Oct.; 1923] ROBBINS — ISOELECTRIC POINT FOR PLANT TISSUE 
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that from solutions 3, 4, 5, and 6, pH 4.9-6.1, less red than 2 but distinctly 
redder than that from 7, 8, 9, and 10, pH 6.95-7.85. The potato from the 
redistilled water, pH 6.5, was about like that from solution 6. The potato 
discs from solutions 3, 4, 5, and 6 were almost alike, with some tendency to 
greater intensity of staining in those from the more acid solutions. The 
potato discs from solutions 7, 8, 9, and 10 were almost alike. With 10 
minutes’ staining but more thorough washing, even better results were 
secured. 
These results show that the acid dye eosin is absorbed and held by potato- 
tuber tissue from buffer mixtures whose average pH is 6.1 or less, and is 
held very weakly by potato discs from buffer mixtures of average pH 6.95 
or above. In other words, the potato acted much as we should expect a 
protein to act with isoelectric point between pH 6.1 and pH 6.95. 
Similar results were secured with the buffer mixtures made from 0.02 M 
H3PO4 and 0.02 M NaOH. After 3 hours’ contact with the buffer mixtures, 
the potato was stained with eosin 1-500 for 5 minutes. The potato from 
solutions 2, 3, 4, 5, and 6, pH 2.0-5.6, was distinctly deeper red than that 
from solutions 7, 8, 9, and 10, pH 6.95-8.95 +. The line of separation 
appeared between 6 and 7. 
With the citrate set of buffer mixtures, the potato was stained with 
1-500 eosin for 10 minutes after 6 hours’ treatment with the buffer mixtures. 
The intensity of color decreased from that of the potato from solution 2 to 
that of the potato from solution 10. The potato from the redistilled water 
stained about like that from solution 5. There was a fairly sharp line of 
separation between the potato from solution 5, pH 5.8, and that from solu¬ 
tion 6, pH 6.2. However, the potato discs from solutions 6 and 7 were a 
little redder than those from 8, 9 and 10, which latter were about alike. 
This would place the isoelectric point either between pH 6.2 and pH 6.3 or 
between pH 6.3 and pH 6.5. 
A number of experiments were also completed with Martius yellow, 
using the 0.002 M H3PO4-0.002 M NaOH buffer mixtures only. For 
example, the potato was treated with buffer mixtures composed of 0.002 M 
H3PO4 and 0.002 M NaOH for 2 hours. The pieces were removed and 
stained with the Martius yellow 1-2500 for 5 minutes. They showed little 
difference in intensity of staining at this time. They were washed with 
redistilled water several times and with the buffer mixtures for 20 minutes. 
An examination of the discs showed that the potato responded as would be 
expected if an isoelectric point existed between pH 6.1 and pH 6.95. The 
potato from the redistilled water stained somewhat less than any of the 
other pieces. 
Orange G at a concentration of 1-500 for 1 minute and for 10 minutes 
was also used. Most of the stain, however, washed out, leaving only a 
little color in the pieces from the more acid solutions. A longer period of 
staining or a stronger solution of the dye might give more favorable results. 
