THE PENTOSES. 197 



and the Methyl-Pentoses 

 Fucose 

 Ehamnose 



Arabinose, CH 2 OH.(CHOH) 3 .COH, probably never occurs 

 as such in plants, but is obtained by hydrolysis from the 

 pentosans, (C 5 H 8 4 ) n , which occur in the gum-like constituents 

 of many vegetable tissues. In some cases these gums yield 

 also galactose or glucose, so that they are probably mixtures 

 or compounds of pentosans and hexosans. Moreover the pen- 

 tose obtained is sometimes arabinose, in other cases xylose. 

 The total amount of the pentosans present in various plants 

 has been determined by Tollens, Chalmot, Giinther, Stone, 

 and others. The following table gives some of their results : 



% 

 Cherry gam ... ... 52 to 59 



Gum tragacanth ... ... 32 to 38 



Gum arabic ... ... 26 to 28 



Wheat bran ... ... 22 to 25 



Meadow hay ... ... 16 to 18 



Clover hay ... ... 8 to 10 



Pea straw ... ... 15 to 17 



Oat straw ... ... 16 to 20 



Wheat straw ... ... 26 to 27 



Barley straw ... ... 22 to 25 



Eye straw ... ... 22 to 25 



Brewers' grains (dry) ... 27 to 31 



Maize bran ... ... 38 to 43 



Jute fibre ... ... 13 to 15 



Wood gums (various) ... 60 to 92 



Humus ... ... ... 1-5 to 4 



Wheat (grain) ... ... 4 to 5 



Maize ... ... ... about 5 



Arabinose is a crystalline solid, with intensely sweet taste, 

 very soluble -in hot though much less in cold water; it is 

 insoluble in alcohol. It reduces copper solutions and forms 

 an osazone, C 5 H H O 3 (N 2 HC 6 H 5 ) 2 , melting at 160. Boiling 

 hydrochloric acid converts it into furfurol 



