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conditions are alike. At first sight already, there can be no doubt 

 but under these circumstances fixation of considerable quantities 

 of nitrogen must take place, and chemical analysis proves that this 

 is really the case. 



The microscopic image of the Azotobacter growth in the malate 

 commonly shows smaller individuals of greater motility than the 

 formerly described forms which are obtained in the mannite solutions. 

 They keep about the middle between A. chroococcum and A. agilis, 

 and remind strongly of a variety found in America, which has 

 received the name of A. vinlandi. The plate cultures of such a malate 

 accumulation again prove not to be pure but to consist of the usual 

 mixture of non spore-forming species. They are best grown on a 

 medium of the composition : 100 tapwater, 1 calcium malate, 0.05 

 K 3 HPO\ J to 2 agar, on which the Azotobacter colonies become 

 already visible after 12 hours at 30° C, which is not the case with 

 any other species of microbes known to me. As these plates are 

 somewhat cloudy by ihe produced calciumphosphate and the imper- 

 fectly dissolved malate. ii is desirable to mix the ingredients in the 

 way as follows. Into a culture tube are first introduced some drops 

 of a neutral, concentrated solution of kaliummalate and herein are 

 dissolved both the calcium malate and the kaliumphosphate, with a 

 little water to dilute, but the smallest quantity possible, as the dissol- 

 ving power of the kaliummalate is much stronger in the concentrated 

 than in the dilute solution. Then the contents of the tube are mixed 

 with the agar solution. 



The malate plates prepared in this way have proved to be better 

 for the growth of Azotobacter germs than the mannite and glucose 

 plates, so that of a definite number of germs there develop more 

 to colonies on the former than on the latter. Hence it has become 

 possible more exactly to compute the number of individuals of our 

 species present in a sample of soil than after the old method, to 

 which circumstance we return below. 



Before going further I wish to notice the following concerning 

 other salts of organic acids as carbon food for Azotobacter. 



Except with calciummalate there could be obtained an abundant 

 or moderate growth with calciumlactate, calciumacetate and calcium- 

 propionate, particularly when using canal water for the first infection. 

 It was remarkable that the transport of a malate culture into lactate 

 appeared to succeed nearly as well as of malate into malate, while 

 even relatively rich crude cultures in propionate- or acetatesolutions, 

 obtained directly from soil or water when inoculated into corresponding 

 media, hardly grow on, if at all. This fact is the more remarkable 



