AMEBOID MOVEMENT 101 



phase resulting in gelation, for observation has failed to detect 

 this process going on to any extent whatever in streaming proto- 

 plasm. Further, an increase in the amount of water in the proto- 

 plasm is associated with more rapid streaming. If streaming 

 therefore depends upon a phase change in a colloidal system, it 

 must be in the direction of liquefaction, that is, changing the 

 internal more fluid phase to the external phase. A phase change 

 in one direction would thus lead to contractility, while a change 

 in the other direction would lead to streaming. 



Theories accounting for the intimate nature of the process of 

 streaming without special reference to ameboid movement, have 

 been offered by many botanists. In most plant cells in which 

 streaming movements occur the ectoplasmic covering does not 

 change shape. Streaming of the endoplasm therefore is a much 

 less complicated process in such a case than in an ameba where 

 locomotion is also present. It is to be expected therefore that a 

 theory of streaming based upon observation of a plant cell such 

 as is found in Chara would be different from one based upon 

 observation of a moving ameba. Such is found to be the case, 

 as the following discussion of some of the principal theories ac- 

 counting for streaming in plant cells strikingly shows. 



(1) The contractility theories. Corti ('74), who was the first 

 to record observations on the process of streaming in plants 

 thought that the movement of the endoplasm was caused by waves 

 of contraction passing around the cell in a way analogous to 

 that in which fluid may be passed through a rubber tube by closing 

 the finger over it and passing it along the tube. Heidenhain 

 ('63), Kiihne ('64), Briicke ('64), Hanstein ('80), in one form 

 or another also have expressed their adherence to the contrac- 

 tility theory. More recently Bellinger ('06, p. 356) postulated 

 contractile fibrillae in rhizopods similar to those postulated by 

 Briicke to explain protoplasmic streaming. The contractility 

 theories are no longer considered tenable, for no waves of con- 

 tractility can be demonstrated, as the theories of Corti, Heiden- 

 hain, et al. demand, and contractile fibers can neither be demon- 

 strated nor can they be conceived to exist in endoplasm which 

 exhibits all the essential properties of a fluid. 



(2) The imbibition theories. Sachs ('65), Hoffmeister ('67) 



