388 
Journal of Agricultural Research 
Vol. XXXI, No. 4 
The writers conclude from the two experiments presented in Table 
1 that potato-tuber tissue in 0.001 M. potassium phosphates acts like 
an amphoteric colloid with an isoelectric point of P H 6.45. On the 
acid side of this point the tissue acts mainly as a base, taking up the 
phosphate ion chiefly; and on the alkaline side it acts mainly as an 
acid, taking up the potassium ion. 
Most of the results in 11 other experiments confirmed those of the 
2 just described. In Table II are summarized 13 experiments made 
with potato-tuber tissue by the methods which have been given. 
The initial P H of the solution, the final P H of the solution, the length 
of time the experiment was run, and the weight of the potato-tuber 
tissue are given for each experiment. In experiments 11 and 13, the 
original lot of tissue was replaced by a fresh lot. Experiments done 
on the same day on the same sample of potato tubers are bracketed 
together. An examination of the data in Table II shows that in 7 
cases the final equilibrium point reached was P H 6.43 to 6.46; in 3 
additional cases P H 6.37^to 6.39. In experiment 3 the final point 
reached was P H 6.2. This probably represents, however, an equili¬ 
brium point due to a deficiency of ampholyte. The solution in experi¬ 
ment 3 was the most acid one used and would necessitate a greater 
absorption to bring its reaction to the isoelectric point than would 
solutions such as were used in experiments 4 and 5. If a fresh lot 
of tissue had been added to the solution used in experiment 3 the 
Sorensen value would probably have been increased. In only two 
cases were the results contrary to the conclusion that potato-tuber 
tissue in 0.001 M. potassium phosphates acts like an amphoteric 
colloid with an isoelectric point near P H 6.4. In experiments 2 and 
9 the addition of potato-tuber tissue to solutions of P H 6.32 and 6.33 
lowered the P H instead of raising it. Whether this is due to experi¬ 
mental errors, to variations in the potato tissue, or to some other 
cause is an unsolved problem. 
Other experiments than those summarized in Table II were made 
with potato-tuber tissue. The results, however, were considered 
misleading because from 25 to 50 c. c. of solution was used in a 150 
c. c. beaker and the hydrogen electrode was unprotected. This 
larger amount of liquid and the larger container did not permit the 
complete and rapid removal of the carbon dioxide by the stream of 
hydrogen. The potato disks crowding together on one side of the 
beaker prevented the ready mixing of the solution with the possible 
development of a local zone the reaction of which would be different 
from that in the vicinity of the electrode. Experiments in which 
potato-tuber tissue was placed in 50 c, c. of 0.001 M. potassium 
phosphates of initial reaction of P H 5.73, 6.37, 6.48, and 6.97, showed 
final Sorensen values, after 6 to 7 hours, of P H 6,30, 6.78, 6.70, and 
6.86, respectively. Hydrogen was bubbled through the solutions 
part of the time during these experiments, and part of the time the 
potato stood quietly m the solutions. These liigh values are no 
doubt due to the presence of carbon dioxide, which kept the reaction 
of the tissue, or a part of it, on the acid side of the isoelectric point, 
and induced a continued absorption of the phosphate ion as m the 
experiments with soy-bean root tips described later in this paper. 
The results of the experiments given in Table II agree fairly well 
with those performed earlier by one of the writers (5) on the effect 
of buffer mixtures on water absomtion and dye absorption by potato- 
