GUMS 189 



4. A solution of neutral sulphate of aluminium (10 per 

 cent) generally gives a precipitate which is, in many cases, 

 soluble in potash. 



5. Basic lead acetate yields a precipitate which is entirely 

 or partially soluble in excess. 



GUM TRAGACANTH. 



This gum occurs in species of Astragalus, and consists 

 of about 8-10 per cent of soluble calcium, magnesium, and 

 potassium salts, together with about 60-70 per cent of in- 

 soluble salts, which only swell up in water to a jelly. The 

 water-soluble portion is said to contain a substance, poly- 

 aribanan-trigalactan-geddic acid, which on hydrolysis breaks 

 up into arabinose, galactose, and geddic acid, an isomer of 

 arable acid. The part soluble in water, when treated with 

 baryta water, gives two isomeric tragacanthan-xylan-bassoric 

 acids, which on hydrolysis yield a pentose sugar tragacanthose, 

 xylose, and bassoric acid C14H20O13. 



Von Fellenberg * has shown that the water-insoluble 

 constituent of tragacanth and bassorin gums is a methoxylated 

 compound ; it dissolves in alkali undergoing hydrolysis with 

 the liberation of methyl alcohol. The de-esterified compound 

 is named bassoric acid and yields on hydrolysis large quantities 

 of galacturonic acid showing it to be allied to the pectic acid 

 derived from pectin. 



WOUND GUM. 



A gum-like substance, termed wound gum, is frequently 

 found in the tracheae of plants, in the immediate neighbour- 

 hood of wounds, and stopping up the lumina ; it is secreted 

 by the surrounding living cells. Wound gum does not swell 

 in water, and is insoluble in sulphuric acid and in caustic soda. 

 On oxidation with nitric acid it yields both mucic and oxalic 

 acids, and it responds to lignin tests ; e.g. on treatment with 

 phloroglucinol and hydrochloric acid a bright red coloration 

 results. 



The origin of gums is as yet unknown ; by some authors 

 they are regarded as decomposition products of cellulose, 

 * V. Fellenberg : " Biochem. Zeit.," 1918. 85, 118. 



