Permeability 15 
and in comparative experiments allowance may have to be made for 
this (Stiles and Jorgensen, 1917 a). 
Using this method Briggs and True and True and Bartlett (1912) 
showed the excretion of salts into distilled water by the roots of seed¬ 
lings of Lupinus albus and field peas, while Stiles and Jprgensen 
(1917 a) investigated the influence of a number of organic substances 
on the exosmosis of electrolytes from potato tuber tissue. 
The method is also usable within limits to determine the intake 
of electrolytes by plant tissue. Here a complication arises because 
intake of the salt by the tissue and excretion of electrolytes by the 
tissue will influence the conductivity in different directions. Inter¬ 
action of the exudate and the constituents of the external solution 
may also be a disturbing factor. Results obtained may thus be some¬ 
times very difficult to interpret (Stiles and Jorgensen, 1915 a). 
Nevertheless, by allowing for the exosmosis it may be possible to 
obtain quantitative results as has been done in regard to the absorp¬ 
tion of a number of salts by carrot root by Stiles and Kidd (1919 a, b), 
who have pointed out the causes making for a fall or rise in the con¬ 
ductivity in the external solution, and have shown that in the results 
published by them the values obtained can be taken with confidence 
as minimum values for absorption. These results will also be dealt 
with in the following chapter. 
7. Determination of Permeability and Absorption by 
Means of Electrometric Measurements with the 
Hydrogen Electrode 
The exosmosis of acids and alkalies can be determined by mea¬ 
suring the concentration of the hydrogen ions in the external solution 
by means of the hydrogen electrode. A description of the method and 
the various forms of electrodes that have been devised will not be 
entered into here; those interested should consult the recent work 
of Clark (1920) in which the measurement of hydrogen ions is ade¬ 
quately treated. Similarly the absorption of hydrogen or hydroxyl 
ions from an acid or alkaline solution can be determined. The method 
has been used for measuring the absorption of hydrogen ions from 
dilute solutions of hydrochloric acid by Stiles and Jdrgensen (1915 b), 
and of a number of other acids by Miss Hind (1916). 
