1919] 



ALLEN AZOTOBACTER CHROOCOCCUM 



37 



Agar 

 no. 



Growth after 



24 hours 



72 hours 





II 



III 



Faint growths 



6 days 



10 days 



Pronounced, 

 raised, srrooth, 

 slimy streaks 



Slight growths 



Abund int 

 rather flat 

 growths, ap- 

 parently best 

 in whole 

 series 



Growths some- 

 what flat and 

 spreading, be- 

 coming dry and 

 slightly wrinkled, 

 dry portions be- 

 coming brownish 



Abundant, spread- 

 ing growths; 

 streaks quite 

 largely run to- 

 gether; consider- 

 able invasion 

 and collection of 

 slime at base of 

 slant 



Good growths 

 now dry and 

 wrinkled ; pig- 

 ment production 

 quite marked 



Abundant, flat, 

 wrinkled 

 growths ; some 

 pigment produc- 

 t i o n ; growths 

 slightly heavier 

 than in No. II 



The following general conclusions may be drawn from the 

 above experiments with different agars : Agar No. I produces 

 the poorest growth and No. Ill the best; agar No. II pro- 

 duces a growth distinctly better than No. I and almost as 

 good as that on No. III. 



Nutrient solutions. — In these experiments only enough agar 

 was added to make the solution slightly viscous. Only one- 

 tenth the amount of agar was used, i. e., 1.5 gms. per liter. 

 Now, if this amount of agar functions as a protective colloid 

 the precipitated phosphate should pass through the filter 

 quite largely. That being true, then the effect of filter- 

 ing the medium reported above (pp. 31-32) would largely dis- 

 appear. To see whether any such action could be detected a 

 modified Ashby medium was prepared, using .15 per cent agar 

 as a protective colloid, and filtered and unfiltered portions 

 tested with and without mechanical agitation. A modified 

 Kaserer's solution was tested in a similar manner. 



The modified Ashby solution was 



pared as described 



above for the use of agar as a protective colloid, except that 

 0.75 gm. instead of 7.5 gms. was added to each of the solutions 



ponding to solutions A and 



were 



mixed, well stirred up with CaCOs, heated for 30 minutes ii 

 the autoclave, and then a portion of the preparation filtered 





