THE POLYSACCHARIDES 375 



nettle (Stachys tuhifera). Upon complete hydrolysis this carbohydrate yields 

 one molecule of glucose, one of fructose and two of galactose. 



The Polysaccharides. — The polysaccharides are carbohydrates formed by 

 the condensation of large numbers of monosaccharide molecules. Those 

 occurring in the plants fall largely into the three groups hexosans, pentosans 

 and mixed petitosan-hexosans. The latter group corresponds roughly to the 

 so-called "hemicelluloses." The polysaccharides are not sweet like the sugars, 

 and they are not soluble in water. All of them, however, are more or less 

 hydrophilic, and hence are frequently found as components of hydrophilic 

 colloidal systems. In this property the pentosans, in general, greatly surpass 

 the hexosans. 



1. The Pentosatis. — These compounds have the empirical formula 

 (C5Hs04)„ and are the condensation products of arabinose and xylose, 

 the corresponding pentosans being the arabans and xylans. The molecules 

 of these compounds apparently consist of long chains of arabinose or xylose 

 residues, similar in structure to cellulose and starch molecules described later. 

 Arabans and xylans apparently occur principally in the cell walls of plants. 

 In some species, such as the cacti, they are also important constituents of the 

 mucilaginous materials present in the cells, and contribute largely to the hy- 

 drophilic properties of such substances. Arabans are one of the constituents 

 of cherry, peach, and plum gum, while xylans are found very commonly in 

 wood, straw, corncobs, and seed coats. Xylans may constitute as much as 

 15 per cent of the woody tissues of some trees. 



2. The Hexosans. — The important polysaccharides which are synthesized 

 in plants from ^-glucose are cellulose, starch, dextrins, and glycogen. All 

 of these, as well as the hexosans formed from other hexoses, are represented 

 by the empirical formula (CgH^^oOs) re- 

 Cellulose is the principal constituent of the cell walls of all of the higher 



plants (Chap. VI). In terms of absolute amounts it is probably the most 

 abundant organic compound present on the earth. Much of the cellulose 

 as it occurs in the plant is in intimate mixture with, or encrusted by, other 

 materials. Some fibers, however, are practically pure cellulose. Fibers of 

 the cotton plant, for example, are composed of about 91 per cent cellulose, 

 8 per cent water, and only i per cent of other substances. 



Cellulose "molecules" are long, ribbon-like structures. These chain-like 

 m.olecules are built up by the linear condensation of /? ^-glucose molecules. 

 Each cellulose molecule therefore consists of a chain of at least lOO and 

 probably many more glucose residues (Chap. VI) linked together by oxygen 

 bridges. 



Chemically cellulose is relatively inert, being insoluble in water and all 



