Permeability 181 
In Table XIX are given the exact values for the isotonic co¬ 
efficients of a number of substances found by de Vries by the plasmo¬ 
lytic and tissue tension methods. 
Table XIX 
Isotonic Coefficients of' a Number of Substances 
(Data from de Vries) 
Isotonic coefficient found by 
Substance 
Plasmolytic 
method 
"N 
Tissue tension 
method 
Sucrose 
i-88 
1*84 
Malic acid 
1-98 
Tartaric acid 
2-02 
— 
Citric acid 
2-02 
— 
Sodium nitrate 
3 -o 
— 
Potassium chloride 
3 '° 
2-84 
Sodium chloride 
3-05 
Ammonium chloride 
3-0 
Potassium acetate 
3 -o 
— 
,, dihydrogen citrate 
3-05 
— 
,, oxalate 
— 
3-93 
,, sulphate 
3-9 
3-92 
,,‘ • monohydrogen phosphate 
— 
3-96 
,, tartrate 
— 
3-99 
,, malate 
— 
4-ir 
„ monohydrogen citrate 
4-08 
,, citrate 
5-01 
474 
Magnesium malate 
i-88 
1-63 
,, sulphate 
1-96 
1-78 
,, citrate 
3-88 
3'53 
,, chloride 
4-33 
Calcium chloride 
4'33 
— 
Determination of the Osmotic Pressure of the Cell 
Accepting the assumptions already made with regard to the 
water relations of the cell, the determination of the osmotic pressure 
of the cell sap is a simple matter. The method usually employed is 
the plasmolytic method of de Vries already described; in a second 
method, recently proposed and used by Hofler (1917), the osmotic 
pressure is also measured in the cell as such; other methods in which 
the osmotic pressure of the expressed sap is measured either directly 
‘‘approximately isotonic with the cell sap.” But in such tissue as the potato 
tuber employed the turgor pressure is low in comparison with the osmotic 
pressure of the cell sap, and the solution in which such tissue neither gains 
nor loses weight is actually approximately isotonic with the cell sap (cf. Stiles 
and Jorgensen, 1919). With Thoday’s ‘‘rejoinder” to this (1919), that it is 
incorrect to call a turgorless tissue turgescent, the joint authors would doubt¬ 
less agree, at the same time pointing out that it is not they, but Thoday, 
who has suggested that the potatoes used had a high turgor pressure, the word 
turgescent not even appearing in any of their papers. 
