ELECTRO-CHEMICAL SURFACE-TENSION THEORY 119 



entiated gymnoplasts. Even although the same natural forces are 

 ultimately responsible for the movement in both cases, their origin, mode 

 of application, and also the intrinsic character of the mechanism itself may 

 differ widely. The dogma that all protoplasmic movement must neces- 

 sarily be produced in a similar manner has even less justification than the 

 older one which caused the existence of anaerobic organisms to be denied 

 for a time. It is, indeed, doubtful whether any streaming movements in 

 mass of the plasma of a muscle-fibre do actually occur, and the small size 

 of most animal cells militates against the existence in them of any active 

 internal currents in mass. The muscle-fibre has become specially differ- 

 entiated for the performance of external work by external change of shape, 

 but in the case of a streaming plant-cell no such active external change of 

 shape is possible, and the work done is entirely internal. In the first case, 

 the work done is intermittent and partly performed against the elasticity 

 of the fibrillar net-work, whereas in the plant-cell the work is continuous 

 normally, and is done against friction only. Moreover, it is by no means 

 contrary to the theory of Natural Selection to suppose that these two forms 

 of protoplasmic movement may possibly have arisen independently, just as 

 various modes of locomotion have been independently acquired by different 

 groups of plants and animals. This might still be the case even although 

 in both cases the same form of energy was utilized for the production of 

 movement. The wings of insects, birds, and bats afford, for example, 

 instances of mechanisms of similar function, but dissimilar origin and struc- 

 ture, all three of which are driven by similar muscular energy and act 

 against the resistance of the same medium. 



It may, therefore, well be impossible to formulate any one theory 

 which shall apply to all the known forms of protoplasmic movement, and it 

 is extremely unsafe to make broad generalizations from data which may be 

 really of limited and special application, or to attempt from observations 

 made upon animal cells to deduce the conditions for protoplasmic move- 

 ment in plants. 



Summary of Results. 



The energy of movement is generated in the moving layers them- 

 selves, and these are retarded by friction against the non-moving ectoplasm, 

 and to a much less extent by friction against the cell-sap which is passively 

 carried with the stream. 



The motor-mechanism is such that no backward reaction is produced 

 on the external or internal layers. 



The velocity of streaming is largely dependent upon the viscosity 

 of the protoplasm, and hence also upon the percentage of water in the 

 latter, but the osmotic pressure exercises little or no direct influence upon 

 streaming. 



