Conditions and Principles of Absorption 69 



stances, so long as an equilibrium has not been established, 

 and water is available, there is absorption of water, and 

 there is manifest always, with adequate water-supply 

 within the cell, an hydrostatic pressure known as turgor. 

 This turgor, existing throughout the plant, is, as already 

 indicated, the chief cause of the rigidity of leaves and suc- 

 culent shoots. Turgor is then the expression of the os- 

 motic pressure of the cell. This turgor may be measured 

 by simple experiments conducted as described below. 



43. Plasmolysis and wilting. - - If the root-hair or any 

 equivalent cell is placed in a solution stronger than the 

 cell-sap, the major current of water will be outward, so 

 that water will be withdrawn from the protoplasm, the 

 latter contracting from the cell-wall. This state of con- 

 traction is termed plasmolysis. Plasmolysis throughout a 



FIG. 19. Lettuce plants in solutions : A, tap water; B, 2.9 per cent 



sodium chlorid. 



