206 The Structure of Protoplasm 



During the time in which the protoplasm flows in the opposite 

 direction, namely from h to a, the air-pressure of A must be lowered 

 below that of B, if the flow is to be accelerated. In practice, however, 

 the pressure of B is increased instead of decreasing the pressure of 

 A, in order to establish a pressure difference in this direction. 



Protoplasmic streaming can be artificially speeded up in this way 

 to maximum velocities which are beyond those of natural conditions. 

 By establishing a pressure difference of, say 20 cm. of water, so as to 

 cooperate with the motive force developed in the protoplasm, one 

 can readily increase the velocity to more than 2 mm. per second 

 through a connecting capillary of 200!^i inner diameter. The normal 

 maximum is about 1 mm. a second. 



Next must be considered the effect of a pressure difference, 

 which is established so as to oppose the protoplasmic force. In this 

 case, as expected, the velocity of the flow is retarded. When, for 

 instance, the protoplasm flows from a into b, the volume of the proto- 

 plasm b is, of course, increased. If the air-pressure of B is made 

 higher than that of A, then it is clear that a part of the motive force 

 generated in the protoplasm must be used in the work required to 

 bring about the expansion of the protoplasmic volume against the 

 higher pressure which has been artificially applied. Should the 

 pressure applied be stronger than the protoplasmic force developed 

 in the plasmodium, then the forward-moving protoplasm is forced 

 backwards. In other words, the motive force of the protoplasm is 

 overcome by the application of a counter-pressure. 



If, now, the protoplasm is flowing as before, namely from b to a, 

 a lower pressure must be appHed to B, if the flow is to be retarded 

 or reversed. 



From the foregoing description it is evident that by changing 

 the pressure of B both on the + and — sides of the manometer, the 

 direction and speed of the protoplasmic flow along the connecting 

 strand can be accurately controlled. Flowing protoplasm thus con- 

 trolled is driven by a resultant force which is the algebraic sum of 

 the shearing stress developed in the protoplasm and that caused 

 by the pressure difference artificially induced. 



One would expect that artificial acceleration, retardation, or 

 reversal of flow would induce serious disturbances in the proto- 

 plasm, but a wholly normal flow is resumed as soon as the pressure 

 is released. It is rather surprising that such a remarkable modifica- 

 tion of the flow does not result in any pathological abnormality. 



