92 TURGIDITY. [CH. VI 



has become flaccid in salt-solution it can be rendered stiff 

 by replacing it in water. 



Or it may be made to collapse and become flaccid by 

 immersion in water at 60 C. But in this case the 

 flaccidity is permanent, because the tissues are killed. 

 When the cells were alive they were tensely filled with 

 cell-sap, which escapes as soon as the protoplasmic lining 

 of the cells is killed by heat. The flaccidity of the dead 

 tissue depends on a loss of fluid from the cells, and this is 

 likewise the cause of the similar though temporary loss 

 of stiffness produced by dry air or immersion in salt- 

 solution. 



To understand the problem more fully it is best to 

 take the case of a single isolated cell capable of standing 

 up and supporting its own weight. The cell is stiff just 

 as an air-cushion, tensely filled with air, is stiff. The 

 air-cushion is filled by blowing air into it with a pair 

 of bellows. The cell is filled by osmosis, which depends 

 (i) on the fact that the cell-sap is denser than water, 

 (ii) on the physical properties of the protoplasmic lining. 



When the fluid surrounding the cell is denser than 

 the cell-sap, the osmotic flow is from the cell to the fluid ; 

 when the reverse is the case, the flow is in the opposite 

 direction, and the cell gains, instead of losing, fluid. 

 Again, when the physical properties of the protoplasm 

 are changed by death, the cell-sap escapes just as the 

 air escapes from a ruptured air-cushion. 



When a cell is tensely filled with fluid by osmosis it is 

 called turgid, and turgidity is the cause of the stiffness of 

 not merely the isolated cell, but also of masses of cells in 



