V] CARBOHYDRATES 55 



and two molecules of glucose) has been isolated from the root of Gentian 

 (Gentiana). Melicitose (fructose and two molecules of glucose) occurs 

 in a manna ^ which exudes from the twigs of the Larch (Larix) and 

 Douglas Fir {Pseudotsuga). 



A tetrasaccharide, stachyose (fructose, glucose and two molecules 

 of galactose) has been isolated from tubers of Stachys tubifera, from 

 White Jasmine (Jasminum) and other plants. 



Polysaccharides. 



These substances are formed by condensation, with elimination of 

 water, from more than three molecules of monosaccharides. 



Pentosans, 



It has already been mentioned that condensation products of the 

 pentoses, the pentosans, are widely distributed. The two most frequently 

 occurring pentosans are xylan and araban. No enzymes are known which 

 hydrolyze the pentosans. It is characteristic of xylan and araban that 

 they form copper compounds in Fehling's solution in presence of excess 

 of alkali. 



Araban. This pentosan may be regarded as a condensation product 

 of arabinose as already indicated. It occurs in various gums (Gum 

 Arabic, Cherry Gum) frequently in combination with other substances. 

 On hydrolysis with acids, araban yields arabinose. (See also gums and 

 arabinose.) 



Expt. 47. Preparation of araban from Gum Arabic. (Salkowski, 30.) Weigh out 

 20 gms. of gum arable and dissolve in 500 c.c. of warm water in a large evaporating 

 dish on a water-bath. Then add 200 c.c. of Fehling's solution and excess of strong 

 caustic soda solution. The araban will be precipitated as a white gummy mass which 

 will settle at the bottom of the dish. Filter off through muslin. Take up the preci- 

 pitate in the minimum quantity of dilute hydrochloric acid (1 pt. of acid : 1 pt. of 

 water), and then add alcohol. The araban separates out as a white precipitate. Wash 

 away the copper chloride with alcohol. 



Expt. 48. Hydrolysis of araban. The araban from the last experiment is put 

 into a round-bottomed flask with about 200 c.c. of 2 ^/q sulphuric acid and heated on 

 a water-bath for 2 hours, the flask being fitted with an air condenser (see p. 46). 



1 Manna is a name given to exudations from the branches of various trees and shrubs. 

 Sometimes the flow is assisted artificially as in the case of the Manna Ash {Fraxinus Ornus) 

 where the product, consisting almost entirely of the polyhydric alcohol, mannitol, is of 

 commercial value as a drug, etc. In other cases, the manna exudes as the result of the 

 attacks of insects. Mannas appear to be readily soluble in water to clear, non-sticky so- 

 lutions, thereby differing from gums and resins. 



