PLANT CONSTITUENTS — CARBOHYDRATES 265 



polysaccharoses like pentosans. They are soluble in water 

 and on boiling the solution, or by the addition of alcohol, they 

 gelatinize. It is these pectins which are present in the juices 

 of succulent fruits, and which on the concentration of the juice 

 by boiling produce a jelly. The other variety of compound 

 celluloses is that found in the corky tissue of plants, and is 

 termed adipo-cellulose. The noncellulose constituent is prob- 

 ably of fatty nature, and is known as cutin. In all plants 

 containing largely hemicelluloses or compound celluloses, the 

 normal cellulose is also present to a greater or less extent. 



Cellulose as Food. — As a constituent of human foods cellulose 

 does not play an important part and is very slightly digested. 

 In the case of herbivorous animals, like the cow, sheep and 

 horse, however, they constitute a large part of their normal 

 food, and are thus of far greater importance than with man. 

 The most easily digested form of cellulose is that of the hemi- 

 celluloses, which, however, are not as abundant as the normal 

 celluloses, which probably rank next in order of digestibility. 

 The least digestible form is that of the lignocelluloses and of 

 these those found in hay and straw are the most valuable. In 

 the digestion of cellulose by herbivorous animals an amount 

 varying from 30 to 60 per cent is considered as digestible. 

 Of this amount, however, a considerable portion is fermented 

 by bacteria in the intestines, and is converted into products, 

 such as methane gas, which are unabsorbed. This necessitates 

 a correction for the digestibility of cellulose constituents in 

 order to determine the amount actually utilized by the animal. 

 This will be considered again (p. 297). The cellulose actually 

 utilized by animals is considered to have the same food value 

 as the other carbohydrates, such as starch and sugar. 



Crude Fiber. — In the analysis of cellulose-containing foods 

 the substance is usually determined in the form of what is 

 commonly termed crude fiber. If such a food is boiled with a 

 1.25 per cent acid solution and then filtered, and the undissolved 

 residue again boiled with a 1.25 per cent alkali solution, the 

 undissolved residue finally obtained is largely cellulose, and is 



