OSMOTIC PRESSURE IN THE CELL 



135 



support, can withstand almost any intensity of pressure. Hence an ex- 

 tremely high osmotic pressure may be maintained so long as the dissolved 

 substances do not diosmose through the semi-permeable plasma. A 

 working model of the relationships existing in the cell is presented when 

 a semi-permeable precipitation membrane is formed within, and adpressed 

 to, an enclosing cell of porous clay, for the latter resembles the cell-wall 

 in so far as it allows crystalloid substances to diosmose through with 

 relative ease. It is not the porous clay cell (or cell-wall), but the 

 precipitation membrane (or plasmatic membrane) which is of primary 

 importance with regard to the production and maintenance of the osmotic 

 pressure. 



The pressure exerted against the clay cell remains unchanged, even 

 if a second smaller precipitation membrane is formed within the first and 

 floats freely in the central fluid. For, provided the membranes are semi- 

 permeable, if the osmotic pressure is higher inside the inner membrane than 

 outside of it, this membrane will grow until sufficient water is absorbed to 

 bring the inner cell to the same osmotic level as the outer, equilibrium then 

 being again restored throughout the system. The living cell is actually such 

 a system of two plasmatic membranes, one enclosing the other (Sect. 18 

 and Fig. 10), nor does it make any essential difference whether the plasma 

 forms an external layer only, or also crosses the vacuole in the form of 

 strands. It is also evident that the appearance of numerous vacuoles in the 

 plasma will not alter the turgid pressure, while the presence of a well- 

 defined vacuole simply adds an additional membrane to the osmotic system. 



All substances present in solution in the plasma exert an equivalent 

 osmotic force, but if such dissolved substances are absent the power 

 of swelling inherent in the plasma must resist the pressure which the 

 osmotic tension exerts upon it. A similar relationship would exist in 

 the porous cell just described, if the space between the two precipitation- 

 membranes were filled with gelatine free from all saline admixture. 

 For the gelatine would diminish in volume until the imbibitory swelling 

 force, which rapidly increases as the amount of imbibed water decreases, 

 is just equal to the osmotic pressure which the inner precipitation-membrane 

 (like the vacuolar membrane) exerts against the gelatine. When plasmo- 

 lysis is produced the same general relationship is maintained, for now the 

 resistance of the cell-wall is replaced by the osmotic pressure which the 

 plasmolysing fluid exerts upon the ectoplasmic membrane. 



The semi-fluid nature of the protoplasm does not presuppose that it has 

 no power of swelling, and this power may counteract the osmotic pressure 

 exerted upon it. It is probable, however, that dissolved substances are 

 present in the plasma, and these may aid, or even be of primary importance, 

 in maintaining internal osmotic equilibrium, so that the protoplasmic power 

 of swelling may not be called into play to counterbalance the pressure 



