THE INFLUENCE OF OTHER OKGANISMS. 53 



EXERCISE XLI. 



THE INFLUENCE OF OTHER ORGANISMS ON NITROGEN-FIXA- 

 TION BY AZOTOBACTER. 



1. Measure ofE twelve 100 cc. portions of "mannite'" solu- 

 tion in 500 cc. flasks. 



2. Plug and sterilize as usual. 



3. Inoculate with Azotobacter and pure cultures of other 

 soil organisms, using 1 cc. of vigorous cultures in every 

 case. 



4. 



1 & 2 — Az. vinelandii. 

 3 & 4 — Az. vinelandii — B. mycoides. 

 5 & 6 — -Az. vinelandii — B. aerogenes. 

 7 & 8 — Az. chroococcum. 

 9 & 10 — Az. chroococcum — B. mycoides. 

 11 & 12 — Az. chroococcum — B. aerogenes. 



5. Incubate 6-7 days at 20° C. 



6. Add Hg. and H2SO4 and digest for total nitrogen by 

 the regular Kjeldahl method. ' 



7. The effect of the non-fixers on the fixation by Azoto- 

 bacter is thus determined. 



