152 THE OSAIOTIC QUANTITIES OF PLANT CELLS 



upon the possible diffusion pressure deficits in the material under investiga- 

 tion. Some investigators have undertaken to measure the diffusion pressure 

 deficit of individual cells (Ursprung and Blum, 1916). Single cells or groups 

 of such cells are isolated from the tissue, and mounted under the microscope, 

 usually in paraffin oil (to prevent changes in diffusion pressure deficit due 

 to evaporation, or osmotic movement of water into or out of the cells from 

 or into an aqueous medium). The linear dimensions of the cells are recorded, 

 after which a sample of the cells is immersed in each sucrose solution in the 

 series. After the attainment of an equilibrium between the cells and the 

 solutions they are remeasured under the microscope. The osmotic pressure 

 of the solution in which the cells show no change in dimensions is considered 

 to be equal to the diffusion pressure deficit of the cell. This method requires 

 technique of considerable precision and has not been widely used. 



A "simplified method" of measuring diffusion pressure deficits has also 

 been introduced by Ursprung and Blum (1923). In this method narrow 

 strips of tissue are cut from such structures as thin leaves or petals. The 

 length of each strip is immediately measured under the microscope, usually 

 while mounted in paraffin oil. Several strips are then immersed in each of a 

 graded series of sucrose solutions in which they are allowed to remain until 

 an equilibrium has been attained, after which they are remeasured. The os- 

 motic pressure of the solution in which no change in the length of the strips 

 occurs is considered to be equal to the average diffusion pressure deficit of the 

 cells in the strip. 



For the measurement of the average diffusion pressure deficit of the cells 

 in bulky tissues such as potato tubers, masses of tissue such as cylinders of 

 approximately equal dimensions can be employed. If possible all of the cylin- 

 ders used in a given determination should be cut from the same organ. The 

 equilibrium point can be determined by measuring changes either in the weight 

 or the volume of the cylinders. The solution in which the cylinder neither 

 gains nor loses weight (or volume) is considered to have an osmotic pressure 

 equal to the average diffusion pressure deficit of the cells in the cylinder. 



Determinations of diffusion pressure deficits by the methods described 

 above should not be confused with plasmolytic determinations of the osmotic 

 pressures of plant cells. In the latter type of determination the critical 

 measurement is the osmotic pressure of the solution with which the cells come 

 to an equilibrium without any turgor pressure, i.e. at incipient plasmolysis. 

 In diffusion pressure deficit determinations the critical measurement is the 

 osmotic pressure of the solution with which the cells come to equilibrium 

 without any change in their turgor pressure, i.e. without any change in the 

 volume of the cells. Only when the cell is initially in a completely flaccid 



