H 4 THEORETICAL AND GENERAL 



of Canada balsam with a solution of sulphur in carbon bisulphide, as the 

 latter evaporates. It does not, however, follow that the cause of movement 

 corresponds even approximately in the two cases, and, as a matter of fact, 

 evaporation is impossible in Diatoms submerged in water, although diffusion 

 or chemical action might play the same part in producing localized 

 changes of surface-tension. 



Berthold regards the regular rotation in vacuolated cells as being 

 due to differences of surface-tension in the limiting layers of cell-sap and 

 endoplasm, and considers irregular streaming to be of similar nature to 

 the movements in emulsions. In the former case, since both protoplasm 

 and cell-sap have each their own surface-tension films, being non-mixible 

 fluids, streaming can only be induced by movements of the films transferred 

 by friction to the neighbouring layers of protoplasm and cell-sap. It is 

 not easy to see how continuous movement in a definite direction could 

 be produced in this manner, for any localized increase of surface-tension 

 would exert a centripetal attracting force upon all the surrounding regions 

 of the film, and if the surface-tension were progressively increased around 

 the cell, after a few rotations the breaking strain of the film would be 

 reached, and a stoppage would ensue. 



If, however, a series of rapid waves of increased surface-tension 

 passed around the cell in a particular direction, this might induce a general 

 movement of the film, and with it the protoplasm in a definite direction. 

 Such differences of surface-tension might be produced by the inward 

 and outward diffusion of dissolved substances, and hence would also affect 

 the surface-tension film of the cell-sap. A surface-tension film of water 

 against air can resist a maximum strain of 80 dynes per linear centi- 

 metre. Taking the force acting on a gramme of moving plasma as being 

 from 10 to aoo dynes, a simple calculation showed that in the case of 

 particular cells, of Nitella, Ckara, and Elodea, the required strains upon 

 the surface-tension films were always much below this limit. For example, 

 in one case a gramme of plasma represented an internal surface of 

 42 sq. cms., which required a strain of from 15 to 30 dynes per linear 

 centimetre to give the required propulsive force. There is, therefore, no 

 physical obstacle to this theory, but there is a very serious practical one. 



The density of the cell-sap is somewhat less than that of the proto- 

 plasm, but on the other hand it has a very much lower viscosity. Hence 

 the rapidity of movement in the outer layers of the cell-sap should be less 

 than that of the inner layers of the protoplasm, since the ' slip ' is greater 

 in the former case. For the same reason the velocity should decrease 

 more rapidly inwardly than that of the protoplasm does outwardly. This 

 is actually the case, but on the other hand, the velocity of small floating 

 particles in the endoplasm of Chara and Nitella can usually be seen to 

 increase from within outwards until a maximum is reached at a 



