Oct., 1923] ROBBINS-ISOELECTRIC POINT FOR PLANT TISSUE 
417 
In the preparation of the tissue the general methods of Stiles and 
Jorgensen (23) were followed. By means of a cork borer, cylinders of the 
potato tissue were cut 1.5 cm. in diameter. These cylinders were then 
sliced into circles about 1 mm. in thickness. At first the slices were cut 
by hand with a razor. Later a slaw-cutter was used with part of the board 
cut away under the knife to permit the slices to cut cleanly. The slaw cutter 
produced pieces of uniform thickness and was much more rapid than the 
razor. After being cut, the potato slices were rapidly washed with distilled 
water to remove the starch set free in the cut cells and then blotted dry with 
filter paper until no evidence of free water was visible on the surface of the 
slices. After thorough mixing, they were weighed out in lots of 10 g. and 
dropped into 150-cc. quantities of the solutions used, contained in 150-cc. 
beakers of Pyrex glass. 
Buffer mixtures of phosphoric acid and sodium hydroxide, of secondary 
sodium citrate and sodium hydroxide, or of potassium hydrogen phthalate 
and sodium hydroxide were used to maintain the desired hydrogen-ion 
concentrations. The disadvantages and advantages of using buffer mix¬ 
tures instead of acids or alkalies only for studying the effect of hydrogen-ion 
concentration are well appreciated and need not be discussed at length here. 
The presence of ions other than the hydrogen and hydroxyl ions at con¬ 
centrations which are necessarily not constant and which are far above those 
of the hydrogen and hydroxyl ions is unavoidable, and must be taken into 
consideration in interpreting the data. The buffer mixtures afford a means, 
however, of maintaining a definite hydrogen-ion concentration which could 
not be done if an acid or alkali alone were used. 
After the potato tissue had stood in the buffer mixtures for 6 to 12 hours, 
the slices were removed, carefully blotted dry with filter paper, weighed, and 
returned to a fresh quantity of the same buffer mixture. After 12 or 24 
hours, and again at the end of 24 or 48 hours, the process was repeated. In 
this way three sets of weighings at different intervals of time were made 
on each lot of potato. 
Each treatment was performed in triplicate, and the average of the 
triplicate weighings is given in the tables or used in constructing the curves 
which follow. The hydrogen-ion concentration of the solutions in contact 
with the potato was also determined by Gillespie’s colorimetric method each 
time the potato was weighed in order to learn how much the presence of 
the potato had affected the reaction. The accuracy with which the colori¬ 
metric method of determining the hydrogen-ion concentration was used 
probably did not exceed 0.1 pH. Difficulty was experienced with brom- 
cresol-purple. This indicator seemed to yield results which were about 
0.2 pH lower than those secured with methyl red or brom-thymol-blue. 
Wherever possible, determinations were checked with a second overlapping 
indicator, but in the lower range of brom-cresol-purple this was impossible. 
All experiments were carried out at room temperature. This varied from 
