246 



SCIENCE 



N. S. Vol. XXX. No. 764 



lation in irritable or contractile tissues de- 

 pend primarily on a temporary and readily 

 reversible increase in the permeability of the 

 surface films or plasma membranes of the 

 constituent cells or elements. This evidence 

 is essentially as follows. 



1. Those motile organs and tissues in plants 

 vchere movement is due to changes in the 

 turgor of the cells present perhaps the clear- 

 est case (osmotic motile mechanisms: sensi- 

 tive plants, Dioncea, stamens of Cynares, 

 etc.). In Mimosa pudica (e. g.) the move- 

 ment results directly from a sudden loss of 

 turgor in the pulvinus cells, due to the es- 

 cape of a fluid containing considerable dis- 

 solved matter. This effect indicates very 

 clearly a sudden increase in the permeability 

 of the protoplast in relation to the dissolved 

 substances of the cell-sap. Movement in 

 these plants is excited by the usual stimu- 

 lating agencies and is accompanied by an 

 electrical change or " negative variation " 

 similar to that observed in the irritable tis- 

 sues of animal during stimulation (Burdon- 

 Sanderson). These conditions show (1) that 

 in resting cells, at least of turgid plants, the 

 normal state is one of almost absolute im- 

 permeability to the dissolved crystalloid 

 substances within the cell combined with free 

 permeability to water — otherwise the main- 

 tenance of turgor would be impossible; and 

 (2) that during stimulation this semi-perme- 

 ability is temporarily lost. 



2. In animal cells the evidence of increased 

 permeability during stimulation is less direct. 

 That in this case also a high degree of im- 

 permeability characterizes the plasma mem- 

 branes is shown by the phenomena of plasmol- 

 ysis, by the inability of many dyes to enter 

 the living cell, and by the failure of many 

 dissolved crystalloid substances (sugars, neu- 

 tral salts) appreciably to enter the suspended 

 cells in fluids like blood (Hedin's researches). 

 That stimulation is associated with an in- 

 crease of permeability is indicated by the fact 

 that most stimulating agencies (heat, various 

 chemical substances, mechanical action, elec- 

 trical shocks) also visibly increase the per- 

 meability of pigment-containing cells like 



blood-corpuscles, as shown by their laking 

 action. Direct evidence of increased perme- 

 ability during strong stimulation is also seen 

 in certain favorable organisms, e. g., Areni- 

 cola larvse (see below). 



3. Evidence that semi-permeable mem- 

 branes are concerned in stimulation is seen 

 in Nernst's proof that the stimulating action 

 (s) of alternating currents decreases with an 

 increase in the number of alternations per 

 second according to an apparently quite definite 

 rule (s <= i/\/m, where i = intensity of cur- 

 rent and m number of alternations). This 

 indicates that changes in ionic concentration 

 at the semi-permeable surfaces of the irritable 

 tissue — i. e., at the plasma-membranes — are 

 an essential condition of electrical stimula- 

 tion. A corollary of this theory is that if 

 during stimulation the permeability is in- 

 creased so ^that the semi-permeability of the 

 membranes temporarily vanishes, stimulation 

 should become temporarily impossible; the 

 existence of a refractory period is thus in- 

 direct, but strong evidence of a marked in- 

 crease in ionic permeability at the height of 

 stimulation. 



4. The assumption that stimulation is as- 

 sociated with an increase in the permeability 

 of the semi-permeable membranes also ex- 

 plains the characteristic electrical phenomena 

 of irritable tissues. If the irritable element, 

 e. g., muscle-cell, be regarded as a concen- 

 tration-element in which the potential-differ- 

 ence between exterior and interior is due to 

 a separation of ions at the plasma membrane, 

 which is assumed to be readily permeable 

 during rest to certain cations (probably 

 hydrogen ions) but not to anions (Ostwald- 

 Bernstein " membrane-theory ") — ^the sudden 

 fall of the demarcation-current potential 

 during stimulation (negative variation or 

 action-current) is at once explained by as- 

 suming that at this time the membrane be- 

 comes freely permeable also to anions. 

 Free permeability to ions during stimulation 

 is indicated by the refractory period, as al- 

 ready pointed out. The increase in perme- 

 ability following death or injury is accom- 

 panied by an electrical change similar to 

 that associated with stimulation — a fact which 



