When boiled with nitric acid, it is oxidized to mucic, saccharic, and oxalic acids. It gives characteristic reactions with alum, basic lead acetate, and other common reagents.
Gum arabic comes on the market as a brittle, glassy mass, which is used in the preparation of mucilages, and as a carrier for essential oils, etc., in certain toilet preparations.
Recent investigations have shown that the so-called "meta-pectic acid," which is often found in sugar beets and interferes with the process of sugar manufacture, is identical with gum arabic in composition and properties.
Gum tragacanth is the soluble portion of the natural gum which is found in several species of Astragalus. It constitutes only 8 to 10 per cent of the total gum-like material which is present, the remainder being composed of insoluble gummy substances of unknown composition. The soluble gum consists of calcium, potassium, and magnesium salts of an acid which, when hydrolyzed, yields several molecules of arabinose, six of galactose, and one of geddic acid (an isomer of arabic acid). It is said to be produced by the metamorphosis of the medullary rays under unfavorable conditions of growth. It comes on the market in globular masses of amorphous material, and is used in the manufacture of cosmetics, etc.
Wound gum is frequently found in the tracheæ of plants, and near surface wounds, which it stanches. It is secreted by the cells surrounding the injured part. It responds to the reactions of other gums and to some of those of woody fiber. Its exact composition is not known, but probably lies between that of the true gums and that of cellulose.
These gums are generally considered to be decomposition products of celluloses, resulting from the action of some hydrolytic ferment, usually stimulated by some unfavorable condition of growth, some injury, or some morbid condition.
The pentosans, araban and xylan, occur normally in the stems and outer seed coats of many common plants. They constitute a considerable proportion of these tissues, as indicated by the following results of typical analyses: Wheat bran, 22 to 25 per cent; clover hay, 8 to 10 per cent; oat straw, 16 to 20 per cent; wheat straw, 26 to 27 per cent; corn bran, 38 to 43 per cent; jute fiber, 13 to 15 per cent; various wood gums, 60 to 92 per cent.
They are white, fluffy solids, which are difficultly soluble in cold water, more readily in hot water. They are very difficult to hydrolyze, and indigestible by animals. When finally hydrolyzed, they yield arabinose and xylose, respectively. The pith of dry corn stalks is a good illustration of their general character.
MUCILAGES
These are characterized by forming slimy masses when moistened with water. They are secreted by hairs on the skin of many plants, so that the external walls of the leaves, fruit, and seeds are often mucilaginous when damp. This is particularly true of aquatic plants. The chemical composition of the mucilages is unknown. When hydrolyzed, they yield arabinose and a hexose; the latter is sometimes galactose and sometimes mannose.