X. 



THE CHEMICAL CONSTITUENTS OF PLANTS 



191 



immersed in it. They are not coloured by iodine, and by boiling with 

 dilute acids they yield sugars, often arabinose or xylose. 



Some of the substances are employed in the arts, e.g., gum arabic, 

 the exudation from the bark of several species of acacia. This substance 

 contains 3 or 4 per cent of ash (mainly lime), and as its principal constit- 

 uent arabin or arabic acid, a white solid soluble in water, of highly com- 

 plex constitution (C 89 H 142 O 72 or C 78 H U() O^ 1 ). Very similar bodies are 

 found in nearly all vegetable tissue. Wood gum, the name given to the 

 substance occurring in wood, the straw of cereals, etc., is a substance of 

 this class. By boiling with dilute acids or alkalies, some gums yield 

 pentose sugars, arabinose, xylose, or lyxose, C 5 H 10 O 5 . They, therefore, 

 belong to the class of bodies for which the name pentosan has been pro- 

 posed, of the composition (C 5 H 8 O 4 ) n . It appears 2 that the effect of 

 boiling arabin with dilute acid is to add gradually the elements of water 

 and to cause the splitting off of a sugar molecule, leaving a residue 

 known as arabinosic acid, which, by further boiling, loses another sugar 

 molecule, giving a lower acid /3-arabinosic acid and so on, until 

 finally an acid of the formula C 23 H 38 O 22 is left. 



C 78 H 120 63 + H 2 = C 5 H 10 5 +C 73 H 112 59 ; 

 finally C 73 H 112 O 59 + 13H 2 = 10C 5 H 10 O 5 + C 23 H 38 O 22 . 



Some gums, on boiling with dilute sulphuric acid, yield not only 

 arabinose or xylose, as above, but galactose, C fi H 12 O r> , as their main 

 product. Hence gum is a name which includes both pentosans and 

 hexosans (i.e., polysaccharoses, which yield pentose and hexose sugars). 



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 gum 



Gum tragacanth 



Gum arabic 



Wheat bran 



Meadow hay . 



Clover hay 



Pea straw 



Oat straw 



Wheat straw . 



Barley straw . 



Eye straw 



Brewers' grains (dry) 



Maize bran 



Jute fibre 



Wood gums (various) 



Humus . 



Wheat (grain) 



Maize 



about 5 



By prolonged boiling with hydrochloric acid, pentosans yield fur- 

 furol (furfuraldehyde) by the removal of water from the pentoses first 

 formed : 



1 O'Sullivan. 



2 0'Sullivan, Jour. Chem. Soc., 1882, 41. 



