268 
Journal of Agricultural Research 
Vol. XXIV, No. 3 
A rather interesting phenomenon was often noted in these liquid 
cultures when treated with a lead-acetate solution. If the heavy white 
membraneous precipitate which forms is collected on a filter, it gradually 
darkens and within several minutes is black. The reaction resembles 
an enzymatic process. No attempt was made to ascertain whether or 
not this process is connected with the characteristic pigment production 
in the living cells. 
The yield, or amount of growth, was determined by centrifuging 50 cc. 
of the culture until the supernatant fluid was clear. This was decanted 
off, care being taken not to disturb the precipitate cells. The centrifuge 
tube containing this precipitated growth was placed in a drying oven and 
desiccated for eight hours at 75° to 80° C. to a constant weight. Dupli¬ 
cate determinations of each culture were made. The yield is recorded 
as a percentage of sugar originally present in the medium. 
A daily comparison of the nitrogen assimilation, dextrose fermentation, 
and the yield of three cultures of Azotobacter is summarized in Table II. 
Four flasks containing 250 cc. of dextrose medium were used for each 
culture. All were aerated vigorously at a temperature of 30° C. One 
flask of each culture was removed daily for analysis. The medium con¬ 
tained I gm. of dextrose per 100 cc. 
Table II. —Yield rate of dextrose fermentation and nitrogen fixation by Azotobacter 
I day. 
2 days. 
Culture No. 
Aver¬ 
age. 
Culture No. 
Aver¬ 
age. 
zo B 
19 
19-399 
zo B 
19 
19-399 
Nitrogen per 100 cc. 
(mgm.)... 
Dextrose per 100 cc. 
/ crTn^ . 
I. 82 
4-31 
.49 
•0743 
.0717 
14. 50 
5. 60 
.67 
.0319 
.0154 
5 - 73 
3-91 
8. 77 
.478 
. 0901 
.0784 
16. 85 
8. 21 
0 
•1939 
. 1741 
34 -80 
10. 20 
. 06 
. 1281 
. 1198 
24. 79 
9. 06 
Yield per 50 cc. (gm.). 
Yield (per cent). 
f . 0920 
1 *0750 
16. 70 
12.31 
28.81 
3 days. 
4 days. 
Culture No. 
Aver¬ 
age. 
Culture No. 
Aver¬ 
age. 
xo B 
19 
19-399 
zo B 
19 
19-399 
Nitrogen per 100 cc. 
(mgm.). 
Dextrose per 100 cc. 
(gm.). 
Yield per 50 cc. (gm.). 
Yield (per cent). 
9 - 36 
.05 
f . 0841 
16. 82 
13. 72 
0 
• 1487 
• 1 S 9 S 
30.82 
12. 50 
0 
. 1234 
24. 68 
II. 86 
24. 10 
12. 0 
0 
. 1043 
. 0889 
19.32 
14-43 
0 
• 1317 
. 1528 
28. 45 
12. 90 
0 
. 1017 
. 1149 
21. 66 
13. II 
23. 14 
There occurs a gradual increase in the quantity of nitrogen fixed. 
The average quantity fixed for the three cultures was for the first day 
3.91 mgm., for the second day 9.06 mgm., for the third day 11.86 mgm., 
for the fourth day 13.11 mgm., per 100 cc. of medium. The 
characteristic rapid disappearance of dextrose occurred. It was practi¬ 
cally all consumed by the three cultures within two days. 
