52
F. B. SHORLAND
mintica (Compositae), has now been found in the Euphorbiaceae as
well as in the Malvaceae. Similarly, the occurrence of a fatty acid with a
cyclopropene ring was thought to be characteristic of the seed fat of
Sterculia foetida (Sterculiaceae). However, a similar acid has since been
located in the seed fats of the two species of Malvaceae. This contrasts
with other members of the Malvaceae family which normally elaborate
seed oils containing palmitic, oleic, and linoleic acids as the main
constituents.
The correlation between the fatty acid composition of seed oils and
the botanical classification thus deserves serious consideration but is not
acceptable as an unequivocal guide as to the family, genus, or species
to which a plant belongs.
V. Fatty Acid Composition and Mode of Combination
Within the same organism and even within the same tissue, wide
differences in the fatty acid composition of different types of lipids are
found. In animal tissues, for example, the phospholipids usually contain more of the higher molecular weight fatty acids than do the glycerides. Again the waxes in plants contain as a rule the higher saturated
acids, while the glycerides and phospholipids are usually rich in oleic
and linoleic acids. Differences in fatty acid composition are also
observed between the different kinds of phospholipids.
In Section IV the fatty acids of the main depot fats, which are predominantly glyceridic, have been discussed. The fatty acids of waxes
and phospholipids will be described in the present section.
A. FATTY ACIDS OF WAXES AND RELATED SUBSTANCES
As mentioned in Section ΙΙ,Β, waxes are defined as the higher fatty
acid esters of the higher fatty alcohols. The naturally occurring waxes,
however, usually have in addition to esters many other components such
as free fatty acids, free fatty alcohols, hydrocarbons, and sterols. Hydroxy acids are often present, giving rise to estolides and to lactones.
The amounts of associated compounds vary with the source from which
the wax has been derived. Although often present in only minor proportions, waxes are of widespread occurrence in plant life. They occur
in tubercle bacilli in considerable amounts. In the higher plants they
are usually found in the outer skin of leaves, stems, flowers, and fruits,
as well as to some extent in the tissues. Waxes are not so common in
animals, but occasionally they form considerable deposits, as in the
head of the sperm whale, in the preen gland secretions of birds, and
in the skin and hair lipids of animals.
The unique high molecular weight mycolic acids, which have been
F. B. SHORLAND
mintica (Compositae), has now been found in the Euphorbiaceae as
well as in the Malvaceae. Similarly, the occurrence of a fatty acid with a
cyclopropene ring was thought to be characteristic of the seed fat of
Sterculia foetida (Sterculiaceae). However, a similar acid has since been
located in the seed fats of the two species of Malvaceae. This contrasts
with other members of the Malvaceae family which normally elaborate
seed oils containing palmitic, oleic, and linoleic acids as the main
constituents.
The correlation between the fatty acid composition of seed oils and
the botanical classification thus deserves serious consideration but is not
acceptable as an unequivocal guide as to the family, genus, or species
to which a plant belongs.
V. Fatty Acid Composition and Mode of Combination
Within the same organism and even within the same tissue, wide
differences in the fatty acid composition of different types of lipids are
found. In animal tissues, for example, the phospholipids usually contain more of the higher molecular weight fatty acids than do the glycerides. Again the waxes in plants contain as a rule the higher saturated
acids, while the glycerides and phospholipids are usually rich in oleic
and linoleic acids. Differences in fatty acid composition are also
observed between the different kinds of phospholipids.
In Section IV the fatty acids of the main depot fats, which are predominantly glyceridic, have been discussed. The fatty acids of waxes
and phospholipids will be described in the present section.
A. FATTY ACIDS OF WAXES AND RELATED SUBSTANCES
As mentioned in Section ΙΙ,Β, waxes are defined as the higher fatty
acid esters of the higher fatty alcohols. The naturally occurring waxes,
however, usually have in addition to esters many other components such
as free fatty acids, free fatty alcohols, hydrocarbons, and sterols. Hydroxy acids are often present, giving rise to estolides and to lactones.
The amounts of associated compounds vary with the source from which
the wax has been derived. Although often present in only minor proportions, waxes are of widespread occurrence in plant life. They occur
in tubercle bacilli in considerable amounts. In the higher plants they
are usually found in the outer skin of leaves, stems, flowers, and fruits,
as well as to some extent in the tissues. Waxes are not so common in
animals, but occasionally they form considerable deposits, as in the
head of the sperm whale, in the preen gland secretions of birds, and
in the skin and hair lipids of animals.
The unique high molecular weight mycolic acids, which have been