68 PLANT COMPOUNDS 



with so-called "encrusting substances" derived from it, 

 such as lignin and pentosans. In the pure form it occurs 

 naturally only in the cotton plant as a mass of seed hairs, and 

 in a very few other plants. As has been stated (Section 24) 

 it is derived from dextrose bv dehydration, thus: 



nCeHioOe — nH 2 = (C 6 Hio0 5 )n. 



Its function is to provide a somewhat elastic, but fairly 

 rigid envelope for plant cells. Where heavy walls are 

 needed and heavy weights are to be supported, as in trees, 

 these walls become "lignified" or thickened, filling the cell 

 almost completely. This thickened part forms the encrusting 

 substances mentioned above. 



Physically, cellulose is a white, amorphous or fibrous 

 solid, insoluble without decomposition in all solvents. 

 Cuprammonium hydroxide solution (cf. Section 216, I, c), 

 called Schweitzer's reagent, dissolves cellulose to a thick, 

 viscous, dark blue liquid in which the cellulose exists as a 

 complex compound with the copper and ammonia. From 

 this solution it may be reprecipitated in the amorphous 

 form by acidification with hydrochloric or sulphuric acid. 

 A strong solution of zinc chloride in concentrated hydro- 

 chloric acid also dissolves cellulose to a complex compound 

 from which it mav be recovered on treatment with alcohol. 

 Cold, concentrated sulphuric acid dissolves cellulose. In 

 so dissolving it is changed to a cellulose sulphuric acid 

 compound from which the cellulose can not be recovered. 

 On diluting such a solution and boiling it, the cellulose is 

 hvdrolvzed to dextrose. 



If cellulose is treated with a cold solution of sodium 

 hydroxide stronger than 10 per cent., a compound is formed 

 which has a formula something like this: Ci2HisOioNa2. 

 On washing with water the cellulose is regenerated as the 

 hydrate, C12H20O10.H2O. If cotton cloth is stretched on a 

 frame and treated in this way there is obtained a cloth of 

 silky appearance called "mercerized cotton." The treatment 

 results in a shrinking of the cotton fibers, and a resultant 

 gloss if the cloth is kept taut during the shrinking process. 



