II2 THEORETICAL AND GENERAL 



of summation with and antagonism to the action currents on the opposed 

 sides of the cell. 



The fatal objection to the theory is that streaming is not directly 

 affected in a strong magnetic field however the cell may be placed. 

 Moreover, streaming in protoplasmic threads is very difficult to explain 

 on the above hypothesis, especially when, as in very fine threads, the 

 streaming is all in one direction 1 . 



In very fine threads, however, the inwardly directed surface-tension 

 becomes considerable relatively to their bulk, so that the threads behave 

 like thin-walled elastic tubes, and a passive propulsion of the plasma 

 through them by the pressure behind becomes possible. 



SECTION 51. Surface-tension Theory. 



This was first propounded by Berthold 2 , who considers that all proto- 

 plasmic movements can be directly referred to simple physical causes, 

 such as differences of surface-tension and the like. Thus he compares 

 the movements of Amoebae and of swarm-spores with the movements 

 of drops of oil in an emulsion, or of water or benzine in the presence 

 of alcohol, or of a piece of camphor floating in water, the movement 

 always occurring towards the side of least surface-tension. This latter 

 statement is, however, misleading. In the case of a piece of camphor 

 floating on water the surface-tension is lowered unequally as the camphor 

 dissolves irregularly. The fragment is drawn to the side where the 

 surface-tension of the water is greatest ; for the surface-tension film 

 moves in that direction, and drags the camphor after it. In the particles 

 of an emulsion, however, a diminution of the centrally-directed surface- 

 tension pressure on one side or an increase on the opposite face causes the 

 particle to move to the side of least surface-tension, and this movement 

 will continue as long as the potential difference is maintained, and will 

 cease as soon as equilibrium is reached. 



A direct physical explanation can, however, hardly apply to organisms 

 which possess definite locomotory organs, such as flagellae or cilia. It is 

 undoubtedly often the case that physical forces such as surface-tension, 

 osmosis, imbibition, &c., when intense, may overpower the organism, but 

 there can equally be no doubt that the latter has acquired the power 

 of directing and controlling these natural forces for its own benefit, so 

 that a simple direct physical explanation can hardly be postulated for 

 phenomena which may be due to a multiplicity of interacting factors, 

 viz. the attraction of swarm-spores by light, and their repulsion when the 

 illumination is intense. 



1 Cf. Velten, Flora, 1873, p. 98. 2 Protoplasmamechanik, 1886, pp. 119 seq. 



