Oct., 1923] ROBBINS — ISOELECTRIC POINT FOR PLANT TISSUE 
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in solution 3, equal parts of 0.1 M H 3 P 0 4 and 0.1 M NaOH were mixed. 
The final buffer mixture contained, therefore, if the two constituents which 
compose it are considered individually, 0.05 M H 3 P 0 4 and 0.05 M NaOH. 
It is recognized, of course, that the H 3 P 0 4 and NaOH do not exist as such 
in the buffer mixtures, but for convenience they are expressed as individuals. 
In the second experiment the concentrations were one tenth of those in 
experiment one. 
In the first experiment of this series the potato was weighed at the end of 
8, 12, and 24 hours. The amount of potato used for the solutions of different 
hydrogen-ion concentration varied from 10.06 g. to 10.16 g., but the gains 
or losses in weight were calculated per 10 g. of original potato tissue. An 
examination of the data showed that little change took place in the pH of 
the solutions as a result of standing in contact with the potato. There was 
a slight change toward greater alkalinity at the acid end. The increase in 
weight for the potato in the two most acid and the two most alkaline solu¬ 
tions was greatest at the end of 8 hours. For the balance of the solutions 
little change took place between 8 and 12 hours. By the end of 24 hours the 
potato in all solutions had begun to decrease in weight below the maximum. 
This loss was greatest in solutions 1, pH 2.0, and 3, pH 4.1, amounting to 
more than 0.9 g., and least in solution 5, pH 5.4, where it amounted to but 
Fig. 1. The gain or loss in weight of 10 g. of potato-tuber tissue in 8 and 24 hours in 
150-cc. quantities of buffer mixtures of 0.1 M phosphoric acid and 0.1 M sodium hydroxide. 
The buffer mixtures changed at 8, 12, and 24 hours. 
