HEMICELLULOSES 185 



MIXED GALACTANS. 



Other sources of galactan which, however, do not yield 

 exclusively galactose on hydrolysis are of common occurrence. 

 Such substances have been variously described as galacto- 

 arabans, galacto-xylan, galacto-mannan, etc., according to 

 the sugars to which they give rise ; these occur notably in 

 the mucilaginous extracts of seaweeds and form the agar and 

 carragheen extracts of commerce (see below, p. 191). Under 

 this heading should be included galacto-araban, sometimes 

 wrongly described as para-galactan, which occurs in the 

 cell walls of the cotyledons of many plants, e.g. Lupinus 

 luteus and other species, Phoenix dactylifera, Cocos nucifera, 

 and other palms, Soja hispanica and Coffea arabica, where it 

 forms a reserve food-material which is digested on germination. 



Galacto-araban is a white solid which is insoluble in water 

 and cuprammonia ; it dissolves in hot potash. On heating 

 with nitric acid it is oxidized to mucic acid. Microchemically 

 it may be identified by its insolubility in the reagents men- 

 tioned, and also by the fact that with phloroglucin and hydro- 

 chloric acid it gives a red coloration on warming. No colour 

 is given in the cold. 



Its association with cellulose prevents the latter exhibiting 

 some of its reactions ; thus the cellulose is unacted upon by 

 cuprammonia unless the galacto-araban be removed ; this may 

 be done by boiling in dilute hydrochloric acid. 



Other substances giving rise to galactose are the pectins 

 (p. 192). 



AMYLOID. 

 Amyloid is the name given to a substance occurring in 

 the seeds of paeonies and certain cresses,* which yields on 

 hydrolysis with dilute sulphuric acid a mixture of galactose, 

 glucose, and xylose. It is a colourless substance, and is in- 

 soluble in cold water, but swells up into a slimy mass in hot 

 water ; it is soluble in cuprammonia solution. Amyloid does 

 not reduce Fehling's solution, but is oxidized by nitric acid to 

 mucic and trihydroxy-glutaric acids. It gives a blue colour 

 with iodine. 



* Winterstein : " Zeit. physiol. Chem.," 1893, 17, 353. 



