Chapter XVI 



THE ROLE OF THE VACUOLAR SYSTEM AND 

 HYPOTHESES CONCERNING IT 



One of the most important functions of vacuoles is to regulate 

 the exchange of water which takes place in the cell by osmotic 

 phenomena. This was brought out by the classical research of DE 

 Vries. We have already mentioned this function (p. 125). Now 

 we must show the applications of it made by DE Vries. 



From his experiments, this investigator thought out a method 

 by which the osmotic pressure of a cell might be determined. This 

 consists in placing fragments of living tissue (for example, stami- 

 nate hairs of Tradescantia, which have been mentioned before as 

 particularly favorable for these experiments) in solutions of a 

 known substance such as sugar, arranged according to concentra- 

 tion. There may then be found a limiting concentration at which 

 plasmolysis is just beginning, i.e., in which separation of the proto- 

 plasm from the angles of the cell wall is first detected. This limit- 

 ing phenomenon is considered as a criterion and it is recognized 

 that it corresponds to an equality in osmotic pressure : the solution 

 is therefore isotonic with respect to the vacuolar sap. 



By this method DE VRIES made known one of the fundamental 

 laws of osmosis. By a series of progressive comparisons of dif- 

 ferent substances, it is demonstrable that they are isotonic when 

 each produces incipient plasmolysis of cells. By this biological 

 method, DE Vries was able to show that isotonic solutions are equi- 

 molecular, i.e., equal osmotic pressures are developed by an equal 

 number of molecules. The method is so sensitive for sugars that 

 DE Vries was able to determine the molecular weight for raffinose 

 about which chemists disagreed. Electrolytes, however, are ion- 

 ized in solution and each ion, acting as a molecule, increases osmotic 

 pressure. Consequently DE Vries had to introduce into this law 

 a coefficient of correction (isotonic coefficient). 



Osmotic pressure of the cell sap varies according to the con- 

 ditions of the life of the plant : the osmotic value is 4-5 atmospheres 

 for aerial parts of aquatic plants, 12-14 atmospheres for cells in 

 the root of the bean, almost 100 atmospheres for the chlorophyll- 

 bearing parenchyma of various plants. 



Normally, cells are always distended by their vacuolar sap. 

 This rigidity is called turgidity. The cells are entirely comparable 

 to a blown up balloon : the internal pressure manifests itself if the 

 membrane is pierced by a microdissecting needle and the proto- 

 plasmic and vacuolar contents escape with force just like the air 

 of a punctured balloon. Turgidity plays a considerable role in the 

 life of plants in maintaining their rigidity. When it is lacking, 

 the plants lose their rigidity and wilt. Cells have, moreover, the 

 means of regulating the concentration of their vacuolar sap in 



