The plant gums have been defined (1) as "those substances of
plant origin which are obtained as exudations from the fruit, trunks or
branches of trees spontaneously or after mechanical injury of the plant
by incision of the bark or by removal of a branch, or after invasion by
bacteria or fungi ". The exudates harden on drying, forrdng nodules of
polysaccharide material, with various impurities such as lignin, protein,
resinous material and bits of bark.
The gums show many similarities in properties and in molecular
structure to the plant mucilages, and are normally reviewed together (1 -6).
They are usually distinguished, in that the mucilages exist inside the
plant in the form of membrane thickening material, or intracellular
substances, and are thought to act either as food stores, or as a means
of retaining water. They have also been divided because of their
different solubilities in water, the gums generally dissolving to give
clear solutions and the mucilages forming colloids.
The plant gums are found on trees and shrubs of many species,
but are most commonly produced by fruit trees, and by the small thorny
trees which are widespread in hot, dry countries. Gum arabic is the
best known and most important commercially of the gums. It has been in
use for many centuries, the Egyptians employing it as a paint thickener,
as far back as 2000 B.C. It is now harvested in considerable amounts in
the Near East, often from trees specially grown in gardens for the
production of the gum.
Under the name of gum arabic go secretions from trees of the
various species of the genus Acacia. This leads to confusion in
structural work, and unless a gum has a known botanical origin, precise
conclusions cannot be drawn about the molecular structure of the species.
The commercially highest quality gtmì comes from Acacia senegal and the
classical work of Smith (24,27,30,31,33,34) on the molecular composition
of arabic acid, was carried out on gum of high quality known as Gum
Kordofan.
The cause of formation of the gums is still in doubt. Although
they seem to be exuded exclusively to seal off the attacked or invaded
area, it is unknown whether they are produced by invading micro- organisms,
or by the plant to protect itself against such attack, or if they are
merely natural secretions of the tree. It has been found that some fungi
will induce the formation of water soluble gums on Acacia trees (7), but,
on the other hand, in the case of other gums, there is evidence against
the secretions being bacterial products. For example, gum tragacanth is
produced immediately the bark of the tree is cut. In addition the
exudates from different trees of the same species, are remarkably similar
in their properties, whereas, one might expect different bacteria to form
different gums on trees of the same species. It is also interesting to
note that gums are produced mainly by trees which are old, in poor health or
short of water. Healthy trees with a plentiful supply of wzter, produce
little gum.
