224 THE CARBOHYDRATES. 



those varieties of gum which do not or only partly dissolve in water, and which 

 swell up therein to a greater or less extent. The natural varieties of gum and 

 mucilage, to which belong several generally known and important substances, 

 such as gum arabic, wood-gum, cherry-gum, salep, and quince mucilage, and 

 probably also the little-studied pectin substances, will not be treated in detail, 

 because of their unimportance from a physiological standpoint. 



The Cellulose Group (C 6 Hi O 5 )x. 



Cellulose is that carbohydrate, or perhaps more correctly, mixture of 

 carbohydrates, which forms the chief constituent of the walls of the plant- 

 cells. This is true for at least the walls of the young cells, while in the 

 walls of the older cells the cellulose is extensively incrusted with a sub- 

 stance called LIGNIN, and with many other cellulose derivatives and 

 compounds. 



The true celluloses are characterized by their great insolubility. They 

 are insoluble in cold or hot water, alcohol, ether, dilute acids, and alkalies. 

 We have only one specific solvent for cellulose, and that is an ammo- 

 niacal solution of copper oxide called SCHWEITZER'S reagent. The cellu- 

 lose may be precipitated from this solvent by the addition of acids, and 

 obtained as an amorphous powder after washing with water. 



Cellulose is converted into a substance, so-called AMYLOID, which 

 gives a blue coloration with iodine, by the action of concentrated sul- 

 phuric acid. With oxidizing agents (nitric acid, etc.) oxycelluloses are 

 produced. By the action of strong nitric acid or a mixture of nitric 

 acid and concentrated sulphuric acid, celluloses are converted into nitric- 

 acid esters or nitrocelluloses, which are highly explosive and have found 

 great practical use. 



The ordinary celluloses when treated at the ordinary temperature 

 with strong sulphuric acid and then boiled for some time after diluting 

 with water are converted into dextrose. In this case it must be observed, 

 according to MAQUENNE, that it is not maltose that is produced as an 

 intermediate step, but another disaccharide, called cellose or cellobiose. 



Hemicelluloses are, according to E. SCHULZE/ those constituents of the 

 cell-wall related to cellulose which differ from the ordinary cellulose by dissolving 

 on heating with strongly diluted mineral acids, such as 1.25-per cent sulphuric 

 acid, and of yielding arabinose, xylose, galactose, and mannose instead of dextrose. 

 Those hemicelluloses which serve partly as reserve food and partly as support- 

 substance, are very widely distributed in the plant kingdom. 



The cellulose, at least in part, undergoes decomposition in the intestinal 

 tract of man and animals. A closer discussion of the nutritive value of 

 cellulose will be given in a future chapter (on digestion). The great- 

 importance of the carbohydrates in the animal economy and to animal 

 metabolism will also be given in the following chapters. 



1 E. Schulze, Zeitschr. f. physiol. Chem., 16 and 19, with Castro, ibid., 36. 



