SURFACE-TENSION THEORY 113 



A satisfactory theory must conform to all the conditions existing 

 within the cell. It appears that the peripheral layer of cell-sap is passively 

 carried along by the vacuolar membrane, which it wets and to which 

 it adheres. The rapidity with which this velocity decreases inwardly 

 depends upon the friction between the successive layers of cell-sap, i.e. 

 upon its viscosity. The energy of motion is derived directly or indirectly 

 from the metabolism of the moving layers themselves, for by sudden 

 plasmolysis it is sometimes possible to cause portions of endoplasm to 

 separate and show streaming for a short time. Since under normal 

 conditions the force or forces inducing movement do not appear to 



FIG. 15. A. Diagram of section of Cham cell, showing rows of emulsion globules in endoplasm. The row o ' 

 s shows the relative velocities of different layers. B. Row of emulsion globules showing surface-tension 



exercise a corresponding reaction against either the ectoplasm or the 

 cell-wall, the movement can hardly have an electro-magnetic origin, but 

 probably belongs to the domain of molecular phenomena. 



The activity of streaming depends upon the magnitude of the forces 

 acting as compared with the resistance to be overcome. As was first 

 shown by Nageli (1. c.), slight localized streaming movements may occur in 

 Desmids and other Conjugatae, as well as in the cells of many Phanerogams. 

 -The movement soon ceases and recommences elsewhere. Berthold (1. c.) 

 has shown that similar movements occur in the interior of an emulsion 



