Jan. 28, 1918 



Nitrogen- A ssimilating Bacteria 



207 



It will be seen that all three nitrates exerted considerable influence 

 on the formation of volutin bodies. Not only was the number of bodies 

 increased, but also the size. The relative increase in size of the granules 

 was much more marked than was the numerical increase. In Azoto- 

 bacter cells grown on mannit agar containing no nitrate the number of 

 volutin bodies in each cell averaged about two ; in the presence of nitrate 

 four to five volutin granules were found. The greatest increase in num- 

 ber, as well as size, occurred where the nitrate concentration was highest. 

 With both strains sodium nitrate apparently caused the greatest increase. 

 This was true in the lower as well as in the higher concentrations. The 

 volutin bodies in strain B seemed to respond to the presence of nitrates 

 more noticeably than did those of strain A, especially in the presence of 

 potassium nitrate. It is evident that nitrates of potassium, sodium, 

 and calcium cause an increase in the number and size of volutin bodies 

 in Azotobacter cells. 



Do these salts tend to hasten the appearance of these bodies, or do 

 they at first retard their development? The following experiment was 

 carried out in an endeavor to determine this point. Only sodium nitrate 

 was used, since this particular salt proved most beneficial to the forma- 

 tion of volutin bodies. Agar slopes were prepared containing the different 

 amounts of nitrate as indicated in Table XIX. The cultures were incu- 

 bated at 28° C. and examined daily for the presence of volutin bodies. 

 The methylene blue — i per cent sulphuric acid — method of staining was 

 employed. The results of the experiment are given in Table XIX. 



Table XIX. — Influence of sodium nitrate on the rate of formation of volutin bodies. in 



Azotobacter 



" Representing an approximate average of two volutin bodies per cell. 

 b Representing an approximate average of four volutin bodies per cell. 



A study of Table XIX shows that it is rather doubtful whether the 

 nitrate present tended to hasten the appearance of the volutin bodies. 

 No convincing evidence has been presented for or against this statement. 

 No granules were seen in the first day's growth of strain A, although 

 the next day they were present in all four cultures. In strain B more 

 convincing proof is furnished that the sodium nitrate hastened the 

 appearance of these reserve food substances. The volutin bodies were 

 not present in the control and lowest nitrate concentration cultures 

 the first day, but they were very noticeable in the culture containing 

 the highest concentration of nitrate and doubtful in the remaining one. 

 On the second day volutin bodies were present in all cultures grown on 

 27807°— 18 5 



