=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 generally 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. =Pecto-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 "muco-celluloses." The exceedingly inert character 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 investigations 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 consideration 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 rare cases, however, is there any evidence that they can be reconverted into carbohydrates to serve as food material. Certain bacteria can make use of cellulose as food, and secrete an enzyme, cytase, which aids in the hydrolysis of cellulose to sugars for this purpose. But this enzyme seems rarely, if at all, to be present in the tissues of higher plants. It has been reported that some cellulose is hydrolyzed during the malting of barley, indicating that this might have some food use for the growing seedling; but this observation has not been confirmed and later investigations seem to throw doubt upon its accuracy.