416 
AMERICAN JOURNAL OF BOTANY 
[Vol. io» 
of a protein as that reaction where the relation of the concentration of the 
hydrogen ions to the hydroxyl ions in the solution is the same as the relation 
of the acid-dissociation constant (Ka) of the protein to its basic-dissociation 
constant (Kb). At the isoelectric point the sum of the number of protein 
anions is equal to the number of protein cations present, and the sum of the 
protein ions in relation to the non-ionized protein is at its minimum. 
The significance of the isoelectric point for the physical and chemical 
properties of proteins has been pointed out by Hardy, Procter and Wilson, 
Loeb (13), and others. At the isoelectric point the physical properties of 
such a protein as gelatin are at a minimum. Thus, if the swelling of the 
same quantity of gelatin in solutions of different hydrogen-ion concentrations 
is measured, it is found that the swelling is least at the isoelectric point, 
pH 4.7. In solutions of greater acidity or alkalinity than this, the swelling 
increases to a maximum. The same is true of the viscosity, osmotic pressure, 
conductivity, and alcohol number of gelatin, as has been pointed out by 
Loeb. A protein can combine with anions only on the acid side of the 
isoelectric point and with cations only on the alkaline side of the isoelectric 
point. In other words, a protein may act as either a base or an acid, the 
reaction of the solution with the isoelectric point as the critical one deter¬ 
mining which it shall be. From the above discussion it is evident that there 
are available several methods of determining the isoelectric point of a pro¬ 
tein. It can be determined by the method of Hardy, i.e., by observing the 
migration of particles in an electric field in solutions of different pH. It 
can be determined by measuring the swelling, viscosity, osmotic pressure, 
or electrical conductivity of the protein. It can also be determined by 
determining in what H-ion concentrations cations or at what hydrogen-ion 
concentrations anions do and do not combine with the protein. These 
methods have largely been developed by Loeb (13) chiefly by a study of 
gelatin. 
Experimental 
The experimental work described in this paper includes experiments 
which deal with the absorption of water by potato-tuber tissue in solutions 
of different hydrogen-ion concentrations and experiments on the absorption 
of dyes by plant tissue which had been in contact with solutions of different 
hydrogen-ion concentration. 
Experiments on Water Absorption 
Potato-tuber tissue was used for the experiments on water absorption 
because it appeared to furnish the most easily available supply of fairly 
uniform material. One experiment was also carried out with beet tissue, 
but the material showed such great variation, probably due to the fact that 
stored beets, somewhat wilted, were used, that no conclusions could be 
drawn from the data and the results are not reported. 
