Walter Stiles 
186 
the cells in a tissue or organ. This may be an advantage or dis¬ 
advantage according to the end in view 1 . 
The Magnitude of the Osmotic Pressure in Plant Cells 
A great number of determinations of the osmotic pressures of 
plant cells and tissues have been made by the plasmolytic method 
and by determinations of the freezing point of expressed saps. With 
the vast array of data so obtained it is not proposed to deal in any 
detail here. Some reference should, however, be made to the magni¬ 
tude of the osmotic pressure in plant cells, and to the variations in 
the osmotic pressure related to the ecological type of plant, the 
habitat of the plant, the position of the organ or tissue, the time of 
day and season of the year. 
Unfortunately different authors have expressed the osmotic pres¬ 
sures of the cells examined in different units. Thus Sutherst simply 
gives the freezing point lowering, Ursprung and Blum give the con¬ 
centration of sucrose or potassium nitrate isosmotic with the sap 
(the “cane sugar value” or “saltpetre value”) while Dixon and 
Atkins and Harris and his collaborators give the pressures in atmo¬ 
spheres and Livingston (1903) in millimetres of mercury. 
Pfeffer (1900), basing his remarks on older observations (de Vries, 
1884a; Stange, 1892; Janse, 1887a), states that the osmotic pressure 
in the cells of land and fresh water plants is usually of the order of 
5 to 11 atmospheres, but the recent observations of Dixon and Atkins, 
Harris and his collaborators, and Ursprung and Blum, clearly show 
that the normal range of osmotic pressures of plant cells is wider, 
extending much further in the upward direction. Thus out of 53 
1 The method of tissue tension has not been included among the methods 
for determining the osmotic pressures of cells for the reason explained in the 
footnote on p. 180; that is, on account of the presence of turgor pressure, 
any results obtained with turgid cells must necessarily be approximate, and 
with highly turgid tissues will be exceedingly incorrect. The same holds with 
regard to the change-in-weight (or change-in-volume) method. But for cells 
without an enveloping elastic envelope like the cell wall, a liquid in which 
cells neither lose nor gain in weight, that is, in which no interchange of water 
takes place, is to be regarded as isosmotic with the cell sap. The method is thus 
applicable to the determination of the osmotic pressure of some animal cells 
(H. J. Hamburger, 1886, 1887, 1889, 1890; Gryns, 1896; Koppe, 1895, 1897; 
Lob, 1894; Hedin, 1895 a, 1895 b, 1896), and also to a few plant cells without 
cell walls. 
By attaching weights to tissue, rendered turgorless by plasmolysis or death, 
until the tissue has regained its original length, it is possible, according to 
Pfeffer (1903), to calculate approximately in some cases the original osmotic 
pressure of the tissue. As such a method can only be very approximate at 
best, no further mention of it need be made here. 
