1923] 



CAMP — CITRIC ACID AS A SOURCE OF CARBON 257 



conditions, 50 cc. of sterile distilled water were added to the mat, 

 allowed to stand 5 minutes, then poured off and added to the 

 culture solution first removed. The 50 cc. of the sterile solution 

 in the 120-cc. flask was then poured into 'the 300-cc. flask with 

 the culture. All manipulations were carried out as carefully as 

 possible, and care was taken to wet the top of the mat as little 

 as possible when adding the water and the new solution. Pyrex 

 flasks were used for the mats so that the necks could be flamed 

 freely to avoid contamination. Three solutions containing 3 

 different nitrogen compounds, E^NOi, NH 4 NO,, and Ca(NO,),, 

 were used for comparison. The P H of the combined solution 

 and washings was determined and the solution used for this deter- 

 mination returned to the flask and the titer of the solution deter- 

 mined, using N/10 NaOH and phenolphthalein. 



The solution used for the growth of the fungi was the regular 

 solution diluted, the following concentrations being used: 

 dextrose,. 25 gms.; KNO„ 40 cc; KH,PO<, 40 cc; MgSO„ 20 cc; 

 FePOi, trace; H a O to make 1 liter. 



The sugar solution tried fermentatively contained: dextrose, 

 25 gms.; N source (KNO„ NrLNO,, or Ca(NO,),), 25 cc; KH,- 

 PO4, 10 cc; MgSO<, 5 cc; FePO,, trace; H,0 to make 1 liter. 

 This solution was adjusted to an acidity of P H 2.5 with HC1 

 before sterilization. The citric acid solution was the same as the 

 sugar solution except that 25 gms. of citric acid were substituted 

 for the dextrose. The schedule of changes to which each of the 

 3 mats of each fungus was subjected was as follows: 



Mat 1. KNO,*-dex.-*KNOrcit.->Ca(NO,)r-dex.->Ca(NO,)r- 

 cit. i 



Mat 2. NH 4 -dex.-»NH 4 -cit.->KNO,-dex.-^KNO,-cit. 



Mat 3. ^Ca(NO,) s -cit.->NH < -dex.-^NH 4 -cit. 



It must be remembered that there must always be a slight 

 difference between the blank of the solution and the solution 

 removed from the fungous mat, even if there has been no reaction 

 on the part of the fungus, since it is manifestly impossible to 

 wash the mat completely free of the solution without unduly 

 exposing it to contamination. 



* The solutions will be differentiated as far as the nitrogen source is concerned by 

 indicating the cation of the compound used, i. e., "Ca-dex"for "Ca(NO»)2-dextrose." 



