OSMOSIS 179 



up in wet weather when abundance of water is conducted to them 

 is sometimes sufficient to burst the cell-walls and the fruits split. 



The osmotic properties of a plant cell are, however, not the 

 same as those of a bladder filled with sugar-solution, for in many 

 instances cells containing sugar or other substances do not allow 

 these to pass out into water in which the cells may be immersed. 



It is obvious that even a very slight permeability of the sub- 

 stances to which turgidity is due would make it practically 

 impossible for any submerged water-plant to remain turgid, and 

 the accumulation and retention of sugars and other soluble 

 substances in the roots of beet and similar plants growing in 

 damp soil would be equally difficult if the protoplasm and walls 

 of the external cells were permeable to such compounds. 



Whatever substances pass into or out of a living plant cell 

 must permeate both the cell-wall and the thin lining of cytoplasm. 

 While pure water finds a ready passage through both membranes 

 the cytoplasm is very frequently either quite impermeable or per- 

 meable in a very different degree to many substances which easily 

 travel through the cell-wall. Moreover, the permeability of the 

 cytoplasm to any particular substance is not the same at all times. 



When a turgid cell is immersed in a solution of a substance 

 whose attraction for water is greater than that possessed by the 

 substances dissolved in the cell-sap, a larger or smaller amount 

 of water is abstracted from the cell and the osmotic pressure is 

 reduced, the cell becoming smaller and more or less limp. If 

 the vitality of the cytoplasm is not destroyed and the osmotic 

 action of the solution continues, more water is abstracted from 

 the vacuole, but the cytoplasm instead of remaining in contact 

 with the cell-wall and allowing the solution to penetrate into the 

 vacuole, shrinks away from the cell-wall and takes the form of a 

 nearly spherical hollow ball in the centre of the cell-cavity : a 

 living cell in this condition is said to be plasmolysed. The space 

 between the cell-wall and the shrunken cytoplasm betomes occu- 

 pied by the solution which has penetrated inwards through the 



