102 AMEBOID MOVEMENT 



and Englemann ('79) conceived of streaming as being caused 

 by certain constituents of the cell imbibing water and later dis- 

 charging it. Sachs and Hoffmeister thought that waves of im- 

 bibition and extrusion of water passing progressively along the 

 cell was able to cause movement of the protoplasm. Ewart ('03) 

 has shown, however, that as much as 2000 times its own volume of 

 water would have to be imbibed by a cell of Nitella in the course 

 of a day to account for the amount of streaming observed, and 

 that no sign of the extrusion of water could be detected by ob- 

 serving small suspended particles in the immediate vicinity of 

 the cell. Englemann's theory involving a change of shape of his 

 hypothetical supra-molecular "Inotagmas," by the imbibition of 

 water and the subsequent release if it, which was supposed to 

 account for the movement of the protoplasm while streaming, has 

 been considered too hypothetical and too far removed from the 

 realm of experiment to be of real value, either as an explanation 

 or as a working hypothesis. 



(3) The oxidation theory of Verworn. Verworn ('92, '09) 

 has postulated a "Biogen Molecule" which exists only in living 

 protoplasm and dissociates when protoplasm dies into a number 

 of chemical molecules of albumin and other substances. Ameboid 

 movement and streaming generally, according to Verworn, is 

 caused by the lowering of the superficial surface tension in the 

 moving mass of protoplasm followed by streaming of the proto- 

 plasm toward the point of lowered tension. The lowering of the 

 surface tension is brought about by a union of the Biogen Mole- 

 cule with oxygen. With the dissociation of the biogen-oxygen 

 compound, presumably through a respiratory process, the surface 

 tension rises again. This theory does not hold for amebas, for 

 we saw in the preceding pages that the surface tension is higher 

 at the anterior ends of pseudopods than elsewhere on the ameba. 

 And in plants, as Ewart ('03) has shown, oxygen does not seem 

 necessary to the streaming process, for the endoplasm of Cham 

 cells continues to stream for many days in the entire absence of 

 oxygen. It is possible that there would be enough loosely fixed 

 oxygen in the endoplasm of Chara to supply the demands of Ver- 

 worn's theory; but the very hypothetical nature of his theory 

 prevents one from discussing this possibility. 



