[Vol. 1 



214 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



to be one of the deciding factors. Nevertheless, for those fungi 

 which cause the ultimate destructive decay which releases the 

 acids contained in the juice sacs of the fruit, there must be meta- 

 bolic adjustment to life in an extremely acid medium. 



Discussion of Citric Acid 



Citric acid (oxy-tricarballylic acid) is a tricarboxylic acid with 

 Le substituted hydroxyl group, of the structure: 



CIL.COOH 



C(OH).COOH. 1 ILO 



CIL.COOH 



It crystallizes from concentrated solutions with one molecule of 

 water of crystallization; but if heated too much in concentrating 

 it turns yellow and does not crystallize, probably due to the for- 

 mation of other compounds. It is extremely soluble in water 

 but less so in most of the organic solvents, such as ether, alcohol, 

 and chloroform. It dissociates in 3 stages, and Blasdale ('18) 

 gives a dissociation constant of 8 X 10~ 4 for the first stage. 

 Davis, Oakes, and Salisbury ('23) point out, however, that in 

 titrating citric acid with alkali a curve corresponding to that for 

 HC1 is obtained instead of one corresponding to the titration 

 curve for H,P04; this would indicate that the di-basic and mono- 

 basic salts are not sharply separated from each other in formation, 

 as is the case in the similar salts of phosphoric acid. Citric acid 

 does not possess an asymmetric carbon atom and consequently 

 does not rotate the plane of polarized light. It forms 3 classes 

 of salts and some mixed salts, but only the tri-basic salts are 



usually prepared. 



Potassium citrate, used considerably in the work here reported, 

 is a soluble, strongly alkaline salt, crystallizable with difficulty 

 from water, owing to its high solubility, but more easily crystal- 

 lized by shaking out with alcohol. Sodium citrate is even more 

 soluble in water, but is easily crystallized out as fine crystals by 

 shaking an aqueous solution of the salt with 95 per cent alcohol. 

 The crystals form at the junction of the 2 liquids and slowly settle 



