Oct., 1923] ROBBINS-—ISOELECTRIC POINT FOR PLANT TISSUE 413 
Hixon (9) investigated the germination and seedling growth of peas, 
corn, wheat, oats, and carrot in Tollen’s solution, in agar made from tap 
water, in tap water, or in Tollen’s solution made up in tap water. In each 
case the reaction of the medium was adjusted with HC 1 or NaOH to a pH 
of approximately 3.0, 4.0, 5.0, 6.0, and 7.0. He found a double-maximum 
curve for the germination and seedling growth in length of peas with the 
minimum at pH 5.0. A double-maximum curve was found for the germina¬ 
tion and seedling growth in length of corn, wheat, and oats with the 
minimum at a pH of 6.0 between the two maxima. For the germination 
of carrots a double-maximum curve with minimum at pH 5.5 was found, 
although the curve for the seedling growth showed but one maximum. The 
wet weight and dry weight of the roots of wheat seedlings grown for 16 days 
showed a minimum at pH 5.25 with increase in weight on either side of the 
minimum as the alkalinity or acidity increased, the increase being greater 
on the alkaline side. The ash content was less in the solutions more acid 
than pH 6.0 and greater in solutions more alkaline than pH 6.0. The 
growth of the plants tended to shift the reaction of the more acid solutions 
toward greater alkalinity and that of the more alkaline solutions toward 
greater acidity. Hixon quotes a suggestion by E. J. Cohn that the minimum 
point is the isoelectric point of a protein of the cell membrane of the seed. 
Olsen (18), according to Arrhenius, found that some plants showed a 
double-topped curve when the growth was plotted against the pH of the 
medium. He explained this as due to the presence of two different strains 
of plants. 
Hopkins (11) in this laboratory grew Gibberella saubinetii in liquid 
media the reaction of which was adjusted by means of H 2 S 0 4 and NaOH 
or the potassium phosphates, H3PO4 and KOH, and on potato-dextrose 
agar the reaction of which was adjusted by means of lactic acid. He found 
that the growth showed a double-maximum curve when plotted against 
the hydrogen-ion concentration of the medium. The minimum for this 
fungus was found at pH 5.5-6.0. In soil cultures in which the reaction was 
adjusted by H 2 S 0 4 and NaOH, or by HC 1 and NaOH, he also found a 
double-maximum curve when the rate of germination of wheat or the seedling 
infection of wheat by Gibberella saubinetii was plotted against the hydrogen- 
ion concentration of the soil. The minimum for the number of wheat 
seedlings up in 4 days was at pH 54-5.6, and for infection at pH 5.2-5.5. 
The conidial germination of Gibberella saubinetii (10) was also found to show 
a double maximum when the percentage of germination was plotted against 
the hydrogen-ion concentration of the solution used as a medium for 
germination. 
Cole (5) found that the increase in length of seedling corn roots in a 
2-4 day period in buffer mixtures of H 3 P 0 4 and NaOH, or KH phthalate 
and NaOH, showed a double-maximum curve when plotted against the pH 
of the solution. The minimum was located at about pH 6.0. 
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