ALLIED AND MISCELLANEOUS PRODUCTS 181 



Acetic and lactic acids are produced in the fermentation of 

 carbohydrates by bacteria and are the familiar acids in ensilage 

 and sauerkraut. 



Oxalic acid is found free in sorrel (Rwnex), Oxalis, and rhubarb 

 (Rheum) . In many other plants it occurs as a salt of calcium and 

 potassium. The calcium oxalate crystals which are often found 

 in plant cells are insoluble, and this is thought to be a method of 

 getting rid of the excess calcium. 



Malic acid is found in apples, cherries, and many small fruits 

 as well as in many vegetables. In boiling down maple syrup, 

 calcium malate is found in the bottom of the vats as ' : sand." 



Tartaric acid is found in many fruits but especially in grapes. 

 In the fermenting of grape juice to make wine, large quantities 

 of the monopotassium salt of tartaric acid precipitate out, from 

 which is prepared the commercial " cream of tartar" used in bak- 

 ing. 



Citric acid is found especially in the citrus fruits, but it also 

 occurs in many small fruits like loganberries, cherries, and cur- 

 rants, where it is associated with malic acid. 



These acids perform important functions : 



1. They regulate the acidity of the cell sap and thus of the 

 medium in which much of the work of the cell is carried on. All 

 reactions, notably those of enzymes, have an optimum acidity; 

 and the acids serve to determine this within the cells where they 

 are found. They may thus regulate the aggregation of colloids and 

 other colloidal phenomena. The amount of acid in a tissue may 

 change with the state of development of the tissue. As fruit ripens 

 the amount of acid generally diminishes. Seeds generally contain 

 small amounts of acid which increase as the seed germinates. 

 According to this conception, there is no reason why different 

 plants should produce different acids if their function is merely 

 to insure a favorable environment for the chemical processes 

 which take place in the growing plant, including the conversion 

 of reserve food into an available form and its translocation. 



2. They serve to make certain ripe fruits more attractive by 

 their pleasant odors and flavors and thus aid in the distribution 

 of the species. 



3. In green fruits their presence may make the fruit unpalatable 

 and unattractive so that it will not be touched by distributing 

 animals before the seeds are mature. 



