5 o PHYSICS OF STREAMING 



If, however, the pole-pieces are placed 5 to 10 mm. apart, and the cells 

 are arranged so that the direction of streaming cuts the lines of force at 

 right angles, distinct effects are produced. The ultimate effect is always to 

 retard streaming, although curiously enough the first effect may be either 

 to produce a slight acceleration or a slight retardation. 



In another case, after one hour streaming was slow and irregular and 

 ultimately ceased, the cell dying. This happened frequently after very 

 long exposures, not only with Chara, but also with Nitella, Elodea, and 

 Vallisneria, provided that the direction of streaming cut the lines of force 

 at right angles, or nearly so. 



The passage of the streaming plasma across the lines of force in the 

 field causes a weak constant current to pass around the short axis of the 

 cell, and since the medium is not homogeneous, a slight electrolytic effect 

 will be exercised at various points. The velocity of streaming is relatively 

 extremely slow (i to 3 mm. per min.), and hence the current is proportion- 

 ately weak, but Hermann a has shown that a current of 0-000,000,030,884 

 ampere suffices to cause a temporary stoppage in a highly excitable cell 

 of Nitella of diameter 610 /x. Moreover, the excitability is increased by 

 previous electrical excitation, and hence it is easy to understand the retard- 

 ing effect produced by prolonged exposure in a magnetic field under the 



1 Protoplasmastromung bei den Characeen, Jena, 1898, p. 65. 



