62 CARBOHYDRATES [ch. 



arabica), Onion {Allium Cepa) and of members of the Leguminosae, 

 Rubiaceae, Coniferae and Umbelliferae. In seeds the mannans may con- 

 stitute, together with cellulose, the thickened cell-walls of the endosperm 

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

 are not strictly celluloses. " Vegetable ivory," which is the endosperm 

 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, 25) 

 and tubers of various genera of the Orchidaceae : they are also found in 

 the roots of the Dandelion {Taraxacum), Helianihus and Chicory, Aspa- 

 ragus 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. 51). 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 {Coffea arabica), the Bean (Faba), the Lupin {Lupinus), the 

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

 the Pea (Ptswm), the Nasturtium {Tropaeolum) and many others (Schulze, 

 Steiger and Maxwell, 32). 



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 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. 



