Oct., 1923] ROBBINS — ISOELECTRIC POINT FOR PLANT TISSUE 
435 
compared with 9 and 10. The stained potato was then replaced in 10-cc. 
quantities of the buffer mixtures and allowed to stand for 16 hours. When 
removed from the solution all pieces were colorless, in consequence of a 
reduction of the methylene blue. The exposure to the air and treatment 
with H 2 0 2 developed the blue color. The pieces from the alkaline buffer 
mixtures were bluer than those from the acid buffer mixtures, although a 
sharp line of separation such as could be made in the case of the eosin-stained 
potato could not be made. The potato from solutions 2, 3, 4, and 5 was 
lighter blue than that from solutions 7, 8, 9, and 10, but the pieces from 
solution 6 were intermediate between those from 5 and 7. 
Clearer-cut results with the basic dyes might be secured by weaker 
staining and with more thorough washing with the buffer mixtures after 
staining. The use of tissue with little or no starch and with large thin- 
walled cells well filled with protoplasm would also be advisable. 
The response of dead potato was also compared with that of living 
potato, using the 0.002 M H 3 P 0 4 -Na 0 H buffer mixtures. It was expected 
that a distinct difference would be found between the response of the dead 
and that of the living potato. The potato was killed by treatment with 
50 percent alcohol. The living and the dead potato were treated with the 
same buffer mixtures for the same length of time and stained in the same 
dishes. No difference qualitatively was noticed between the dead and the 
living material. The dead potato, however, stained more deeply than the 
living potato. 
Experiments were also carried out with Spirogyra with methylene blue 
and orange G. Difficulties at once arose, due to lack of uniformity of the 
material and the trouble of comparing the intensities of color in microscopic 
preparations. The Spirogyra was placed in buffer mixtures of 0.002 M 
H3PO4-0.002 M NaOH to which methylene blue at a concentration of 
1-100,000 was added at once or after a period. The results of 5 experiments 
indicated that the greatest intensity of color was found in the Spirogyra 
from solutions 8, 9, and 10, that is, from solutions having a pH of 8.0 or more. 
Variations in the staining of duplicate lots and the difficulty of comparing 
the color intensity made accurate conclusions impossible. The same was 
true in experiments with orange G, although no difficulty was found in 
determining that the orange G was taken up more strongly from the acid 
solutions. Sprigs of Elodea, instead of Spirogyra, or sections of young 
tomato stems or Kudzu plants were also found unsatisfactory material. 
The small size of the sections and the presence of the natural green pigment 
made the difficulties too great to encourage experimentation with them. 
To summarize briefly the results secured with the experiments on dyes: 
Discs of potato 1x15 mm. which were allowed to stand in 0.002 M H 3 P 0 4 - 
0.002 M NaOH buffer mixtures of varying hydrogen-ion concentration for 
from 2 to 6 hours took up and retained acid dyes like eosin more strongly 
when the buffer mixtures had a pH of 6.1 or less than they did when the pH 
