Oct., 1923] ROBBINS — ISOELECTRIC POINT FOR PLANT TISSUE 415 
the concentration of NaOH inhibited the growth. Although the hydrogen- 
ion concentration was not measured, it would appear that the growth of 
pollen tubes of Phaseolus odoratus in solutions of varying hydrogen-ion 
concentration should produce a double-maximum curve if plotted against 
the pH of the solutions. 
Cohen and Clark (4) figure a double-maximum curve for the number of 
viable cells of B. dysenteriae (Shiga) at the end of 5.5 hours in media of 
different hydrogen-ion concentrations. Hopkins (11) has indicated that 
the curve given by Brightman, Meecham, and Acree (2) for the growth of 
Endothia parasitica at various hydrogen-ion concentrations may be in¬ 
terpreted as having a double maximum. 
Although there is an increasing body of evidence to indicate that the 
occurrence of a double-maximum curve is to be expected when the influence 
of hydrogen-ion concentrations upon various physiological functions or 
processes of plants is investigated, a considerable amount of work has been 
reported which shows no such phenomenon. This should not be considered 
as necessarily indicating the non-existence of a double-maximum curve. 
Many conditions may conceivably conceal or obliterate the minimum 
located between the two maxima. Thus the pH values of the solutions may 
be separated so far that the minimum is not found. Growth conditions 
such as temperature, water supply, container size, or salt content of the 
medium may so limit the development that the values of the two maxima 
may lie close to the minimum value and be indistinguishable from it. 
A minimum located between two maxima also occurs when the swelling, 
the osmotic pressure, the viscosity, the electrical conductivity, and the 
alcohol number of gelatin in solutions of varying reaction is plotted against 
the reaction of the solution expressed as pH. This minimum is located at 
the isoelectric point of gelatin, pH 4.7. The possibility that there might 
be an' isoelectric point for living tissue and that the existence of an iso¬ 
electric point might account for the double-maximum curves and the 
minimum to which attention has been called in the discussion given above 
has led to the investigation which is reported in this paper. 
The Isoelectric Point 
Before describing the experiments involved in this investigation, a 
brief statement of the meaning of the isoelectric point will add to the clear¬ 
ness of the later discussion. 
The conception of an isoelectric point is due to Hardy, who found that 
the direction of movement of a protein in an electric field is determined by 
the reaction of the fluid in which it is suspended. Since a hydrosol in which 
the particles are electro-positive can be changed by the addition of free acid 
until the particles become electro-negative, it is evident that there must be 
some point at which the particles and the fluid in which they are immersed 
are isoelectric. Michaelis and Mostynski (15) defined the isoelectric point 
