134 PLANT PHYSIOLOGY 



Among the substances which yield these uronic acids are gums 

 and pentosans. 



Linkage. — The sugars are easily connected to each other, with 

 the elimination of water, to form more complex compounds. It 

 is thus that the monosaccharides are connected to form a disac- 

 charide or a trisaccharide : 



2C 6 H 12 6 = CijHjjOn+HaO 



and these in turn to form the polysaccharides. When hydrolyzed 

 the reverse process takes place: 



C12H22O11+H2O = 2C6H12O6. 



Monosaccharides. — The monosaccharides are sugars which 

 cannot be split or hydrolyzed into simpler compounds with the 

 properties of sugar, that is, they are the simplest sugars. One 

 group of monosaccharides, the pentoses, contains five carbon atoms 

 while the other group, the hexoses, has six carbon atoms. In the 

 laboratory, monosaccharides have been made with three and four 

 atoms of carbon, but these do not occur free in plants. 



Pentoses. — The pentoses have the formula C5H10O5. They have 

 been reported in very small amounts in the free state in corn, 

 turnips, bamboo, and other plants, but they generally exist in the 

 combined state as condensation products from which water has 

 been eliminated. These products, which are commonly gums and 

 mucilages, yield pentoses when hydrolyzed, in much the same way 

 that starch yields glucose. The two pentoses commonly found in 

 plants are arabinose and xylose ; they are not so readily fermentable 

 by yeast as the hexoses. 



Arabinose is found in cherry gum, peach gum, and gum arabic. 

 It has a very sweet taste and is dextrorotary. Xylose is obtained 

 by hydrolysis from wood gum and straw as well as from grains 

 such as barley and corn. It is not so sweet as arabinose and is less 

 dextrorotary. The formulae for these pentoses are: 



