and in
considerable proportions in the leaves of several species of nettle, where
it is responsible for the unpleasant effects of the "sting." It may be
detected in small amounts in the vegetative parts of many, if not all,
plants, especially during periods of rapid growth, and is probably one of
the intermediate products in the photosynthesis of carbohydrates (see
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