GUMS, PECTINS, AND CELLULOSES 69 



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 tracheae 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 com- 

 position 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 hydro- 

 lyzed, they yield arabinose and xylose, respectively. The pith 

 of dry corn stalks is a good illustration of their general char- 

 acter. 



