60 CARBOHYDRATES AND THEIR [CH. 



Rubiaceae, Coniferae and Umbelliferae. In seeds the mannans may 

 constitute, together with cellulose, the thickened cell-walls of the endo- 

 sperm and are included in the term " reserve- or hemi-cellulose " though 

 they are not strictly celluloses. " Vegetable ivory," which is the endo- 

 sperm of the Palm, Phytelephas macrocarpa, contains considerable 

 quantities of a mannan and is used as a source of mannose. Mannans, 

 in addition, form constituents of certain mucilages, as for instance those 

 in Lily bulbs (Lilium candidum, L. bulbiferum, L. Martagon and others) 

 (Parkin, 23) and tubers of various genera of the Orchidaceae : they are 

 also found in the roots of the Dandelion (Taraxacum), Helianthus and 

 Chicory, Asparagus and Clover, and in the wood and leaves of various 

 trees. 



Many of the mannans, unlike true celluloses, are readily hydrolyzed 

 by dilute hydrochloric and sulphuric acids. The mannan in the Coffee 

 Bean, however, is hydrolyzed with difficulty. 



GALACTANS. 



These substances bear the same relationship to the hexose, galactose, 

 as the mannans to mannose, that is, they are condensation products of 

 galactose (see p. 49). Similarly they frequently occur, together with 

 the condensation products of other sugars, as galactoaraban, galactoxylan, 

 galactomannan, etc. As such they form constituents of many gums and 

 mucilages and of the cell-walls of the reserve tissue of seeds, i.e. the 

 Coffee Bean (Goffea arabicd), the Bean (Faba), the Lupin (Lupinus), the 

 Paeony (Paeonia), the Kidney Bean (Phaseolus), the Date (Phoenix), 

 the Pea (Pisum), the Nasturtium (Tropaeolum) and many others (Schulze, 

 Steiger and Maxwell, 29). 



GUMS. 



These substances occur widely distributed among plants, especially 

 trees. Some gums are wholly soluble in water giving sticky colloidal 

 solutions : others are only partially soluble. They are all insoluble in 

 alcohol. In the solid state they are translucent and amorphous. 



Chemically the gums are varied in nature ; they may in general be 

 regarded as consisting of complex acids in combination 1 with condensa- 

 tion products of various sugars, such as araban, xylan, galactan, etc. 

 On hydrolysis they give mixtures of the corresponding sugars, arabinose, 

 xylose, galactose, etc., in varying proportions, though in some cases one 

 sugar preponderates. 



