Chapter III).

Higher members of the formic acid series (as acetic, CH_{3}·COOH; propionic, C_{2}H_{5}·COOH; butyric, C_{3}H_{7}·COOH; etc.) are often found in small quantities in the leaves of many plants and seem to be characteristically present in certain species. They are easily produced from carbohydrates by bacterial action and, hence, are always present in fermenting tissues, such as silage, sauerkraut, etc. Furthermore, the glycerol esters of higher members of this and other monobasic acid series are constituents of all natural fats and oils (see Chapter X).

=Oxalic acid=, HOOC·COOH (H_{2}C_{2}O_{4}), is found in small amounts in nearly all plants and in relatively large proportions in those of _Oxalis_, rhubarb, etc. It occurs both as the free acid and as neutral, or acid, oxalates of calcium, potassium, and, perhaps, of magnesium and sodium. Solid crystals of insoluble calcium oxalate are often found in plant cells, and it has been shown that when so deposited the calcium cannot become again available for metabolic uses. It is stated, further, that such 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 tissues. 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}O_{6}), is found in many fruits, but most characteristically in the grape, where it occurs as the mono-potassium salt. During the fermentation 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 | =Citric acid=, HOOC·CH_{2}·COH·CH{2}·COOH (H_{8}C_{6}O_{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 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.