1909] CURRENT LITERATURE 475 



Miss Lutz 15 studied forty individuals of 0. lata X 0. gigas and describes a 

 group of hybrids which probably include both the types of DeVries, and in 

 addition plants like 0. lata and like O. gigas, having their respective numbers of 



chromosomes. — R. R. Gates. 



Nitrogen fixation by Azotobacter. — Krzemieniewski has contributed a paper 16 

 that seems to throw much-needed light on the problems of nitrogen fixation by 

 Azotobacter in the soil. Perhaps its most valuable feature is the demonstration 

 of the accelerating influence of humus on the process. He finds that Azotobacter 

 in pure cultures in ordinary nitrogen-free media can fix little atmospheric nitrogen, 

 but that the addition of sterile soil or of humic acids or their calcium, potassium, 

 or sodium salts to such solutions multiplies the amount of nitrogen fixed many 

 times. It is interesting to note further that the humus derived from different soils 

 does not yield uniform results, and that artificial "humus" formed by the action 

 of acids on carbohydrates is of little or no value. Lipman 17 in this country has 

 anticipated in part some of these results, for he found that the amount of nitrogen 

 fixed by Azotobacter growing in solutions to which different types of soils had 

 been added varied greatly. Krzemieniewski further reaches the interesting 

 conclusion, as a result of repeated experiments, that humus does not serve either 

 as a source of nitrogen or of carbon for Azotobacter. He finds that the various 

 nitrogen compounds used in an effort to duplicate the stimulating influence of 

 humus are without such results. When these compounds were used in conjunction 

 with humus they were found to be even decidedly inhibitory in action. Why 

 the humus should thus stimulate growth of Azotobacter he fails to explain, 

 although he seems to have had abundant experimental evidence of the fact. The 

 author was able to demonstrate as much as seventeen milligrams of nitrogen fixed 

 per gram of glucose used in carbohydrate solutions to which humus or humic 

 acid^ had been added. This is a higher ratio than has been reported by other 

 investigators. He further succeeded in demonstrating by gas analysis for the 

 Azotobacter what was long ago demonstrated for the legume, an actual decrease 

 in the amount of nitrogen supplied to the culture. The organic nitrogen fixed 



the culture solution was found at the close of the experiment to check very 

 closely with the amount which disappeared from the air. 



The organism is a strict aerobe, neither alcohols, acids, nor hydrogen gas are 



CO 

 found as products of metabolism. The ratio -^ approaches unity. The 



temperature optimum for nitrogen fixation is 28° C Prolonged cultivation of 

 Azotobacter on artificial media the author finds has little influence on its "viru- 



15 Lutz, Anne M., Notes on the first generation hybrid of Oenothera lata X O. 

 gigas. Science N. S. 29:263-267. 1909. 



.6 Krzemieniewski, S., Untersuchungen uber Azotobacter chroococcum. Beij. 

 Bull. Acad. Sci. Cracovie CI. Sci. Math, et Nat. 1908:299-1050. pi. 1. 



■7 Lipman, Jacob G., Bacteriological indications of the mineral requirements of 

 soils. Ann. Rep. N. J. State Agr. Exp. Sta. 27:177-187- I9°6- 



in 



