[Vol. 9 

 37() ANNALS OF THE MISSOURI BOTANICAL GARDEN 



growth in solutions containing sugar over those in which it is 

 absent, in the amino acid series which was previously presented. 



Hydrogen-ion concentration in relation to growth. — It has al- 

 ready been reported that the concentration of hydrogen ions plays 

 an important part in the inhibition or enhancement of growth of 

 the foxtail organism. In addition to the experiment on acid 

 broths, nutrient broth was rendered acid or alkali by adding hy- 

 drochloric acid and sodium hydroxide aseptically after the broth 

 had been sterilized. The following series was used: Pn 4.8, 5.0, 

 6.6, 7.2, 7.4, 7.6, 8.5, 8.6, and 9.8. There was good growth in P H 

 6.6, 7.2, 7.4, and 7.6, and no growth in P H 4.8, 5.0, 8.5, SXk and 9.8. 

 This series in combination with the other studies indicate that the 

 optimum reaction lies between P„ 6.0 and Ph 8.0, that the lower 

 limiting reaction for growth is around P H 5.0, varying somewhat 

 with different media, and that the upper limit for growth to 

 occur is somewhat below P H 8.5. 



NUTRIENTS IN K ELATION TO PRECIPITATE PRODUCTION 



Production of a colorless zone followed by a white discolora- 

 tion has already been reported on nutrient agar and on whey 

 agar when these gave a slightly acid reaction. It has also been 

 reported that no precipitate was obtained on nutrient gelatin. 

 In an effort to determine which material in the media contained 

 the ingredient which gave rise to the precipitate a number of ex- 

 periments were tried. Beginning with agar an attempt was made 

 to substitute other plant materials for the algal product and the 

 gums arabic and tragacanth were tried without success. The 

 first of these when dissolved in water does not give a stiff me- 

 dium even at very high concentrations, and the second is ex- 

 tremely difficult to dissolve in water. Attempts were then made 

 to thoroughly wash the agar according to the method used by 

 Ayers, Mudge, and Rupp (II, '20). These authors found that 

 unwashed agar contained various calcium and magnesium salts 

 as well as proteins and that by a thorough washing considerable 

 part of these impurities were removed. Shredded agar was used, 

 and after washing the nutrients peptone and beef extract were 

 added in the usual amounts; part of the medium was allowed to 

 remain slightly acid, Ph 6.6, and part was rendered slightly al- 

 kaline with sodium hydroxide, Ph 7.2. Good growth was obtained 

 on both, and while no precipitate was obtained in the alkaline 

 medium the characteristic white discoloration occurred on the 



