The origin of the gums is still uncertain. They are commonly, although not exclusively, Produced in hot, dry climates, and healthy trees tend to exude less gum than those in poor condition. For this reason, it has been suggested that they are the result of infection, and a few gums, including chagual (10) and honey locust gums (11), are in fact known to be pathological products. On the other hand, some gums, such as gum tragacanth, are exuded copiously immediately after incision of the bark, and are obviously natural products of the plant's metabolism. It also seems unlikely that gums produced on a commercial basis are the products of infection. In general, it is probable that a tree exudes gum in order to seal off the injured part, and to prevent the spread of infection. The similarity of the gums and the bacterial polysaccharides, and the cross -reactions which can take place between gums and some Pneumococcus sera (12), may be significant in this context, and the complexity of structure of the gum polysaccharides may be connected with the necessity for dealing with a variety of attacking bacteria.

Many trees, of a wide variety of species, respond to injury by exuding yellowish viscous fluids, which harden on exposure to the atmosphere, producing glassy nodules. These are the plant gums (1), which are among the most complex polysaccharides known. In molecular structure, they resemble the mucilages and the bacterial polysaccharides; in fact, there are no general structural differences between the gums and the mucilages(2 -8). The only distinction lies in their mode of origin, since the mucilages are isolated only by the extraction of seeds or other plant material, in which they apparently serve as food stores or as moisture reservoirs. It is therefore necessary, for the purpose of this thesis, to give a restricting definition of plant gums as uronic acid containing polysaccharide exudates. This definition also excludes resinous exudates of terpenoid structure, and non- exuded neutral polysaccharides which are known colloquially as gums, e.g. carob seed gum, which is a galactomannan (9).

A typical gum possesses a highly branched structure, containing anything from two to four different neutral sugar residues, and a uronic acid residue; each may exist in more than one type of linkage. Several gums recently investigated are further complicated by the fact that they contain two different uronic acid residues. The most common neutral sugars are D-galactose, D-mannose, L-arabinose, D-xylose and L-rhamnose, but L-fucose and D-tagatose have also been observed. The uronic acids of gums are D- glucuronic acid, 4 -0- methyl-D-glucuronic acid and D-galacturonic acid.

In the natural state, many guns exist as neutral salts of such cations as calcium and magnesium. Some, e.g. the Sterculia gums and Cochlosoermum gossypium, are acetylated, and give off a distinct odour of acetic acid. Varying molecular weights have been quoted, ranging from 2 - 300,000 for gum arabic, to 9,500,000 for Karaya gum (13). The usual means of measurement is by sedimentation techniques. Because of the structural complexity of the molecules concerned, chemical methods of molecular weight determination are in general unsatisfactory.

The commercial use of gums is almost as old as civilization. The Egyptians used them in embalming, and for the last few hundred years they have been common ingredients of medicines and of 'aids to beauty'. Today, they are still used in the fields of pharmaceuticals and cosmetics, but a wide range of manufacturing processes also employs them as emulsifiers, adhesives, thickeners, binding materials, etc. Being harmless and tasteless, they find many uses in the food industry.

In view of their commercial importance, the scientific study of gums is worthwhile, and of course is also important for its intrinsic biochemical interest. But perhaps the most important reason for carrying out investigations lies in the resemblance to bacterial polysaccharides. It may be possible, by drawing analogies, to gain. insight into the structure and formation of the latter, and thus to proceed to a fuller understanding of the bacteria themselves.