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