AMEBOID MOVEMENT 103 



(4) The electrical theories. These fall into three classes: 

 (a) The galvanic theory. Amici ('18) suggested that the chloro- 

 plastids floating in the endoplasm of plant cells acted as galvanic 

 cells, setting up currents in the endoplasm which in some way 

 caused the endoplasm to move. Dutrochet and Becquerel ('38) 

 also held to this explanation. A fatal defect of this theory is 

 that streaming occurs in a great variety of cells, myxomycete 

 plasmodia, amebas, stamen hairs of Tradescantia, etc., in which 

 no chloroplastids occur; and there is no ground for assuming 

 that the causes of streaming in cells with chloroplastids is funda- 

 mentally different from that in other cells, (b) The electro- 

 magnetic theory. Velten ('72, '73) and Hormann ('98) are 

 chiefly responsible for the development of the electromagnetic 

 theory. They hold that chloroplastids have an independent move- 

 ment of their own ; but the principal postulate of this theory is 

 that there is electric repulsion between the ectoplasm and the en- 

 doplasm. Ewart ('03) has pointed out, however, that this theory 

 is contradicted by the fact that when streaming becomes very 

 active in Elodea, the ectoplasm becomes exceedingly thin and 

 therefore would show movement in the direction opposite to that 

 of the endoplasm if there were magnetic repulsion between these 

 layers. Moreover, the formation of threads of endoplasm across 

 the central vacuoles in plant cells, and the much branched network 

 of pseudopods in plasmodia and foraminifera would be very dif- 

 ficult if not quite impossible to explain on this assumption, (c) The 

 electro-chemical surface-tension theory of Ewart. As the result 

 of a considerable amount of experiment and observation on endo- 

 plasmic streaming in plants, Ewart ('03) has come to the con- 

 clusion that there are differences in electrical potential between 

 the protoplasm- vacuole boundary and the protoplasm-cell wall 

 boundary, and that as a consequence electrical currents are pass- 

 ing between these points, traversing the protoplasmic stream. If 

 now it is assumed that the particles in the endoplasm, which are 

 electrically polarized, have the surface tension of their correspond- 

 ing ends decreased when electric currents traverse the endoplasmic 

 stream, the particles and, of necessity, the whole stream of endo- 

 plasm would move in the direction of lowered surface tension 

 (Figure 31, p. 96). Continuous chemical actions would be 



