1923J 



CAMP — CITRIC ACID AS A SOURCE OF CARBON 245 



culture solution used, with one exception, was based on a solution 

 developed at this laboratory by Dr. Duggar. 1 The final concentra- 

 tions of chemicals were as follows: M/4 dextrose, M/5 KNO f , 

 M/20 KHJPO*, M/100 MgSO, and a trace of FePO,. For obtain- 

 ing these dilutions the following stock solutions were used : M/2 

 dextrose, M/l KN0 3 , M/4 KH,PO„ M/10 MgSO<, and M/1000 

 FePO<. For 50 cc. of medium the following amounts of these 

 solutions were used: 25 cc. dextrose, 10 cc. KNO,, 10 cc. KH2PO4, 

 5 cc. MgS0 4 , and 6 drops FeP0 4 . In some instances M/l NH<NO« 

 or a peptone solution containing 8.8 gms. per liter and considered 

 as M/l for nitrogen was used instead of the KNO, solution. 

 Two sizes of flasks were used, 300 cc. and 100 cc, 50 cc. of medium 

 being used in the larger and 25 cc. in the smaller. Pyrex flasks, 

 frequently cleaned with chromic acid cleaning solution, were 

 used almost entirely. Care was taken at all times to have the 

 glassware scrupulously clean. 



Inoculations were made into liquid culture media by the use 

 of spore suspensions where spores were produced. Where no 

 spores were produced the fungus was grown in plate culture on 

 potato agar, and squares about 3 mm. in dimensions, cut around 

 the periphery of the colony, were used for inoculation. In one 

 series of experiments the cultures were kept at 20° and 30° C. 

 but for the most part they were kept at 25° C. The latter tem- 

 perature is probably very near the optimum for most of the fungi 

 used, and it seemed the most suitable temperature as far as the 

 entire group was concerned. For determining the amount of 

 growth, the fungous mat was filtered off on a filter-naner which 



had been dried 



C, weighed, and 



labelled. The mats were weighed after a similar drying. Weights 

 were determined to milligrams as rapidly as possible to prevent 

 absorption of moisture while on the balance. 



The relation of acidity to growth. — In order to gain an idea of 

 the limiting hydrogen-ion concentration for the growth of the 

 fungi used some germination and growth tests were made. In 

 a preliminary way spores of Penicillium stoloniferum and P. sp. 

 were tried in a citric acid solution containing no other nutrients. 



1 A paper embracing the work from which this was taken will appear in a sub- 

 sequent number of the Annals. 



