THE CARBOHYDRATES 31 



ligno-celluloses, and all woody tissues have practically the 

 same elementary composition notwithstanding the difference 

 in their structural complexity. 



Cork and cuticular tissues generally exercise a protective 

 function. They are adapted to resist the action of air (oxygen) 

 and water. Ordinary cork is made up of a complex mixture 

 of oils, waxes, tannins, nitrogenous compounds, and the cork 

 substance proper. The former can be separated from the last 

 by mechanical solvents, e.g. alcohol, ether, etc. They are, 

 therefore, merely mixed with the cork, but their presence 

 greatly increases its power of resisting the natural forces of 

 disintegration. 



The cork substance proper contains cellulose of the cotton 

 type, ligno-cellulose and two characteristic compounds called 

 respectively cerine, C 2 oH 32 O, and suberine, C22H 42 O 3 . The 

 latter are fatty or waxy products. They are not removable 

 by mechanical solvents, but are saponifiable by drastic methods 

 of oxidation and treatment with alkalis. 



Crude Fibre. In the ordinary analysis of feeding stuffs, 

 the carbohydrates are classed as soluble carbohydrates and 

 crude or woody fibre. The former includes the starch, sugars, 

 gums, mucilages, and all the easily hydrolysable compounds. 

 They are estimated by the method of difference. 



To estimate the crude or woody fibre, the substance is 

 extracted with ether to remove the fat, then boiled first with 

 dilute acid, and afterwards with dilute alkali solution. The 

 insoluble residue which remains is next dried and incinerated, 

 and the loss is called crude fibre. 



The method is purely conventional. The results do not 

 correspond to the total cellulose in the material. They do 

 not even show the amount of indigestible cellulose. It was 

 at one time thought that they did, but more recent investiga- 

 tions have shown that some proportion of this crude fibre is 

 always digested by animals. The results are not, however, 

 entirely without value. They serve to distinguish, roughly, 

 between those feeding stuffs which contain a large proportion 

 of the more resistant compounds, which are more difficultly 

 digestible, and those which contain a smaller proportion. It 



