Permeability 73 
may be, can appear in the external solution, this hydrogen-ion or 
hydroxyl-ion being derived from the solvent, in which case the excess 
of the absorbed ion is accompanied into the tissue by an equivalent 
quantity of hydroxyl-ion or hydrogen-ion according as the excess 
of absorbed ion carried a positive or negative charge. In the second 
way an equivalent quantity of some other ion carrying the same 
charge as the ion absorbed in excess, diffuses out from the tissue. 
To take a definite example, let us suppose that a particular tissue 
immersed in a solution of potassium nitrate absorbs the nitrate ion 
in excess. Either an equivalent quantity of hydroxyl ions appears 
in the solution to balance the excess of potassium ions left there, the 
excess of nitrate being accompanied into the tissue by an equivalent 
quantity of hydrogen-ions; or an equivalent quantity of some anion 
or anions, as, for example, sulphate, malate, citrate, diffuses out 
from the tissue. If it can be shown that an unequal absorption of 
ions takes place a complete view of the process can only be obtained 
by determining the quantities of hydrogen, hydroxyl or other ions 
that appear in the external solution. 
Unequal absorption of ions by isolated tissues. Direct observations 
on the intake of the different ions of a salt by isolated tissues are 
few. Some have, however, been made by Meurer (1909) on the 
absorption of a number of salts by slices of beetroot and carrot 3 mm. 
in thickness. A few observations have also been made by Ruhland 
(1909 b) on beetroot in the form of slices 3 mm. and 1 mm. in thick¬ 
ness. Some of the results of these authors are summarised in the 
following table. The absorption in every case is given as the pro¬ 
portion of the concentration of ion absorbed to the concentration 
of the ion in the external solution. 
Table XXX 
Absorption of Ions of a Number of Salts by Plant Tissue 
Duration 
Tissue 
Salt 
Concen- of 
tration of absorption 
solution in days 
Relative Absorption 
,-^ % 
Ration Anion 
Observer 
Carrot 
KC 1 
N/ 15 
2 
o -374 
0-287 
Meurer 
,, 
NaCl 
N /12 
2 
0-411 
0-258 
,, 
CaCl 2 
iV/14 
2 
0-270 
0-229 
Beetroot 
KN 0 3 
NJ 20 
4 
0-524 
0-570 
Ruhland 
CaCl 2 
o *4 % 
2 
0-258 
0-0354 
These results, as far as they go, give clear indications of an 
unequal absorption of ions by the storage tissues examined. The 
