Oct., 1923] ROBBINS — ISOELECTRIC POINT FOR PLANT TISSUE 427 
when the first change in weight was measured, the solutions were clear and 
the potato was in good condition. At the time of the second weighing the 
solutions were clear, but the potato was flaccid in the acid solutions but 
turgid and in good condition in the alkaline solutions. At the end of 24 
hours, solutions 3-11 were quite cloudy, indicating the growth of bacteria, 
and all the potato was flaccid save that in redistilled water. It can be 
noted in table 4 that solutions of pH 4.2-5.85 became more alkaline in 
contact with the potato, the greater change being at the acid end. Solu¬ 
tions of pH 6.2 and 6.3 showed no change, while the two most alkaline, 
pH 6.9 and 7.6, showed a decided change toward greater acidity. The 
changes in weight were quantitatively much the same as those found with 
the more dilute citrate buffer mixtures. By the end of 24 hours, the potato 
in all the buffer mixtures weighed less than it did originally. When the 
change in weight is plotted against the pH of the solution, a curve (figure 6) 
Fig. 6. Gain or loss in weight of 10 g. of potato-tuber tissue in 6, 12, and 24 hours in 
150-cc. quantities of buffer mixtures of 0.01 M potassium hydrogen phthalate and 0.01 M 
sodium hydroxide. See table 4. 
