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 spaces and 

 distends the cells until the pressure is equihbrated by 

 the elastic tension of the stretched cellulose cell walls. 

 Evidently the continued maintenance of 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 pulvini contract, 

 the leaves fall, and the leaflets fold together. Apparently 

 stimulation renders the plasma membrane suddenly 

 permeable to the osmotically active intracellular sub- 

 stances which maintain turgor. This effect is reversible, 

 and under normal conditions turgor is gradually regained. 

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

 tractile stamens of the Cynarece show a behavior essen- 

 tially similar to that of Mimosa. Temporary loss of 

 semi-permeability 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 apply to 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. 



