OSMOSIS— TURGESCENCE 413 



difference between the two experiments ; the salt sohition does 

 not pass through the protoplasm, though it does go through the 

 cell-wall. If, now, the salt solution be replaced by water, the 

 latter is graduallj- attracted again, of course osmotieally, into 

 the cell. It passes through the protoplasm ; the vacuole is re- 

 established, and the protoplasm again comes to line the cell- 

 wall, pressed against it by the .water. 



The protoplasm thus allows water to pass through its sub- 

 stance, but can oppose the passage either way of the -^-arious 

 osmotic bodies with wliich it may be brought into contact. In 

 the last narrated experiment it prevented the salt solution from 

 entering the vacuole, and, as shown by the return of the water 

 into the latter, it prevented the osmotic substances originally in 

 no the cell from leaving it. 



That this is due to the vital power of the protoplasm can be 

 shown by repeating the experiment after killing the living sub- 

 stance by a short immersion of the cell in alcohol. Then the 

 process of osmosis goes on exactly as in the first experiment 

 quoted. The salt solution penetrates into the vacuole as if only 

 a cellulose septum were present, the dead protoplasm exerting 

 no regulating influence. 



The modified osmosis, which is thus the mode of entrj' of 

 water into a cell containing no vacuole, continues after the vacuole 

 is formed, and is the cause of the transfusion of water from cell to 

 cell. We can see that this must be the case, for if two cells are 

 considered which are separated from each other by a common oeU- 

 wall, it is evident that, unless the proportion of water to osmotic 

 substances in the vacuoles of both is the same, osmotic currents 

 will flow from one to the other till this equilibrimn is reached. 



The quantity of osmotic substances present in any cell will 

 depend upon the behaviour of the protoplasm from time to time. 

 Such substances are usvially being continually- produced in all 

 growing cells and in most others in which chemical changes are 

 proceeding. Hence such cells are continually absorbing water, 

 and are consequently so full that a certain stretching force is 

 exerted on the cell-wall which bounds them. Cells in such a, 

 condition are called turgid, and the condition itself is kno"5Pn 

 as turgescence. The equilibrium, which is attained by such a 

 cell, is reached when the distension caused by the osmotic sub- 

 stances is balanced by the elastic recoil of the extensible cellulose 

 wall. In some cases the tension set up in a tissue by the 

 turgescence of the cells is sufficient to force the water, by a pro- 

 cess of filtration, through the walls of the outermost ones, so 



