THE LIVING CELL 



17 



The shrinkage of the protoplast of the plasmolysed cell continues 

 until the sap is concentrated to such a degree that its osmotic pressure 

 becomes equal to that of the external solution : equilibrium is then 

 established, provided that the dissolved substance of the external 

 solution is unable to enter the cell. If it docs enter, the osmotic activity 

 of the cell-sap is gradually increased and eventually the protoplast is 

 able to take up water from the outer solution and so to recover its 

 turgor. This recovery or de-plasmolysis can in any case be rapidly 

 induced by transferring the cells from the plasmolysing solution into 



Fig. 19. 

 Cells from the prothallus of Nephrodium villosum after treatment with 3 per cent, 

 solution of common salt ( x 550). Drawn as observed about 15 minutes after plasmo- 

 lysis ; the threads are very fine, but appear proportionally thicker in the figure 

 than they actually are. 



water. The osmotic properties of the cell-sap now result in a rapid 

 intake of water, just as water passed into the parchment thimble 

 in the experiment shown in Fig. i;a. The protoplast swells and soon 

 regains contact with the wall : through continued entrance of water 

 the elastic wall becomes distended. As the wall now exerts a pressure 

 on the expanding protoplast, the capacity of the cell for further 

 absorption of water is thereby curtailed. The effective osmotic 

 pressure conducive to further absorption is clearly the difference 

 between the osmotic pressure of the cell-sap and the inward 

 pressure of the wall. This effective osmotic pressure is distinguished 

 as Suction Pressure. As the absorption of water continues a st. 



