Oct.. 1923] ROBBINS-ISOELECTRIC POINT FOR PLANT TISSUE 
425 
the alkaline end of the series where the concentration of the citrate present 
was less. The loss between the 12- and 24-hour periods was least in solution 
3 , pH 5 - 5 , where it was 0.46 g. as compared to losses of 0.658-1.125 g. in 
the other buffer mixtures. If we plot the gains or losses in weight against 
the average pH of the solutions, as in figure 4, we obtain a curve with a 
minimum at pH 5.5-5.67. This series would also probably have given us a 
double-maximum curve had we been able to continue it to greater extremes 
of acidity and alkalinity. 
Table 5. The amount of change in weight expressed as percentage of the total weight and as 
percentage of gain in weight found between the minimum and the acid or alkaline 
maximum in the experiments on water-absorption by potato-tuber tissue 
pH of 
Acid or 
Alkaline 
Maximum 
pH of 
Minimum 
Time 
Amt. of Change in Weight in 
Reaching Minimum 
% Maximum 
Total Weight 
% Maximum 
Gain 
0. i M H3PO4. 
4.85 
5-75 
8 hrs. 
2.1 
17 
0.1 M NaOH. 
7-95 
5-75 
8 hrs. 
3-8 
28 
0.01 M H3PO4. 
5-5 
5-75 
12 hrs. 
4.9 
26 
3-5 
5-75 
12 hrs. 
5-5 
29 
0.01 M NaOH. 
6.2 
5-75 
12 hrs. 
5.2 
27 
0.01 M H3PO4. 
5-6 
5.85 
12 hrs. 
5-7 
26 
0.01 M NaOH. 
6.4 
5.85 
12 hrs. 
5-1 
24 
0.01 M secondary Na citrate. 
5-3 
5-5 
6 hrs. 
0.9 
73 
0.01 M NaOH. 
6-75 
5-5 
6 hrs. 
8.5 
93 
0.002 M secondary Na citrate 
5-6 
5-8 
6 hrs. 
1.6 
11 
0.002 M NaOH. 
6-35 
5-8 
6 hrs. 
2.1 
10 
0.01 M KH phthalate. 
5-55 
6.2 
6 hrs. 
0.6 
4 
0.01 M NaOH. 
6-3 
6.2 
6 hrs. 
1.4 
11 
A still more dilute series of citrate buffer mixtures was used because of 
the rapid incidence of loss in weight which took place in the stronger series 
used first. The second series comprised the original mixtures diluted 50 
times instead of 10 times. This was equivalent to mixing 0.002 M secondary 
sodium citrate and 0.002 M NaOH. The potato tissue was weighed at the 
end of 6, 12, and 24 hours. The solutions at the end of 6 hours were clear, 
and the potato was in good condition. There was .slight evidence of 
bacterial action at the second weighing, and, while most of the discs were 
in good condition, some flaccid ones were present in each lot. By the end 
of 24 hours all the solutions were turbid, indicating the growth of bacteria, 
and the potato was flaccid. In this case solutions 1, 2, 3, 4, and 5, pH 
54-5.8, became more alkaline as a result of standing in contact with the 
potato, the change being greater in the more acid solutions. In solutions 
6, 7, 8, 9, 10, pH 6.1-6.8, the shift was toward greater acidity, the greater 
change taking place in the solutions of higher alkalinity. 
