THE FUNCTIONS OF ROOT-HAIRS. 47 



clearer by the researches of Pfeffer and De Vries 

 in 1877. 



De Vries showed that the turgescence of the 

 living cell can be diminished, and even reduced 

 to nothing, by placing the cell in contact with 

 solutions of substances which attract water from 

 the cell-sap : as the turgescence diminishes, the 

 cell contracts, owing to the elasticity of the cell- 

 wall, which was previously distended ; if the 

 abstraction of water continues, the living proto- 

 plasmic membrane lining the cell-wall contracts 

 away from the latter. He then proved that no 

 injury need accrue to the cell by this process of 

 plasmolysis, since the turgescence can be restored 

 by washing out the salt with a more dilute 

 solution, or with pure water ; and the cell may 

 go on living and even growing as before. These 

 phenomena can only be produced in cells where 

 the protoplasmic lining is intact and alive. 



Pfeffer showed that the whole matter depends 

 on the properties of the living protoplasmic 

 membrane, which, so long as it is alive, has the 

 power of governing the entrance or exit of dis- 

 solved substances, but is as a rule freely permeable 

 for water. If, then, substances with a powerful 

 attraction for water are formed in the cell cavity, 

 and of such a nature that the protoplasm does 

 not permit their free diffusion to the exterior, 

 they attract water, and hold it fast, and so set up 

 the condition of hydrostatic pressure known as 

 turgescence, the limit of which depends on the 

 attainment of a state of equilibrium between the 



