ABSORPTION OF WATER 135 



condition of wilting, or of complete loss of turgor, the suction ten- 

 sion reaches its maximum. In this case it is equal to the magni- 

 tude of the osmotic pressure of the cell sap, as may be seen from 

 the formula S = P — T, where S is the greater the smaller T is, 

 and S has the highest value when T = 0. 



In order to determine the magnitude of the suction tension 

 Ursprung has devised a method somewhat similar to the plas- 

 molytic one. The cell is immersed in solutions of different con- 

 centration, whose osmotic pressure approaches the suction tension 

 of the cell. In solutions whose osmotic pressure is greater than the 

 suction tension, the cell loses water and decreases in volume. In 

 weaker solutions, on the contrary, the cell imbibes water and 

 increases in volume. Only in a solution whose osmotic pressure 

 equals the suction power of the cell does the volume of the latter 

 remain unaltered. 



Ursprung's method, though simple in principle, requires pains- 

 taking work when put into practice. Before immersing the cell 

 into any of the solutions, its volume must be measured, for which 

 purpose an ocular micrometer is used. To prevent the desicca- 

 tion of the cell during the time of measuring, it is immersed in 

 liquid paraffin, which is harmless and checks evaporation. With 

 sufficient attention and careful manipulation, the method gives 

 excellent results. For rougher and more rapid determinations of 

 suction tension, instead of single cells, whole sections of tissues may 

 be used. The length of these sections is measured before and 

 after immersion in the test solutions. The solution in which the 

 length of the section does not change, is equal in its osmotic value 

 to the suction tension of the cell. 



The relationship between turgidity, osmotic pressure, and suc- 

 tion tension may be graphically illustrated by the following dia- 

 gram (Fig. 51). On the abscissa are marked the various stages of 

 tension of the cell wall at different degrees of saturation of the cell 

 with water, or at different volumes of the cell. On the left, is 

 seen the condition of a cell when completely plasmolyzed or wilted 

 and, therefore, at its smallest volume, which is denoted by 1. The 

 concentration of the cell sap, and consequently its osmotic pres- 

 sure 0, attains its maximum. The suction tension, then, equals 

 the osmotic pressure. When the cell imbibes water, its volume, 

 and consequently the degree of tension of the cell wall, increases, 

 and a gradually increasing turgidity T sets in. When the cell sap 



