240 Latham : Assimilation of Sterigmatocystis 



amount of nitrate that varied from 205.1 milligrams to 1.6 mg.; 

 the amount supph'ed varying at the same time from 1 15.4 mg. to 

 156.3 mg. When the supply passed beyond the optimum point, 

 when in this case 1 60. 3 mg. were given the fungus at the outset, 

 nitrogen was consumed. This would seem to indicate that the 

 critical point with regard to nitrogen supply is slightly below 

 160.3 ™S' ^^ 50 c.c. of solution, although no attempt was made 

 to determine the critical point with accuracy. It is known in the 

 case of other nitrogen-fixing organisms that with an increasing 

 nitrogen supply, the ability to utilize free nitrogen becomes less 

 and less. At the point where it ceases, nitrogen must, of course, 

 be consumed in the growth of the fungus. It will be noted how 

 great is the difference between the amount of nitrogen fixed by the 

 series just below the one in question and those with a much less 

 amount supplied in the culture fluid. As the curve of nitrogen 

 supply rises the curve of nitrogen-fixing ability falls, until the two 

 cross at a point at which nitrogen consumption will begin. It 



r 



may very well be that along such lines lies the inability of Azoto- 

 bacter to fix nitrogen, ascribed by Beijerinck '^ to unexplained 

 internal causes. 



■ F 



The average of the normals of all six series is 54.7 mg. or 

 199 mg. per gram of dry weight of the fungus felt; excluding 

 from the average the series in which nitrogen was consumed, that 

 Is, series 6, the average becomes 72.2 mg. of nitrogen fixed, which 

 is equivalent to -260. 1 mg, per gram weight of dry crop. This 



w 



union of atmospheric nitrogen diminishes with stimulation, being 

 fairly constant around the point of optimal stimulation — .0005N 

 ZnSO^ — but finally disappearing. With a .0005N solution of 

 ZnSO^, the amount of nitrogen brought into combination varies 

 from 34.6 mg. to 71.4 mg.,the average being 48.3 mg, and this is 

 66.6 mg. per gram of dry felt produced. The effect of chemical 

 stimulation upon nitrogen assimilation is therefore obvious at a 

 glance if we compare normal and stimulated results, 72 mg. or 260. i 

 mg. per gram of dry felt in the unstimulated as against 33.2 mg. or 

 66.6 mg. per gram of dry substance in the stimulated. And again to 

 compare the highest figures obtained at each point, a total gain of 

 174.2 per cent of nitrogen over the amount supplied the normal 



*Beijeritick, W. Centralb. fiir Bakt. II. 7 : ^61, 1901. 



