136 THE OSMOTIC QUANTITIES OF PLANT CELLS 



mature, thin-walled plant cells with prominent vacuoles — the possibility that 

 other mechanisms may also operate in the cell-to-cell movement of water 

 should not be disregarded. The cytoplasm may participate actively in the 

 passage of water into or out of plant cells. Bennett-Clark, et al. (1936) 

 present evidence which they believe indicates that cytoplasm may "secrete" 

 water into the vacuoles of some types of cells. Electro-osmotic phenomena 

 (Chap. VIII) may also be involved in the cell-to-cell movement of water. 

 Furthermore, in some types of cells the vacuoles are very small or are entirely 

 filled with mucilaginous materials of one type or another. In such cells 

 imbibitional phenomena probably play a greater proportionate role in the move- 

 ment of water than in cells in which the vacuole occupies most of the cell 



volume. 



Methods of Determining the Osmotic Pressures of Plant Cells and 

 Tissues. — The plasmolytic method and the cryoscopic method are in common 

 use for determining the osmotic pressure of plant cells and tissues. Both 

 methods have many limitations and neither is as accurate as might be desired. 



DeVries (1884) was the first to use the plasmolytic method (Chap. 

 VIII) which in principle is very simple. A series of solutions (usually of 

 sucrose) graded according to volume molar concentration is first prepared. 

 The range of concentrations to be used depends upon the tissue to be studied. 

 Comparable strips or sections of the tissue are then immersed in each solu- 

 tion and left until an osmotic equilibrium is attained. After immersion in 

 the solution the pieces of tissue are observed under a microscope. In the 

 stronger solutions it will be found that all of the cells are severely plasmolyzed, 

 while in the weaker ones little or no sign of plasmolysis can be detected. 

 Somewhere in the series will be found a solution in which about one-half 

 of the cells are not plasmolyzed, and about one-half are more or less com- 

 pletely plasmolyzed. The average osmotic pressure in the cells of the tissue 

 under investigation is considered to be equal to the osmotic pressure of the 

 solution in which this condition obtains. 



The value obtained by the procedure just outlined is called the osmotic 

 pressure at incipient plasmolysis. This value is often greater than the true 

 osmotic pressure of the cells since plasmolysis of many kinds of cells is pre- 

 ceded by a shrinkage in their total volume. A cell which has an osmotic 

 pressure of 15 atmos. at incipient plasmolysis might have had an osmotic 

 pressure of (for example) 12 or 13 atmos. in its normally distended state. 



Certain more precise methods of determining the osmotic pressure of cells 

 by the plasmolytic method, which take into consideration their volume changes, 

 have also been devised. If incipient plasmolysis is determined to occur for 

 cells of a certain tissue in a sucrose solution with a volume molar concentra- 



