w 



238 Latham: Assimilation of Sterigmatocystis 



sitive to traces of gases in the atmosphere. Thej;ecipe of the 

 culture solution is this : 



i.oo gm. NH^NOj S-oo gm. sugar 



0.50 ** KH^PO^ 100 c.c. water 



0. 25 ' * MgSO^ trace of iron 



The solution was made up in bulk in carefully cleaned and steril- 

 ized '' non-sol " glass, boiled, cooled, and a suitable quantity taken 

 at once for determining the nitrogen content. The rest was sown 

 with spores, and zinc sulphate added in measured quantities where 

 desired. The cultures were then allowed to grow for five or six 

 days to reach a proper state, when they were reaped, the sub- 

 stratum analyzed at once for nitrogen, and the felts washed and 

 dried preparatory to their analyses. In making all measurements, 

 accurate pipettes and burettes were used, and for weighing the dry 

 felt, a good Becker balance. To ascertain the nitrogen present 

 the Gunning-Jodlbauer modification of the Kjeldahl process was 

 followed. Two felts were cultivated at each stimulation and two 

 determ'inations of nitrogen were made on each solution and on 

 each felt. The procedure was to analyze the solution as provided 

 to the fungus and again immediately after the crop was reaped, 

 any difference being attributed to the action of the fungus. Then 

 the dried felt was analyzed and the results obtained here added to 

 the others and thus the full amount of nitrogen present in chemical 

 union was determined. The most marked change is visible in the 

 fluid substratum, because Sterigmatocystis^ like nitrogen-fixing 

 bacteria, may under favorable conditions excrete as a waste prod- 

 uct some of the nitrogen compounds which it has formed from 

 gaseous atmospheric nitrogen. The amount to be found in the 

 make-up of the fungus itself being, as we find, relatively the same 

 under all circumstances. 



From six series grown after methods had been mastered, it was 

 seen that (i) Sterigmatocystis grown normally does fix free nitro- 

 gen which is found to be in combination if nitrogen compounds be 

 supplied in favorable quantity ; (2) the amount of nitrogen so com- 

 bined decreases if the culture be subjected to stimulation, both 

 absolutely and relatively, /. ^., both in absolute quantity and in 

 amount per gram of dry weight of crop produced, the diminution 

 being shown most markedly by the decrease in combined nitrogen 



