THE CARBOHYDRATES 141 



these substances are commonly classified with the reserve celluloses. 

 Mannan is the most common of the mannosans, but secalan from 

 rye is also one. 



Galactosans are similar complexes which yield galactose along 

 with some other hexose or pentose. They occur together with the 

 mannosans in reserve cellulose and have been found in the testas 

 of beans and peas as well as in the regular reserve regions of the 

 coffee bean, date seed, etc. Among the galactosans are galactan 

 and paragalactan, both of which are white amorphous solids 

 slightly soluble in water and hydrolyzed with difficulty. 



Pentosans. — These substances are found in the stems and seed 

 coats of many plants, where they are considered to be of prime 

 importance in conserving moisture and in aiding the plant to resist 

 low temperatures. Thus Hooker and Rosa (1920) found a distinct 

 correlation between hardiness and pentosan content in fruit trees 

 and vegetables. Corn bran contains about 40% and wheat straw 

 about 25% of pentosans. They are white solids which are difficult 

 to hydrolyze and are only slightly soluble in water. When finally 

 broken down they yield the pentoses, arabinose and xylose, and 

 thus bear the same relation to the pentoses that the hexosans 

 do to hexoses. 



Araban occurs in various gums such as gum arabic and cherry 

 gum in combination with other materials. Xylan is the chief 

 pentosan in lignified cell walls and is found in straw, corncobs, 

 nutshells, seed testas, and similar regions, probably occurring 

 wherever lignified tissue is found. 



The gums are amorphous translucent substances, which are 

 widely distributed in plants, especially in trees and shrubs of 

 warm regions. They are composed of sugars (chiefly pentoses) 

 united with an organic acid, which permits them to form salts 

 with metals. Some are soluble in water, while others simply swell 

 up forming a jelly like mass. Because they are so hard to hydrolyze, 

 they are practically indigestible and are of little food value for 

 animals. 



Gum arabic is used commercially in the preparation of mucilage 

 and as a base in many toilet articles. It is a brittle, amorphous, 

 glassy exudate from the branches of Sudan acacias (A. Senegal 

 and A. virek) although other acacias yield inferior gums. When 

 hydrolyzed it yields arabic acid, galactose, and arabinose. The 

 formula, C 91 Hi5o0 7 8, gives an idea of its complexity. 



