Walter Stiles 
182 
or by one of the indirect methods indicated in Chapter VI have also 
been much employed for determining the osmotic pressures of plant 
cells, as, for example, by Dixon and Atkins (1910-1916), Harris and 
co-workers (1916-1921) and Mason (1919). These various methods 
will now be briefly described. 
Plasmolytic method. This method is that of de Vries for deter¬ 
mining isotonic coefficients and has already been described. Solutions 
of sucrose and potassium nitrate are the media generally used to 
produce plasmolysis of the cells investigated. The solution which 
just brings about plasmolysis, or that which just fails to bring it 
about, is regarded as isotonic with the cell sap, and accordingly has 
the same osmotic pressure. The values of osmotic pressure of solu¬ 
tions of sucrose over a wide range of concentrations have been ob¬ 
tained by Berkeley and Hartley and by Morse and his collaborators, 
and the existence of these determinations renders sucrose particu¬ 
larly useful as a plasmolysing substance in the plasmolytic method 
for the determination of the osmotic pressure of plant cells. A table 
of these values is given by Ursprung and Blum (1916 a). Sucrose is 
also to be recommended as it enters the majority of cells with extreme 
slowness and so does not materially affect the determinations on 
account of increased concentration of the cell sap resulting from 
endosmosis. 
Immersion of the cells for some time is necessary for plasmolysis 
to be observed in many cases. Ursprung and Blum ( l.c .) left herba¬ 
ceous parts of a number of plants immersed in the solutions for 
25 to 40 minutes or longer. Sections of stem and root of Fagus 
required at least 40 minutes. This rather long immersion involves 
the possibility of concentration changes in the cell sap owing to 
exosmosis from the interior into the surrounding solution v so that 
an error may arise on this account. As plasmolysis and deplasmo- 
lysis may result in tearing and stretching of the protoplasm it is 
advisable to use fresh cells for every determination. Change in volume 
which the cell may undergo when the tissue examined is isolated from 
the plant may also influence the result and should certainly be looked 
for and taken account of, and the same remark holds with regard to 
change in the volume of the tissue taking place during its immersion 
in the solution. The value of the osmotic pressure obtained is that of 
the cell at the moment when plasmolysis commences. If the osmotic 
pressure of the external solution is then P z> the volume of the tissue 
V z and its original volume in the turgid condition V, its osmotic 
pressure in that condition is P Z VJV. 
