184 
Walter Stiles 
plast change its volume by a fraction ap the first equation becomes 
V 
T r~“P 
P = P LS _ 
* 6 ‘ I -ap * 
The quantity VJV Z is called by Hofler the degree of plasmolysis. In 
cells of regular geometric form in which there is a regular contraction 
on plasmolysis, both V (and V z ) and V p are easily measured. In 
measuring the original volume of the cell it is necessary to measure 
the cell in a medium in which the cell undergoes no change in volume. 
For this purpose water and air are both ruled out, for in the former 
swelling may occur owing to intake of water, and in the latter 
evaporation of water may take place resulting in a diminution of 
volume. In air, moreover, the actual measurement is not easy. 
Ursprung and Blum (1916 d) recommend the use of liquid paraffin 
(" paraffin oil ”) for this purpose, as this substance neither reacts with 
nor enters the cell (Heller, 1904; Schilling, 1915). The cover glass 
must always be supported so that its weight does not fall on the 
cells or tissue and induce changes in form. The replacement of the 
paraffin by the plasmolysing liquid may be effected by irrigating the 
material on the slide, or by removing the material from the slide and 
washing in water. The former involves several successive washings 
to rid the cells or tissue of the paraffin, while the latter requires care 
to avoid deformation of the cells. Sucrose is to be recommended as 
the most generally useful plasmolysing agent for the reasons already 
given. 
In a series of experiments in which parenchymatous cells from 
the stem of Tradescantia elongata were used, Hofler showed that the 
degree of plasmolysis was inversely proportional to the concentration 
of the plasmolysing solution, in the case of sucrose solutions varying 
in concentration from 0-30 to o-6o gram-molecules a litre. With a 
number of solutions of different concentrations consistent values for 
the osmotic pressure of the cells were obtained, thus providing 
grounds for confidence in the correctness of the method. 
Methods involving the determination of the osmotic pressure of the 
expressed sap. Many determinations of the osmotic pressure of the 
liquids in plant cells have been made by expressing the sap from the 
tissues and then determining the osmotic pressure of the sap by one 
of the methods mentioned in Chapter VI. The first point that calls 
for attention here is the method of extracting the sap from the tissues. 
Until comparatively recent the practice was to press the sap from 
the living untreated tissue (cf. Maquenne, 1896; Sutherst, 1901; 
