• i 7 8 
Walter Stiles 
Table XVIII 
Approximate Isotonic Coefficients of de Vries 
Substances 
Isotonic coefficient 
Organic metal-free compounds ... ... ... ... 2 
Alkali salts with one metallic atom in molecule ... 3 
„ „ two „ atoms ,, ... 4 
„ ,, three „ atoms „ ... 5 
Alkaline earth salts with one metallic atom in molecule 2 
„ „ ,, two „ atoms ,, 4 
The value of the isotonic coefficient depends, of course, on the 
osmotic pressure, which depends, as pointed out in Chapter VI, on 
the degree of ionisation of the solution, the high values of the isotonic 
coefficients of electrolytes as compared with organic compounds such 
as sugar being explained on this ground. Since, as pointed out by 
Livingston (1903), the coefficients are only true values within limits 
(cf. Chapter VI), and as more accurate methods are available for 
the determination of the degree of ionisation, we may agree with 
Livingston that “it will be well for physiology when the practical 
use of these coefficients dies out entirely.” Nevertheless, the work 
of de Vries on this subject was nearly as important for both physio¬ 
logy and physical chemistry as that of Pfeffer on osmotic pressure, 
and diagrams of unplasmolysed and plasmolysed cells taken from 
de Vries are to be found in standard text-books of botany and physics 
(e.g. Poynting and Thomson, 1905). Moreover, the methods elabo¬ 
rated by de Vries (1884 a, 1884 b) for the determination of isotonic 
coefficients have found a wide use for physiological problems con¬ 
nected not only with the water relations of the cell, but also with the 
passage of dissolved substances into and out from the cell. Some 
description of these methods is therefore desirable. They are the 
plasmolytic methods (the comparative plasmolytic method and the 
plasmolytic transport method) and the method of tissue tension. 
Comparative plasmolytic method. Certain experimental conditions 
are desirable for the successful working of this method. These are 
(1) that the smallest trace of plasmolysis should be easily recognisable, 
(2) that all the cells of the tissue used should just plasmolyse in 
solutions of the same salt of exactly the same concentration, and 
(3) that the substances examined should not penetrate the cells. 
De Vries specially recommended three particular kinds of cells for 
this work, namely (1) the epidermal cells of the outer side of the 
growing leaf sheath of the dark red form of Curcuma rubricaulis, 
(2) the violet cells of the under epidermis at or near the midrib of 
