Chapter VII — 97 — Osmotic Quantities of Cells 



the walls of the capillaries, it is useful where only small volumes of sap are 

 available. Wildervanck (1932) preferred it over other methods for de- 

 termining the osmotic pressure of Nitella translucens sap. 



Modifications of this method are described by Signer (1930) and by 

 Berl, et al. (1930). The capillary method of Ursprung and Blum 

 (1930) dififers in that the expressed sap or other solution to be investigated 

 is placed in the shallow depression of a small moisture chamber, and that 

 the standard reference solutions are contained in a series of capillaries at- 

 tached to an upper glass cover (Figure 33). Technical details may be 

 found in the original paper. 



Procedures have been perfected for measuring osmotic pressure thermo- 

 electrically, based upon differences in vapor pressure between the solution 

 to be investigated and some standard reference solution (Hill, 1930; 

 Baldes and Johnson, 1939). Either a sensitive thermopile or thermo- 

 couple is used, and very small amounts of liquid can be investigated. The 

 principle involved {cf. Baldes, 1939) is that differences in temperature 

 between the two droplets under standard conditions for evaporation can be 

 measured by a delicate galvanometer. This reading is converted to vapor 

 pressure or osmotic pressure by standardizing the apparatus with known 

 solutions. RoEPKE (1942) has provided refinements in apparatus and 

 method. 



Osmometric Methods : — Direct determination of osmotic pressure by 

 means of osmometers is not particularly adaptable to plant saps. Bour- 

 dillon (1939) describes a micro apparatus utilizing a flat horizontal mem- 

 brane, which will accommodate as little as 0.2 cc. of protein solution in 

 concentrations down to 0.025 mm. Blegen and Brandt Rehberg (1938) 

 used specially prepared collodion tubes. Results compared well with Hill's 

 vapor pressure method. Ordinarily osmometric procedures are successful 

 only where the sample is a fairly pure colloidal solution, where significant 

 amounts of dissolved electrolytes are absent. Levitt (1946) has investi- 

 gated the colloid osmotic pressure of proteins extracted from potato tuber 

 tissue in this manner. 



Other Methods for Osmotic Pressure: — The "simplified" or 

 "minimum cell volume" method, designed by Ursprung (1923), consists 

 of measuring the change in length of strips of tissue immersed in solutions 

 (usually sucrose) of varying concentration. The lowest concentration of 

 sugar that produces minimum length is considered to be isotonic with the 

 cell sap at limiting plasmolysis. There are two serious objections to the 

 method. First, it is not valid to assume that a tissue strip in equilibrium 

 with the most dilute solution effecting "minimum cell volume" is in a con- 

 dition of limiting plasmolysis. There is assumed to be a point, as the con- 

 centration is increased, where shrinkage ceases, and volume remains con- 

 stant. Table 25 shows, however, that within the concentration range 

 used, potato tissue may continue to shrink with increasing concentration 

 of the bathing solution. Although a constant condition is approached, the 

 fact that higher and higher concentrations are capable of causing reduc- 

 tion in imbibition of the walls indicates that the method cannot produce 

 highly accurate results. On the other hand the simplified method is a 

 valuable means of determining DPD values at normal volume (see later). 



The second objection is that change in length of a tissue is not always 

 a true indication of change in volume. Many strips exhibit distortion 



