104 AMEBOID MOVEMENT 



necessary to maintain the conditions as outlined. This theory 

 accords with the facts so far as it goes, but it does not explain 

 the streaming in threads across the vacuole in the plant cell, thus 

 necessitating two theories for the explanation of streaming within 

 a single cell at the same moment. Moreover a central vacuole 

 of cell sap seems always to be required to fulfill the conditions 

 of this theory, and this, as is readily seen, makes it impossible 

 to apply it to streaming in amebas, myxomycetes, foraminifera and 

 ciliar.es. 



The fundamental cause of streaming is therefore still to be 

 discovered, for neither the theories of streaming as applied to 

 ameba, nor those described above which refer especially to plant 

 cells, are satisfactory. But a significant point in these theories 

 is that with increasing information, they come more and more 

 to demand a colloidal structure in the protoplasm. It is the sur- 

 face energy in the interfaces in the colloidal system which comes 

 to be regarded as the primary source of the energy. But all at- 

 tempts thus far to explain exactly how this energy is utilized 

 have been unsuccessful. Gaidukov's ('10) observation is of some 

 interest, however in this connection. He found that the occa- 

 sional stopping of streaming in cells of Vallisneria is accompanied 

 by a cessation of Brownian movement, which indicates a change 

 from a sol to a gel state. This proves therefore that colloidal 

 changes are possible in streaming protoplasm, and that the gen- 

 eral search for an explanation of streaming along this line is 

 proceeding in the right direction. The researches of Bancroft 

 ('13, '14) and especially of Clowes ('13, '16) on the nature of the 

 change of phase in emulsions are very instructive in this connec- 

 tion ; and it is undoubtedly true that as rapid progress is now being 

 made by the investigation of colloidal solutions as by the direct 

 study of protoplasm, in solving the problem of streaming. 



The problem of the control of the streaming process, which is 

 of course much the most important feature of streaming, will 

 probably be solved, at least in part, when the mechanics of stream- 

 ing is understood. 



