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 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 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 roles; 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 role 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 organ