THE VOLATILE PART OF PLANTS. 61 



It may be that metarabin is identical with the "pec- 

 tose " of the sugar beet, since both yield arabin under the 

 influence of alkalies. It is evident that the composition 

 found for dried arabin properly belongs to metarabin, and 

 it is probable that arabin consists of metarabin Ci 2 H 22 O u 

 plus one or several molecules of water, and that metara- 

 bin is an anhydride of arabin. 



Arabin and metarabin, when heated with dilute sul- 

 phuric acid, are converted into a crystallizable sugar 

 called arabinose, C 5 H 10 5 . The gums that exude from 

 the stems of cherry, plum and peach trees appear to con- 

 sist chiefly of a mixture of freely soluble arabates with 

 insoluble metarabin. Gum Tragacanth is perhaps mostly 

 metarabin. All these gums yield, by the action of hot 

 dilute acids, the sugar arabinose. 



Galactin, C 6 H 10 5 , discovered by Miiutz in the seeds 

 of alfalfa and found in other legumes, has the appearance, 

 solubility in water and general properties of arabin, and 

 is probably the right-polarizing ingredient of gum arable. 

 Boiled with dilute acids it is converted into the sugar 

 galactose, C 6 H 12 6 . 



Paragalactin, 6 H 10 5 . In the seeds of the yellow 

 lupin exists up to 20 per cent of a body that is insoluble 

 in water, but dissolves by warming with alkali solutions, 

 and when heated with dilute acids yields galactoso. Ac- 

 cording to Steiger it probably has the composition C 6 Hi 5 . 

 Maxwell has shown it to exist in other leguminous seeds, 

 viz., the pea, horse-bean (Faba vulgaris) and vetch. 



In the "Chinese moss," an article of food prepared in 

 China from sea-weeds, and in the similar gum agar or 

 "vegetable gelatine" of Japan, exists a substance which 

 is insoluble in cold water, but with that liquid swells up 

 to a bulky jelly, and yields galactose when heated with 

 dilute acids. This corresponds to metarabin. 



Xylin, or Wood Gum. The wood of many decidu- 

 ous trees, the vegetable ivory nut, the cob of Indian 



