t60 CHEMICAL COMPOSITION OF PLANTS 



It has been customary to term the material forming the cell- 

 wall cellulose, as if it were a single chemical substance. A 

 variety of celluloses are, however, now known and the cell-walls 

 of plants invariably consist of mixtures or compounds of these 

 with several other substances. 



What may be named the typical cellulose can be readily 

 obtained from cotton-wool and flax-fibre by treating the latter 

 with various chemical reagents to eliminate the substances com- 

 bined or mixed with it : it is a carbohydrate possessing the 

 empirical composition represented by the formula n (Cg Hjq O5). 

 This t3rpical cellulose is insoluble in dilute acids and alkalies, 

 but is soluble in ammoniacal cupric oxide, hot concentrated 

 solutions of zinc chloride and other solvents. 



It stains blue when treated with sulphuric acid and iodine or 

 with ' chlor-zinc-iodine,' and when acted upon with concentrated 

 sulphuric acid yields dextrose sugar. 



Another type of cellulose is present in the cell-walls of 

 lignified tissues. When obtained free from the substances with 

 which they are combined or mixed, these celluloses differ from 

 the cellulose obtained from cotton fibre not so much in empirical 

 composition as in chemical structure. They contain a slightly 

 higher percentage of oxygen, are less resistant to hydrolysis, and 

 yield only small quantities of dextrose and mannose sugars when 

 treated with sulphuric acid ; moreover the aldehyde furfural is 

 produced when celluloses of this type are hydrolysed with dilute 

 hydrochloric acid. 



The cell-walls of the cells of the endosperm-tissue and coty- 

 ledons of seeds are formed of substances termed hemicelluloses, 

 which are so different in chemical properties from the two types 

 just mentioned that they have little right to be considered cellu- 

 loses at all, except that they resemble the latter in appearance 

 and are the materials of which certain cell-walls are composed. 

 Hemicelluloses are very easily hydrolysed by dilute acids and 

 alkalies into galactose, mannose and pentose sugars. 



