THE OSMOTIC QUANTITIES OF PLANT CELLS 145 



discussed in the remainder of this chapter. These are : ( I ) that the cells are 

 at the same temperature, (2) that the cytoplasmic membranes are permeable 

 only to water while the cell walls are freely permeable to both water and 

 solutes and (3) that the walls of all the cells are equally elastic. Further- 

 more, changes in the osmotic pressure of the cell sap of these three cells due 

 to the changes in the volume of the cells will be disregarded. 



Diffusion of water into cell J begins immediately upon its immersion. 

 This inward movement of water is osmosis. The diffusion pressure deficit 

 of the water in the cell sap is 10 atmos. (since its osmotic pressure is 10 atmos., 

 and it is not subjected to pressure) ; that of the surrounding water zero, hence 

 the diffusion pressure of the external pure water is 10 atmos. greater than 

 that of the water in the cell sap. The entrance of water into the cell exerts 

 a gradually increasing pressure against the protoplasm which is in turn trans- 

 mitted to the cell wall. This pressure is called the turgor pressure {cf. dis- 

 cussion in Chap. VIII) and is the cause of the gradual distension of the cell. 

 As soon as any turgor pressure is exerted against the walls of the cell they 

 exert a counter pressure — the wa/l pressure — against the protoplasm and cell 

 sap. The wall pressure is always equal to but acts in the opposite direction to 

 the turgor pressure. The maximum turgor pressure which can be developed 

 by this particular cell is 10 atmos., since, as already shown in Chap. VIII, 

 the osmotic pressure of a solution is a measure of the maximum turgor pres- 

 sure which it can develop. At this point the wall pressure of the cell will also 

 be equal to 10 atmos. The wall pressure of a cell will have exactly the same 

 effect upon the diffusion pressure of the water in the cell sap that a pressure 

 of equal magnitude exerted by a piston would have on water subjected to it 

 (Chap. VIII). In this cell, therefore, the wall pressure will increase the 

 diffusion pressure of the enclosed cell sap by 10 atmos. Since the initial 

 diffusion pressure deficit of the cell sap was 10 atmos. and the development 

 of a wall pressure of 10 atmos. has raised the diffusion pressure of the water 

 in the cell sap by 10 atmos. the diffusion pressure deficit of the water in the 

 cell sap is reduced to zero. Since this is also the diffusion pressure deficit of 

 the surrounding water, a dynamic equilibrium has been established due to the 

 influence of the wall pressure upon the diffusion pressure of the water in the 

 cell sap. When this condition of dynamic equilibrium has been attained, equal 

 numbers of water molecules will be passing across the membrane in both 

 directions per unit of time. 



It is the turgor pressure of the cell acting in opposition to its wall pres- 

 sure which imparts to plant cells their usual rigid, distended condition. This 

 condition is termed turgor, turgidity, or turgescence; the first of these terms 

 being in most general use. Plant cells, although generally turgid, seldom 



