238 



The Structure of Protoplasm 



connecting tube. In order to prevent a flow from a to h, additional 

 air-pressure must be applied to compartment B. This additional 

 pressure is the same as the pressure difference corresponding to the 

 level difference h between the two reservoirs. Like this model, the 



Fig. 18. 



balance-pressure of protoplasm is not concerned with the absolute 

 inner pressure of a plasmodium, but with the difference in pressure 

 between the two parts. 



When water in the connecting tube of the model is kept motion- 

 less, because it is opposed by a balance-pressure, the pressure along 

 the horizontal tube is the same as the hydrostatic pressure corres- 

 ponding to the height of the water level of reservoir a above the tube 

 (H in Fig. 18) . When, however, the balance-pressure is released and 

 water begins to flow, the inner pressure of the tube varies in accord- 

 ance with its horizontal position between a and b. The farther the 

 position of the tube is from reservoir a, the lower is the inner pres- 

 sure. This is true because the potential energy, represented by the 

 difference in level, must in part be expended in giving velocity to the 

 water (velocity head) and also, in overcoming the viscosity (resist- 

 ance head) . This is a well-known fact in hydraulics, and must also 

 in part be true in the protoplasmic system. 



