140 THE CELL 



Thus the salt or sugar solution must make its way through the 

 cellulose membrane, after which it continues to abstract more 

 water from the protoplasm, which shrinks more and more accord- 

 ing to the concentration of the solution, so as to occupy a smaller 

 and smaller space. The sap which it encloses becomes corre- 

 spondingly more concentrated. In spite of these changes, which 

 are grouped together under the same plasmolysis, the protoplasm 

 may remain alive for weeks, and exhibit its usual streaming 

 movements ; it may even surround itself with a new peripheral 

 layer, although it remains in its contracted condition. 



Two conclusions may be deduced from the process of plas- 

 molysis : (1) that the cellulose membrane is pervious to the salt 

 solutions which were used ; (2) " that the amount of dissolved salt 

 which diosmoses through the peripheral layer is not worth 

 mentioning, for if a considerable quantity penetrated into the 

 protoplasm, or into the cell sap, an increase in the quantity of the 

 substances setting up osmosis would be produced within the proto- 

 plasmic membrane, and thus an increase in the volume of the 

 protoplasmic body would result " (Pfeffer). 



If the cells which have become flaccid through plasmolysis are 

 carefully removed and placed in pure water, the reverse process 

 occurs. The sugar solution which was enclosed within the cellu- 

 lose membrane becomes diffused into the water. In consequence, 

 the peripheral protoplasm layer becomes distended, because its 

 cell sap is now richer in osmotolytic substances than its environ- 

 ment, and so water is caused to flow in the opposite direction. 

 This distension gradually increases, as the water becomes ab- 

 sorbed, until the peripheral layer of protoplasm comes into close 

 contact with the cellulose membrane, and until finally the cell has 

 dilated to its original size. 



Other experiments have shown that the sap contained in the 

 plant cell is under a considerable pressure, often of several atmos- 

 pheres. This produces the natural turgescence of certain por- 

 tions of plants. The cause is, that powerfully osmotolytic sub- 

 stances are present in the cell sap, such as saltpetre, vegetable 

 acids, and their potassium salts, which have a strong affinity for 

 water (Pfeffer V. 23 ; de Vries V. 36). 



Therefore under these conditions the protoplasmic coating con- 

 taining the cell sap may be compared to a very elastic thin-walled 

 bladder, which is filled with a concentrated salt solution. If such 

 a bladder is put into pure water, the solution attracts the water, 



