138 PLANT PHYSIOLOGY 



cules of a monosaccharide with the elimination of water according 

 to the equation: 



3C6H12O6 = C18H32O16+2H2O. 



When hydrolyzed they yield three molecules of a monosaccharide 

 or one each of a monosaccharide and a disaccharide. About five 

 are well established as present in plants. 



The only reducing sugar of this group is xanthorhamnose. This 

 peculiar sugar, which is obtained from members of the buckthorn 

 family (Rhamnaceae), is composed of two molecules of the pentose, 

 rhamnose, combined with one of glucose. 



Raffinose is obtained from cotton seeds, barley, eucalyptus, and 

 the sugar beet, which forms the chief commercial source. It is 

 associated with sucrose and resembles it in many properties. 

 When completely hydrolyzed it yields a molecule each of levu- 

 lose, dextrose, and galactose. 



Gentianose, from the yellow gentian, yields fructose and gentio- 

 biose. Melezitose, an isomer of gentianose, is a very sweet sugar 

 which comes from the sap of several conifers, including the Eu- 

 ropean larch (Larix europaea) and Douglas fir (Pseudotsuga). It 

 collects on the twigs in small white flakes, and when hydrolyzed 

 it yields glucose and levulose. 



Stachyose, which comes from the tubers of a mint (Stachys 

 tuberifera), is a faintly sweet sugar which yields a molecule of 

 levulose, one of dextrose, and two of galactose, and is thus seen 

 to be a tetrasaccharide. 



Polysaccharides. — The polysaccharides are composed of many 

 molecules of a monosaccharide linked together with the elimina- 

 tion of water. The general formula may be expressed as (C 6 Hio05)n 

 or more correctly (C 6 ) n (H20)5n+i and (C 5 )n(H 2 0)4n+i, depending 

 upon whether the substance is derived from hexoses or pentoses. 

 The value of n is probably between 30 and 200 but, owing to the 

 slight solubility of these materials, their actual molecular weight is 

 very hard to determine. 



Hexosans. — The hexosans have the general formula given 

 above, and are built up from the hexose sugars, which they yield 

 on hydrolysis. There are four different groups depending upon 

 the hexose which composes them; and upon this basis the hexo- 

 sans may be classified as dextrosans, levulosans, mannosans, and 

 galactosans. 



