14 
Walter Stiles 
mined the relative absorption of potassium and calcium by wheat 
and maize, Colin and de Rufz de Lavison (1910 a, b) who investigated 
the absorption of barium, strontium and calcium, Pantanelli (1915 a, 
b y Cy 1918) and Pantanelli and Sella (1909) who paid particular atten¬ 
tion to the unequal absorption by a number of species of the two ions 
of the same salt, and Miss Redfern (1922 a) who investigated the same 
problem critically in the case of the absorption of calcium chloride 
by the edible pea and maize. The absorption of glucose by the roots 
of the latter plant was shown by Laurent (1897). 
Absorption of salts by disks of storage tissue was examined by 
Nathansohn (1903,1904 a, b), Meurer (1909) and by Ruhland (1909 b). 
The results obtained by these various workers will be dealt with in 
the next chapter. 
The exosmosis of any particular substance can, of course, be 
examined by this method. Thus it has been shown that sugars, chiefly 
sucrose, can diffuse out of the cells of the leaf of the sugar beet when 
they are immersed in water (Puriewitsch, 1898; Ruhland, 1911). 
6 . Determination of Permeability by Measurement of the 
Electrical Conductivity of the External Solution 
The exosmosis of electrolytes from plant tissue into a surrounding 
medium consisting of pure water or a solution of a non-electrolyte 
will result in an increase in the electrical conductivity of the external 
solution. Measurement of the electrical conductivity of the latter can 
therefore be used as a criterion of the permeability of the cell mem¬ 
branes to the electrolytes of the cell. The method cannot of course 
be used to determine the exosmosis of any particular electrolyte, 
but it can be used for approximate quantitative determinations 
of exosmosis by assuming the electrical conductivity of the ex¬ 
ternal solution to be a measure of the concentration of electrolytes 
in the solution. This is, of course, only approximately true, since the 
mobilities of different ions are different and with increasing concen¬ 
tration of the electrolytes the degree of dissociation will decrease, 
and the electrical conductivity is dependent on the mobilities of the 
ions and the degree of dissociation. However, with the exception 
of hydrogen and hydroxyl ions, the mobilities of ions likely to diffuse 
out of plant cells do not differ greatly among themselves, while the 
degree of dissociation will not be depressed greatly in the dilute 
solutions likely to be involved. If this external solution contains a 
non-electrolyte the presence of this will depress the conductivity, 
