1919] 



ALLEN — AZOTOBACTER CHROOCOCCUM 11 



assimilated, i. e., removed from solution, by the growing 



forms. 



Thus it seems that phosphorus nutrition is associated more 

 or less intimately with the growth and development of the 

 Azotobacter. The problem then to us seems to be in part a 

 question of supplying phosphates in the presence of calcium 

 in a slightly alkaline solution. To test our hypothesis the set 

 of experiments described below has been carried out. The 

 theory is naturally quite difficult to prove or disprove, es- 

 pecially in view of the experimental difficulties encountered in 

 work of this kind, and while our results have not led to pos- 

 itive conclusions one way or another it seemed that in view of 

 the uncertainty of carrying work to completion at the present 

 time it was best to report what findings we have, with the hope 

 that they may be of value as suggestions to others as well as 

 to ourselves. 



Experimental 



Culture used. — The Azotobacter used was a subculture from 

 a strain of Azotobacter chroococcum isolated by Mr. A. Bo- 

 nazzi from Wooster, Ohio, soil. It was repeatedly plated dur- 

 ing the isolation until it gave a uniform microscopic picture 

 and produced on Ashby 's mannite agar plates circular colonies, 

 edges entire, moderately raised and shiny. Its purity was as- 

 sured during this work by repeated platings on mannite agar 

 similar to Ashby 's in composition. The colonies on agar plates 

 appear, in 24 hours, small, round, and translucent. They grow 

 rather slowly and become opaque, grayish white, and up to 4 

 or 5 days are of almost butyrous consistency, while as the cul- 

 ture ages they gradually become dry and at 10 days present a 

 slightly wrinkled condition, while at 12 to 14 days the growth 

 is dry, distinctly wrinkled, and shows a faint production of 

 black pigment. The growth of Azotobacter on agar slant is 

 entirely similar to the above, although, of course, in conse- 

 quence of its slimy consistency it shows a fairly marked in- 

 vasion at the base of the slant. Grown on modified Ashby 's 

 soil extract (1:2) mannite agar, it soon acquires a marked 

 pigmenting power, which it loses again when grown on purely 



