PART IV. 



ECONOMIC BOTANY. 



THE GUMS. 



207. The Gums are usually structureless and homo- 

 geneous vegetable products, or exudations from certain 

 trees, which arise mainly by a metamorphosis of tissue, 

 and seldom by a chemical change of the starch contained 

 in the cells. But few vegetable products occur so commonly 

 as the gums. They are soluble (or soften) in water, and 

 insoluble in alcohol. The color may be yellowish, yellow, 

 brownish, or reddish, but seldom white or black. The powder 

 is white. The gums are seldom pure, more or less of organic 

 impurities always being present. They are, according to 

 their chemical composition, carbo-hydrates, that is, they 

 contain carbon, hydrogen, and oxygen, but no nitrogfen ; 

 they are of little or no dietetic value. The important 

 constituents are Arabin, Cerasin, and Bassorin. They are 

 often mixed with resins, and are then resinous gums. The 

 true gums of most importance are Gum-Arabic, Gum- 

 Senegal and Gum-Tragacanth. 



208. Gum-Arabic, called also Gum-Acacia, is obtained 

 from several species of Acacia, a genus belonging to the 

 Pulse family (Leguminosce). The plants are generally 

 small, spiny trees, or shrubs, growing in the deserts of Africa, 

 Arabia, and other countries, sometimes forming vast forests. 

 It was for a long time supposed that the species A. vera 



C244) 



