n6 THEORETICAL AND GENERAL 



side would exert a propelling force in the given direction, which would 

 continue as long as the difference of surface-tension was maintained. 

 If the particles were close together and the emulsion confined in a spherical 

 vessel, a regular motion of rotation would be exhibited so long as these 

 conditions were maintained (cf. Fig. 15). 



Now in the typical cell we have the following factors (i) a magnetic 

 membrane, cellulose, surrounding (2) a double layer of a protoplasmic 

 emulsion containing paramagnetic (albumin, chlorophyll, iron salts, &c.), 

 and diamagnetic substances (water, starch, various proteids, oil, salts, &c.) ; 

 (3) the outer and inner surfaces of this layer are at different potentials, 

 since they touch different substances and saline solutions ; (4) continuous 

 chemical action goes on in the layer. We have therefore all the conditions 

 for the production of inwardly or outwardly directed electrical currents, 

 and for the maintenance of a directive action on floating particles composed 

 of dissimilar materials. Suppose that the surface-tension of the ends 

 pointing in a given direction always decreases when an electric current 

 traverses the particles, then the emulsion as a whole will move in this 

 direction, provided that the particles affected are close together and the 

 viscosity of the emulsion not too great. 



The presence of a cell-wall does not form an essential factor for the 

 existence of regular streaming movements, but an organized arrangement 

 of the protoplasmic particles is equally possible in its absence. Streaming 

 in threads presents some difficulties, for the surface of the thread being 

 bathed all round by cell-sap will be practically equi-potential (electrically) 

 at all points. It is, however, possible that electrical currents might 

 traverse the thread longitudinally. Moreover, since the surface-tension 

 pressure is considerable in thin threads, passive propulsion by pressure 

 from behind is possible through them, just as though they were very 

 thin-walled tubes. In this case the streaming would be most active in 

 the centre of the thread, or in the deeper layers if two opposed currents 

 are present. On the other hand if streaming in protoplasmic threads is 

 due to movements of the surface-tension films, then it would be most active 

 in the outer layers. Unfortunately it has not been found possible to 

 determine the existence of any constant difference of velocity between 

 the outer and inner layers, even when the most minute floating particles 

 are used as indicators. Differences of velocity are often shown, but without 

 any apparent common origin ; and in fact in thick threads a portion may 

 be at rest or nearly so. 



In any case, the detailed behaviour of streaming cells in response to 

 weak (acceleration) and strong (stoppage or retardation) electrical stimu- 

 lation lends support to the view that electrical currents are in part concerned 

 in the production of streaming movements. The proof is, however, by 

 no means so certain as Hormann supposes (1. c., pp. 73 seq.), for precisely 



