272 ORGANIC AGRICULTURAL CHEMISTRY 



Mannan and Galactan. — These two starch-like poly- 

 saccharoses are similar substances and are associated with 

 cellulose in the form of hemicelluloses already discussed. 

 They are found in wheat and in numerous other seeds, especially 

 legumes, but not in amounts sufficient to deserve more atten- 

 tion. A particular source of mannan is the substance known 

 as vegetable ivory. Agar-agar is a galactan. On hydrolysis 

 mannan yields mannose, and galactan, galactose. 



Pentosans 



The poly saccharoses which we have just considered are all 

 hexosans, i.e. they yield hexose sugars on hydrolysis. Analo- 

 gous to these we have polysaccharoses which on hydrolysis 

 yield pentose sugars (arabinose, xylose, etc.). They are termed 

 pentosans. They have already been referred to as constituting 

 the noncellulose part of lignocelluloses and probably also 

 pectocelluloses. They constitute the gummy or resinous 

 material which on hardening within the cell wall produces the 

 lignocelluloses of woody fibers in trees and the stems of cereals 

 and grasses. 



They are found in purer form in what are known as natural 

 gums, such as gum Arabic, gmn tragacanth, wood gum, etc. 

 These natural gums are not simple pentosan polysaccharoses, 

 but are of a complex glucoside nature which readily yield first 

 the pentosan and then the pentose sugar on hydrolysis. Some 

 gums also yield galactans and galactose. They are also con- 

 sidered as acid compounds which are present in the gums as 

 salts of potassium, calcium or magnesium. 



Whether, however, they are present as constituents of lig- 

 nocelluloses, pectocelluloses or natural gums, the pentosans 

 yield pentose sugars on hydrolysis and probably possess a 

 food value close to that of other polysaccharoses. Their wide 

 distribution in plants gives them an importance as food con- 

 stituents, and their determination is a usual part of food analysis, 

 especially in connection with ordinary farm crops used as food 

 for domestic animals. The principle of the analysis is that 



