Permeability 77 
acid or alkaline, while in other cases the equality of positively and 
negatively charged ions in the . external solution is maintained by 
exosmosis of ions from the plant cells. The acidity and alkalinity 
that may be developed in water culture solutions, and which has 
been noticed from the time of Knop onwards, can be explained as 
a case of the former happening. On the other hand, Hoagland (1918) 
found that the culture solution in which barley was growing main¬ 
tained a neutral reaction, and, if the reaction was acid to start with, 
the solution became neutral after contact with the roots for a time. 
Pantanelli considered that the unequal absorption of the two ions 
resulted in the development of acidity or alkalinity at first, but that 
after a time this might disappear and the excess of absorbed ion 
become balanced by excretion of oppositely charged ions from the 
tissue. 
Miss Redfern found that the calcium chloride solutions used in 
her experiments in which the calcium ion was absorbed in excess 
remained approximately neutral throughout the experiment, while 
tests of the external solution showed that magnesium and potassium 
ions had diffused out of the tissue. Therefore the excess of calcium 
ion absorbed is replaced in this case by the diffusion out of the tissue 
of ions carrying a similar charge, and not by hydrogen ions from the 
water of the solution. 
Stoklasa, Sebor, Tymich and Cwacha (1922) also conclude that 
the absorption of aluminium and ferric ions by the roots of living 
plants of Eriophorum vaginatum, Phragmites communis and Carex 
riparia is accompanied by the excretion of calcium, magnesium and 
sodium ions. Their conclusion is based on water culture experiments 
in which the analysis of the solutions was made after the experimental 
plants had been growing in them for 13 days. Since the aluminium 
ion is absorbed much more rapidly than the anion, this excretion of 
other kations must necessarily take place if the solution does not 
become highly acid or alkaline. 
The Position of the Equilibrium Attained in the Intake 
of Dissolved Substances by Plant Cells 
It is a remarkable fact, and one indicating how little the com¬ 
plexity of the problems involved in the phenomena of permeability 
and absorption have been realised by workers in these fields, that 
scarcely any of those who have attempted to obtain quantitative 
data with regard to permeability have concerned themselves with 
the equilibrium attained in the passage of dissolved substances into 
