72 CHEMISTRY OF PLANT LIFE 



hydrolyzed yield various sugars, chiefly galactose, mannose, and 

 the pentoses. They bear the same relation to these sugars that 

 starch does to glucose, and are generally supposed to serve as 

 reserve food material, although it is difficult to conceive how the 

 shells, etc., in which they appear can be utilized by a growing seed- 

 ling. They differ in structure from the fibrous celluloses and are 

 probably not cell-wall building material. They appear to be a 

 form of reserve carbohydrates, which differ from the glucose- 

 polysaccharides in being condensed in, or as a part of, the external 

 structural material rather than in the internal storage organs. 

 They are soluble in water and exhibit the properties of gums, and 

 are often classified with the gums and described under the names 

 " galactans," " mannosans," " pentosans," etc. 



The normal celluloses, of which the fibers obtained from cotton, 

 flax, hemp, etc., are typical examples, are widely distributed in 

 plants and form the commercial sources for all textile fibers of 

 vegetable origin. Ordinary cotton fiber contains 91 per cent of 

 cellulose, about 7.5 per cent of water, 0.4 per cent of wax and fat, 

 0.55 per cent of pectose derivatives, and 0.25 per cent of mineral 

 matter; or a total of only 1.2 per cent of non-cellulose solids. 

 Filter paper is practically pure cellulose. 



Pure cellulose is a white, hygroscopic substance, which is insol- 

 uble in water and in most other solvents. If heated with water 

 under pressure to about 260 9 C., it dissolves completely without 

 decomposition. If boiled with a strong solution of zinc chloride, 

 or treated in the cold with zinc chloride and concentrated hydro- 

 chloric acid, or with an ammoniacal solution of copper hydroxide 

 (Schweitzer's reagent), it dissolves to a clear solution from which 

 it may be reprecipitated without chemical change by neutralizing 

 or diluting the solution. 



Cellulose has the formula (C6Hi2O5) n . When hydrolyzed 

 under the influence of the enzyme cytase, it breaks down, first into 

 cellobiose, an isomer of maltose, and then into glucose. It is, 

 therefore, chemically like, but not identical with, starch; and 

 structurally it is arranged in fibrous form instead of in granules. 

 Under the action of fermentative enzymes, as when vegetable 

 matter decays under stagnant water, in swamps, etc., cellulose 

 breaks down into carbon dioxide and marsh gas, according to the 

 equation 



