysis'' of diatoms 

 and other plant cells as cases of this kind, although he 

 apparently hesitates to apply this explanation to the 

 pulvinus of Mimosa} 



The general rules of stimulation ai)])ly U^ these 

 osmotic motor mechanisms of plants, in the same 

 manner as to the excitation-processes of animal tissues. 

 Electrical stimulation, summation, and anaesthesia occur 

 under conditions similar to those described above, 



^Physiology oj Plants, English translation, III, 75- 



