68 CHEMISTRY OF PLANT LIFE 



THE NATURAL GUMS AND PENTOSANS 



The natural gums, when hydrolyzed, yield large proportions 

 of sugars, but most of them also contain a complex organic acid 

 nucleus, by means of which they form salts with calcium, mag- 

 nesium, etc. Some of them, such as cherry gum and those which 

 are found in the woody stems of plants (wood gum, and those 

 found in corn stalks, the straw of cereals, etc.) yield practically pure 

 pcntoses. These are known as pentosans. They bear the same 

 relation to the pentose sugars as do the dextrosans to glucose, etc. 

 The wound gums, for example, yield arabinose, and the wood 

 gums yield xylose. But most of the natural gums yield a mixture 

 of galactose, some pentose, and some complex organic acid. 



The gums are translucent, amorphous substances, whose solu- 

 tions in water are levorotatory. They are precipitated out of 

 solution by alcohol and by lead subacetate solution. 



Gums are extremely difficult to hydrolyze,the laboratory process 

 of hydrolysis usually requiring from eighteen to twenty-four hours 

 of continuous boiling with acids for its completion. Because 

 of this difficulty of hydrolysis, gums are practically indigestible 

 by animals and of little use as food. 



The following common examples will serve to illustrate the 

 general nature of these compounds. 



Gum arable, found in the exudate from the stems of various 

 species of Acacia, is a mixture of the calcium, magnesium, and 

 potassium salts of a diaraban-tetragalactan-arabic acid. Arabic 

 acid has the formula C23H3 8 C>22, and one molecule of this acid 

 serves as the nucleus for the union of eight galactose and four 

 arabinose groups, linked together in some unknown way. The 

 formula for the compound, exclusive of the metallic elements with 

 which it is loosely united is CgiHisoOrg. This gives some idea of 

 its complexity. 



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 



