228 THE PLANT. X. 



of the cellulose envelopes on the one hand and the water- 

 distended state of their protoplasmic contents on the other. 

 When this turgescence relaxes, e.g., by evaporation, the plant 

 wilts and droops, becoming quite flaccid. 



A solution within a cell composed of a semi-permeable 

 membrane and closed by a manometer, if immersed in the 

 pure solvent, takes in the latter until the osmotic pressure 

 is in equilibrium with the pressure produced by the mercury 

 column of the manometer. If more mercury be added to the 

 manometer some of the solvent will be forced out ; on the 

 other hand, if the mercury column be shortened more solvent 

 will enter and the volume of liquid within the cell will in- 

 crease. It is found that the pressure produced by a given 

 weight of dissolved substance per litre of solution at a given 

 temperature is exactly equal to the pressure which would be 

 exerted by the substance if it could exist as a gas under the 

 same conditions as to volume and temperature. This latter, 

 in the case of different substances, varies inversely with the 

 molecular weight of the substance. Consequently it is found 

 that two solutions exert an equal osmotic pressure when there 

 are present in a given volume of the solution the same number 

 of molecules of the dissolved snbstances. In the case of most 

 metallic salts and the stronger acids in aqueous solution, this 

 law is subject to a correction, because of the dissociation of 

 these compounds into ions, each of which acts as a molecule." 



A living vegetable cell has been employed as a means of 

 detecting the equality of the osmotic pressures existing in two 

 solutions. If the cell be surrounded with a solution in which 

 the number of molecules of dissolved substance per unit 

 volume be greater than that in the sap within it, the water will 

 pass through the protoplasm out of the cell, and the proto- 

 plasm will shrink from the rigid cellulose wall. This phe- 

 nomenon is known as plasmolysis and can be observed under 

 the microscope. With a certain strength of liquid the cell 

 contents will be in equilibrium, i.e., no water will leave or 

 enter. Solutions of different salts have the same osmotic 

 pressure or are isotonic if they are in equilibrium with the sap 

 of the same cell. 



* v. p. 82. 



