THE COMPONENTS OF PLANTS AND ANIMALS 15 



small amounts a carbohydrate called glycogen. Glycogen has 

 the same percentage composition as starch and has sometimes 

 been called animal starch, although improperly, since its proper- 

 ties are quite different from those of starch. It has important 

 functions in the animal, as will appear later. It is not found in 

 the plant. It is readily soluble in water, yielding an opalescent 

 solution. The empirical formula of glycogen is the same as 

 that of starch. When hydrolyzed it yields only dextrose, and 

 is therefore a dextran. 



26. The gums. Familiar examples of this class of sub- 

 stances are gum arabic and the gums of the cherry, peach and 

 plum. The mucilage of flax seed closely resembles the gums, 

 and other seeds also contain gum-like "bodies. Upon hydrolysis, 

 the gums yield hexoses, especially galactose, showing that they 

 contain galactans. In addition to hexoses, however, they yield 

 sugars belonging to the pentose group. 



27. The pentosans. The pentosans may be regarded as 

 polymers or anhydrids of the pentoses, corresponding in this 

 respect to the polysaccharids of the hexose group. Their 

 general formula is (CsHgC^),,, but their molecular structure is 

 unknown. 



Araban. This is a constituent of gum arabic and other 

 gums, as shown by the fact that these gums, as already noted 

 (10), yield /-arabinose when hydrolyzed. 



Xylan. This compound is also known as wood gum. It 

 can be extracted from various woods, from the cob of maize 

 and from various other vegetable materials by the action of 

 dilute alkalies, and yields /-xylose when hydrolyzed. In the 

 plant, araban and xylan appear to be in a more or less close 

 chemical combination with hexosans, especially in the cell walls 

 of the more mature plant, constituting the so-called hemi- 

 celluloses (18). 



Pectins. Most ripe fruits, as well as the flesh of beets, 

 turnips and similar roots, contain a group of substances called 

 the pectin group. As they exist in the roots or fruits they are 

 insoluble in water, but by cooking they are converted into sub- 

 stances which form the basis of fruit jellies. On hydrolysis 

 they yield pentoses, chiefly arabinose. 



