430 
AMERICAN JOURNAL OF BOTANY 
[Vol. io t 
Discussion 
A consideration of the results of the foregoing experiments shows that 
the analogy between the absorption of water by such a protein as gelatin 
in solutions of different hydrogen-ion concentrations and the absorption of 
water by potato-tuber tissue under the same conditions is good if we assume 
that the isoelectric point of the potato tissue is in the vicinity of pH 6.0. 
In each case the water absorption by the potato tissue is at a minimum in 
the vicinity of pH 6.0 and increases in amount as we proceed toward greater 
alkalinity or greater acidity, passing through a maximum and then decreas¬ 
ing. This is what would be expected if the potato tissue acted as gelatin 
does and had an isoelectric point in the vicinity of pH 6.0. The shifts in 
the reaction of the buffer mixtures resulting from contact with the potato, 
to which attention was called in describing the individual experiments, are 
also what would be expected. As was pointed out in summarizing the 
experiments, those buffer mixtures in the vicinity of pH 6.0 (5.5-6.2) 
showed little or no change in reaction due to the presence of the potato. 
Those more acid showed changes in general toward greater alkalinity, and 
when those more alkaline than about pH 6.0 showed a change it was toward 
greater acidity. On the acid side of the isoelectric point a protein reacts 
with anions, with the result that the reaction of the solution in which the 
protein is placed should shift toward greater alkalinity. On the alkaline 
side of the isoelectric point it reacts with cations, resulting in a change of 
the reaction of the solution toward greater acidity. Part of the change in 
reaction in the alkaline solutions was probably due to the production of 
carbon dioxide by the potato tissue. 
Two apparent difficulties prevent the analogy from being complete. 
The first of these is the fact that the position of the minimum point of water 
absorption, which should be the isoelectric point, is apparently affected by 
the acid radicle of the buffer mixture. Thus, with the phosphates it was 
apparently located at pH 5.8, with the citrate series at 5.5-5.7, and with the 
phthalate series at 6.2. These differences are apparently too great to be 
accounted for by errors in the determination of the hydrogen-ion concentra¬ 
tion. It should be pointed out, however, that Michaelis and Rona (17) 
found that anions shifted the pH most favorable for precipitation of de¬ 
natured albumin toward the acid side and that cations shifted it toward the 
alkaline side. 
A second unexpected but interesting fact is that the dead tissue showed 
the same minimum for water absorption as the living tissue. This might 
suggest that it was not the living protoplasm with which we were dealing, 
but that either dead protoplasm killed in cutting the potato or non-living 
protein contained in the cells was concerned in both the living and the 
completely dead material. There is no complete evidence to demonstrate 
the truth of either of these possibilities. It would appear, however, that 
the differences obtained in the experiments with the dilute HsPO^NaOH 
