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[Vol. 10 



158 



ANNALS OF THE MISSOURI BOTANICAL GARDEN 



ion concentration, total acidity, and amount of sugar were 

 determined. Total acidity was determined by titrating a 10-cc. 

 aliquot of the 50 cc. of filtrate with 0.10055 N KOH, using phenol- 

 phthalein as an indicator. The micro method of Shaffer and 

 Hartmann ('21) was employed for the sugar determination. 

 Determinations were made on each member of the duplicate 

 cultures. 



Fig. 11. 



Experiment I. — In this test, cultures were started at the initial 

 H-ion concentrations represented by P H 1.8, 2.1, 3.1, 4.0, 5.1, 

 5.9, 6.5, 7.2, 8.5, and 9.4, and were allowed to grow for a period 

 of 18 days. At the end of this time, best growth, as indicated 

 by dry weight of mat, was found to have occurred in the cultures 

 started at P H 4 and 5.1. No growth took place at P H 1.8 and 2.1, 

 and growth was less at P H 3.1 than at any higher P H exponent. 

 A second minimum occurred at P H 8.5, but there was very little 

 difference between the amounts at P H 7.2, 8.5, and 9.4. Growth 

 was accompanied by a shifting of the reaction of the culture 



