Citric Acid,
, 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 rôle 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 normal salts have a significant effect in regulating the acidity or alkalinity of plant juices, or body fluids, and so determining the nature of the enzymic activities and colloidal conditions of the biological systems (see Chapters XIV and XV). It is probable that other organic acids, such as formic, acetic, oxalic, and succinic acids, in plants and sarco-lactic acid, in animal tissues, perform similar regulatory rôles; but there seems as yet to be no indication as to why different acids should be used for this purpose by different species, or organisms; or as to the methods by which they perform their specific functions, whatever these may be.
In plants, the organic acids are usually in solution in the sap. When the plant ripens, they generally disappear, either being neutralized by calcium, or other bases, and deposited as crystals in the leaves or stems, or else used up in the synthesis of other organic compounds. Small proportions of these acids are usually present in mature seeds, and the percentage increases materially during germination, indicating that they play an important rôle in insuring the proper conditions for the conversion of the reserve food of the seed into soluble materials available for the nutrition of the young growing plant.
BIOLOGICAL SIGNIFICANCE OF FRUIT ACIDS, ETC.
The occurrence of organic acids, or their derivatives, which have pronounced odors or flavors, in the flesh surrounding the seeds of fruits, in the endosperm of vegetable seeds, or in the tubers, etc., of perennial plants, thus making them attractive as food for animals and men, undoubtedly serves to insure a wider distribution of the reproductive organs of these plants; a fact which has unquestionably had a marked influence upon the survival of species in the competitive struggle for existence during past eras and in the development and cultivation of different species by man. Indirect evidence that the proportion of these attractive compounds present in certain species may have been considerably increased by the processes of "natural selection" in the past is furnished by the many successful attempts to increase the percentage of such desirable constituents in fruits or vegetables by means of artificial selection of parent stocks by skillful plant breeders.