198 THE CONSTITUENTS OF PLANTS. IX. 



H C OH 0=C H 



I I /Ov 



HO C-H HC/ X CH + 3H 2 



I II II 



HO C H CH CH 



Arabinose. Furfurol. 



By 'the action of sodium amalgam a pentahydric alcohol, 

 arabite or arabitol, C 5 H ]2 O 5 , is produced a colourless, crystal- 

 line, sweet substance, resembling mannite. 



Xylose, wood sugar. This substance does not occur already 

 formed in plants, but results from the hydrolysis of xylan, the 

 main constituent of wood gum. The latter is very abundant 

 in certain woods, e.g., beech, and in oat straw, maize 

 cobs, brewers' grains, &c. Xylan often occurs in plants in 

 union with araban, hexosan, &c., as araboxylan, glucoxylan, 

 &c. These substances upon hydrolysis yield mixtures of 

 xylose with the other sugars. 



Pure xylan, C 5 H H O 4 , is a white porous solid nearly insoluble 

 in cold water but soluble in hot water. Xylose crystallizes in 

 white needles, which have a sweet taste and dissolve in water 

 or in hot alcohol. It reduces copper salts, and on treatment 

 with sodium amalgam yields xylite or xylitol, C 5 Hj. 2 O r> . 



Ribosc is another isomeric pentose obtained artificially. By 

 the action of reducing agents upon it a pentahydric alcohol 

 Adonite, C 5 H 12 O 5 is produced identical with that found in 

 certain plant saps, e.g., in Adonis vernalis. 



The Methyl Pentoses. Fucose, C 5 H (J (CH 8 )0,, has been 

 obtained from a sea-weed (Fucus nodosis). It is a crystalline, 

 sweet, very soluble substance, yielding methyl furfurol on 

 distillation with hydrochloric acid. 



Rhamnose, C 5 H a (CH 3 )0 5 .H 2 0, is not found in the free state 

 in plants, but occurs very frequently in compounds analogous 

 to the glucosides, from which dilute acids or alum solution set 



