Oct., 1923] ROBBINS- ISOELECTRIC POINT FOR PLANT TISSUE 
431 
buffer mixtures were too great to be accounted for by assuming that dead 
protoplasm or non-living protein was involved. Of course a comparatively 
small change in the isoelectric point with death would not be revealed by 
the methods used. Michaelis and Davidsohn (16) measured the isoelectric 
point of genuine albumin and of albumin denatured by heat by precipitation 
with sodium acetate. They found that the denaturing shifted the iso¬ 
electric point from pH 5.39 to pH 4.69. 
MacDougal (14) has reported, in contradiction to earlier statements 
by the same author, that purified agar swells more in dilute HC 1 , pH 4.2, 
and dilute KOH, pH 11, than in water. The agar at 0.75 percent which is 
near the limit of its gelation at 15 0 C. had a pH of 6.5. These results indicate 
that pentosans may also show a double-maximum curve when their swelling 
is plotted against their pH. The confirmation of this possibility would 
permit an analogy to be drawn between the water absorption by potato and 
that by a pentosan. This analogy, with our present knowledge, would not 
hold for the changes in reaction which the potato produced in the buffer 
mixtures, nor would it explain the results obtained on the absorption of dyes 
reported later in this paper. Rosa (21), in some incomplete studies made 
in this laboratory, did not confirm MacDougal’s results. 
The Absorption of Dyes by Plant Tissue 
It was pointed out in discussing the relation of the isoelectric point to 
the properties of gelatin that the amphoteric gelatin on the acid side of the 
isoelectric point forms salts with anions only, existing as gelatin chloride, 
gelatin sulfate, gelatin citrate, etc., while on the alkaline side of the iso¬ 
electric point it forms salts with bases existing as sodium gelatinate, potas¬ 
sium gelacinate, calcium gelatinate, etc. Since the colored radicles of acid 
dyes like eosin are in the acid radicle, gelatin of different hydrogen-ion 
concentrations takes up and retains the dye on the acid side of the iso¬ 
electric point and does not take it up or does not retain it on the alkaline 
side of the isoelectric point. With basic dyes like safranin, on the other 
hand, gelatin will be stained on the alkaline side of its isoelectric point 
(pH 4.7) and will not be stained on the acid side of pH 4.7. This method of 
determining the isoelectric point has been used by Loeb (13) for gelatin and 
by Thomas and Kelly (24) for the protein, collagen, of hide powder. 
In applying the dye method to the investigation of the isoelectric point 
of plant tissue, potato-tuber tissue was used chiefly and found most satis¬ 
factory. A few experiments were also carried out with Spirogyra, leaves of 
Elodea, and sections of tomato and Kudzu stems. In general it was found 
that plant tissue which had been treated with buffer mixtures of different 
H -ion concentration responded to dyes as though it had an isoelectric 
point. 
The potato used in these experiments was prepared as previously de¬ 
scribed for the experiments on water absorption. From 2 to 25 discs of 
