n8 THEORETICAL AND GENERAL 



in certain cases or under certain conditions may accelerate it in others. Not 

 only may the viscosity of the protoplasm or cell-sap, and hence also the 

 resistance to be overcome, vary, but in addition the relative amount of 

 energy expended in streaming may alter within certain limits. Very prob- 

 ably also the manner in which the energy is utilized is liable to modifica- 

 tion ; and important changes must occur when the protoplast separates into 

 streaming fragments, some of which may contain endoplasm only, or when 

 the direction of streaming is altered, reversed, or undergoes a permanent 

 local retardation. For all these reasons it is impossible to define precisely 

 the conditions which a permanent theory of streaming must fulfil, and it 

 must also take into account the possibility that the movement may not 

 always be produced in an exactly similar manner. 



Assuming that streaming in plants is the result of surface-tension 

 forces, it is interesting to notice that animal physiologists incline to refer 

 muscular contraction to the same natural agency. Thus \ according to 

 Bernstein, muscular contraction is the result of changes of surface-tension 

 produced by chemical action. The changes of surface-tension are supposed 

 to take place in the limiting layers between the walls of the fibrillae and 

 the enclosed sarcoplasm, the effect being to cause the fibrillae to become, 

 shorter and broader. Possibly also an increase in the surface-tension of the 

 sarcoplasm tends to make the cylindrical tubes into which it is broken up 

 assume a slightly more globoidal shape. The purpose of the subdivision 

 of the muscle-fibre into fibrillae is to increase the total surface at 

 which surface-tension forces are active. Even then, however, surface- 

 tension forces amounting to 0-304 gram per centimetre would be required 

 to produce the maximal total force of contraction of which a frog's muscle 

 is capable. The maximal force which a surface-tension film between mer- 

 cury and water can exert is 0-42 gram per cm., and between olive oil and 

 water is 0-021 gram per cm., and is probably even lower than this in the 

 case of the media existing in the muscle-fibre. Hence Bernstein assumes 

 that the fibrillae are broken up into still smaller units, of a probable radius 

 of io~ 6 cm. The resistance to rapid flow of a viscous liquid in tubes of this 

 radius would be enormously great, and indeed it is doubtful whether the 

 highest surface-tension forces would be sufficiently powerful to produce such 

 flow. The theory does not, however, necessarily involve any flow in mass 

 of plasma into and out of the tubes, or even from one part of the tube to 

 another, since both the fibrillar walls and the enclosed sarcoplasm might 

 have the same tendency to shorten and broaden. 



There are, however } fundamental differences between the phenomenon 

 of contractility as exhibited by a muscle-fibre and that of protoplasmic 

 streaming as exhibited in plant-cells covered by a cell-wall or in undiffer- 



1 Naturwiss. Rundschau, 1901, Bd. xvi, Nos. 33-5.. 



