178 OSMOSIS : ABSORPTION OF WATER 



passes inwards through the walls of the bladder a certain amount 

 of the sugar or the other soluble compounds employed passes 

 outwards into the pure water within the vessel : and it is noticed 

 that the process of diffusion or passage of the dissolved substances 

 goes on through the membrane until the percentage, composition, 

 or strength of the solution is the same inside and out. 



Certain membranes are, however, known which allow water to 

 pass through them but which are not permeable to sugar and 

 other dissolved compounds. 



The diffusion or passage of liquids and solutions of sub- 

 stances through membranes in which no visible openings are 

 present is termed osmosis : the pressure set up in the interior 

 of closed permeable membranes is spoken of as osmotic pressure, 

 and the dissolved substances upon which the pressure is primarily 

 dependent may be designated osmotic substances. 



A bladder or other structure distended by osmotic pressure 

 becomes firm or rigid instead of limp and flabby and in this 

 condition is spoken of as turgid. 



Dissolved in the cell-sap of all Hving plant cells are osmotic 

 substances, such as sugars and salts of various kinds, which have 

 the power of attracting water into the interior, and when plant 

 cells are immersed in pure water they become turgid. 



In all living parts of plants which are adequately supplied 

 with water, and especially in those regions in which active growth 

 is going on, the cells are distended by osmotic pressure, and this 

 state of turgidity is the cause of the elasticity and firmness ex- 

 hibited by the thin-walled living parenchymatous tissues of leaves, 

 growing-points, and other delicately-constructed portions of plants. 



The pressure within young turgid cells usually amounts to five 

 or ten atmospheres and under its influence the cytoplasm is forced 

 outwards into close contact with the cell-wall at all points ; the 

 cell-wall becomes stretched until its elastic recoil equals that of 

 the outward pressure. In the cells of fruits containing consider- 

 able amounts of osmotic substances in the cell-sap the pressure set 



