74 CHEMISTRY OF PLANT LIFE 



ligno-celluloses, (2) pecto-celluloses, and (3) adipo-, or cuto- 

 celluloses. As the names indicate, the non-cellulose component 

 in the first group is lignin; in the second, pectic substances; and 

 in the third, fats or waxes. 



Ligno-celluloses. In the young plant cell, the cell-walls 

 consist of practically pure cellulose; but as the plant grows older, 

 this becomes permeated with lignin, or woody fiber, until in the 

 stem of a tree, for example, the proportion of cellulose in the tissue 

 is only 50 to 60 per cent. In the preparation of wood pulp for 

 the manufacture of paper, the lignin materials are dissolved off by 

 means of various chemical reagents, leaving the cellulose fibers 

 in nearly pure form for use as paper. The lignin material gen- 

 erally consists of two types of substances, one of which contains a 

 closed-ring nucleus of unknown composition and the other is 

 probably a pentosan. These materials are so extremely difficult 

 to hydrolyze that their composition has not yet been definitely 

 determined. 



P.ecto-celluloses are found in various species of flowering plants; 

 those which are present in the stems and roots being true pecto- 

 celluloses, while those which are found in fruits and seeds contain 

 mucilages rather than pectose derivatives, and are generally 

 designated as " mu co-celluloses." The exceedingly inert char- 

 acter of these compounds makes their study difficult and their 

 functions uncertain. 



The term cuto-celluloses is applied to the group of substances, 

 including suberin and cutin, which constitute waterproof cell- 

 walls. These were formerly supposed to consist of true cellulose 

 impregnated with fatty or wax-like materials. Recent investiga- 

 tions seem to indicate, however, that there is really no cellulose 

 nucleus in such walls as these, but that they are compound glyceryl 

 esters resembling the true fats (see chapter X) in composition. If 

 this view should finally be established as a fact, this sub-group of 

 supposed compound celluloses should be dropped from consider- 

 ation as such. 



PHYSIOLOGICAL USE OF CELLULOSES 



There seems to be no question that the sole use of celluloses is 

 to serve as structure-building materials. They are undoubtedly 

 elaborated from the carbohydrates as the cell grows. In only 



