The number of species that yield gums or resins in greater or less amount throughout the Vegetable Kingdom must be many thousands. For this reason a selective treatment has been necessary in this work. Special emphasis is placed on those gums and resins that are of commercial importance, or which for some reason or other are of special interest. Particular attention has been given to those that have only become of commercial importance in comparatively recent years, and which are not dealt with in older works of reference. Examples of these are carob seed or locust gum, karaya or Stercullia gum and other tragacanth substitutes, also certain Acacias now known to be exploited for gum, particularly in East Africa.
Owing to the fact that the true gums as a group have entirely different properties and uses from the resins and interest different classes of users, it has been considered desirable to treat the two groups separately. The book has, therefore, been divided into two parts.
Among peoples all over the world, including the aboriginals of Australia and primitive African and Asiatic tribes, certain gums have been used for food as far back as history relates. The adhesive properties of gums have also been utilized from early times, particularly in preparing paints and pigments. Gum Arabic in North Africa has been an article of commerce from at least the first century of the Christian era and the trade existed throughout the Middle Ages. The Sudan gum Arabic trade with European and other countries developed steadily during the last century, in spite of temporary set-backs through political disturbance, and reached still greater proportions in the present century thanks largely to improved transport facilities (railways) in the producing areas.
With regard to modern uses of vegetable resins the paint, varnish, linoleum, paper sizing and soap making trades use considerable quantities. The relative amounts of the different kinds of resins used for industrial purposes in the past have varied considerably as perusal of the following pages will show. At one time it was thought synthetic resins would completely replace natural resins for many purposes, particularly in the paint and varnish industries, but this has not materialized and the natural resins continue to hold their own and to be imported into manufacturing countries in large quantities. What the future will hold no one can foretell, but in the light of recent developments there are strong indications that the use of natural resins will increase, rather than diminish. Recently research has developed new types of resin which are combinations of synthetic and of natural resins, the best examples being the so-called ``copal type synthetics''. These resins are proving very promising. They combine desirable features of both the synthetic and the natural or fossil resins.