78 THE FEEDING OF ANIMALS 



tubers and bulbs, algae, lichens, molds, and the wood and 

 bark of many species of trees. On hydrolysis they yield 

 galactose, mannose, levulose, and dextrose respectively. 

 The compounds are designated as hemi-celluloses. 



They make up the least valuable part of certain vege- 

 table foods. 



106. The pectin bodies. Another class of compounds 

 much like the gums and perhaps related to them chemi- 

 cally, is the pectin bodies. Some of these substances are 

 gelatinous in appearance. The jellying of fruits, such as 

 apples and currants, is made possible by their presence. 

 They exist in greater abundance in unripe fruit than in 

 the ripe, consequently the former is selected for jelly- 

 making. When such fruits are cooked, the pectin which 

 they contain takes up water chemically and is transformed 

 into a gelatinous substance known as pectose. Mucilages 

 not greatly unlike the gums and pectins exist in certain 

 seeds and roots, the most notable instance being flaxseed. 



107. Dextrin, which is sometimes spoken of as a gum, 

 is made by heating starch to about 200 C. It may also 

 be produced by treating starch with a dilute acid. Dex- 

 trin is formed on the outer part of the loaf when wheat 

 bread is baked. It is soluble in water. 



108. Cellulose. This is found in the tough or woody 

 portion of plant tissue. In tables of food analyses we 

 find the term crude fiber, which consists largely of cellu- 

 lose, a familiar example of which in a nearly pure form is 

 the cotton fiber used in making cloth. Crude fiber is 

 separated from associated compounds by the successive 

 treatment of vegetable substance with weak acids and 

 alkalies, and as so determined is sometimes improperly 

 taken to represent the amount of cellulose in a plant. 

 While crude fiber is mainly cellulose, it contains a small 



