OSMOTIC PRESSURE AS A FACTOR 



169 



of dissolved material between the water of the wall and that of 

 the protoplasm in so far as this is not prevented by impermeability 

 of the outer protoplasmic film or by adsorption forces resident in 

 one or other of these two phases. The outer surface of the wall 

 must be considered as permeable to all the solutes of the environ- 

 mental solution, and consequently the wall solution should tend 

 to be in concentration equilibrium with the solution of the envir- 

 onment. It thus comes about that the wall exerts an osmotic 

 pressure inward upon the protoplasm, and this osmotic force is 

 to be added to the forces of cohesion and of imbibition alread)^ 

 mentioned. 



Since the cohesion of the cell wall is very much greater than 

 the force resulting from the cohesion and surface tensions of the 

 protoplasm, it is clear, as every one knows, that of two cells in 

 distilled water, the one with and the other without sl wall, there 

 can be developed within the walled cell, without explosion, a 

 much higher osmotic pressure than could occur in the naked cell. 

 As the wall is progressively stretched its force of elasticity in- 

 creases, until this force balances the outward pressure within, and 

 a static condition is attained. Long before this could happen in a 

 naked cell with the same vacuolar pressure, rupture of the proto- 

 plasmic envelope would usually occur. The outward pressure 

 upon the inner wall surface is, in the case we have now before us, 

 numerically equal to the vacuolar osmotic pressure diminished 

 by the inwardly directed forces already mentioned as resident 

 within the protoplasmic mass, and also diminished by the like- 

 wise inwardly directed imbibitional and osmotic forces of the wall 

 itself. Opposed to this remainder is the force of elasticity of the 

 cell wall, and the action and reaction here implied result in the 

 condition of turgidity with which we are so well acquainted. If 

 the solution of the wall be concentrated enough, in proper solutes, 

 to develop at the outer protoplasmic surface an inwardly directed 

 force of sufficient magnitude so that the outwardly directed 

 pressure remainder just considered becomes nill, then no turgid- 

 ity is manifest ; the wall is not stretched, and the cell is already in 

 the wilted condition. With a still higher concentration of envir- 

 onmental solution and of cell wall, the protoplasmic envelope 



