ORGANIC ACIDS, ACID SALTS, AND ESTERS 127 



crystals form only when calcium is in excess in the plant sap; 

 hence, the deposition of crystallized calcium oxalate seems to be a 

 device for the avoidance of excessive calcium rather than excessive 

 oxalic acid, in the plant juices. 



Succinic acid, HOOC CH 2 CH 2 COOH (H 6 C 4 O 4 ), occurs in 

 many fruits and vegetables, and is also found in some 'animal tis- 

 sues. In fruits, it is usually associated with its derivatives, malic 

 and tartaric acids. 



Malic acid, HOOC CH 2 CHOH COOH (H 6 C 4 O 5 ), occurs in 

 apples and in many small fruits, and in many vegetables. Acid 

 calcium malate is now produced commercially as a bye-product 

 from the manufacture of syrups from fruit juices, and is used as a 

 substitute for " cream of tartar " in the manufacture of baking 

 powders. 



Tartaric acid, HOOC CHOH CHOH COOH (H 6 C 4 6 ), is 

 found in many fruits, but most characteristically in the grape, 

 where it occurs as the mono-potassium salt. During the fermen- 

 tation of grape juice into wine, this salt is deposited in considerable 

 quantities in the bottom of the wine-casks. This crude product is 

 collected and sold under the name " argols." From these argols, 

 pure acid potassium tartrate is obtained by decolorization and 

 recrystallization, and constitutes the " cream of tartar " of 

 commerce. 



COOH 



I 



Citric acid, HOOC -CH2-COH-CH2- COOH (H 8 C 6 7 ), occurs 

 in large proportions in lemons, and associated with malic acid in 

 strawberries, cherries, currants, etc. It is also found in small 

 quantities in the seeds of the common leguminous vegetables, beans, 

 peas, etc. 



Tannic acid occurs widely distributed in the plant kingdom as 

 a constituent of the special type of glucosides known as tannins, 

 whose properties and functions have already been discussed (see 

 Chapter VII). 



PHYSIOLOGICAL USES OF ORGANIC ACIDS 



No conclusive evidence concerning the role of organic acids in 

 plant, or animal, growth, has yet been produced. There can be 

 no doubt that the hypothetical carbonic acid and its acid and nor- 



