418 
AMERICAN JOURNAL OF BOTANY 
[Vol. io. 
24 to 28° C., but was fairly constant for a given set and was of course uniform 
in its effect since one set was completed at the same time. 
The amount of water absorbed or lost by the potato tissue was assumed 
to equal the difference between the original weight and that found at the 
given interval. This is not strictly correct. The absorption of salts from 
the solution or the loss of salts or organic matter from the potato will affect 
the weight of the potato. It is believed, however, that the amount of such 
exchange plays but a small part in the changes in weight which the potato 
tissue shows in these experiments. Ash determinations made in some of the 
experiments indicate that fact. 
A second source of error, and one which can not be overcome, is the 
relation of the hydrogen-ion concentration of the interior of the potato cells 
to that of the solution by which they are bathed. Are they the same or 
different, and how long does it take for equilibrium to be reached? It is 
very probable that the H-ion concentration of the interior of the cells is not 
the same as that of the solution in which they are immersed. Because of the 
Donnan equilibrium, as has been pointed out by Loeb (13), the H-ion con¬ 
centration of gelatin chloride in a collodion sac immersed in HC 1 is less than 
that of the HC 1 which surrounds the sac. We should, therefore, expect that 
the pH of the interior of the potato cells would not be the same as that of the 
outer solution when equilibrium is reached. No attempt has been made to 
discover the rate at which the H and OH ions penetrate the potato cells. 
However, it is probable that they enter rapidly if we can judge from the 
rapidity with which the color of the cell sap changes when pigmented cells 
are mounted in dilute acid or alkali. The use of discs about a millimeter in 
thickness avoided a mass of tissue in the center which would be slowly 
affected and assisted rapid equilibrium. 
Phosphoric Acid—Sodium Hydroxide Series 
The buffer mixtures in this series were prepared by mixing 0.1 M H 3 P 0 4 
and 0.1 M NaOH in the proportions indicated by the titration curve for 
phosphoric acid and potassium hydroxide given by Clark (3). One experi¬ 
ment was performed using the concentrations secured by the use of the 
0.1 M H3PO4 and NaOH. A second experiment was performed in which 
the original mixtures were diluted ten times with distilled water, and a third 
experiment was completed in which the diluted buffer mixtures were used 
but the range of hydrogen-ion concentration covered included only the region 
of pH 54-6.6. In the first two experiments the distilled water used con¬ 
tained calcium salts carried over in distillation sufficient to raise the pH to 
9.0. In the third experiment water redistilled from acid and alkaline 
potassium permanganate was used in preparing the solutions. The calcu¬ 
lated concentrations of H 3 P 0 4 and NaOH in mols per liter present in each 
of the buffer mixtures used in the first experiment are given in table 1. The 
concentrations given there represent those in the final mixtures. Thus, 
