Page:EB1911 - Volume 05.djvu/631

 

 CELLULOSE, the name given to both an individual—cellulose proper, in the restricted sense of a chemical individual—and to a group of substances, the celluloses or cellulose group, which constitute in infinitely varied forms the containing envelope of the plant cell. They are complex carbohydrates, or “saccharo-colloids” (Tollens), and are resolved by ultimate hydrolysis into monoses. The typical cellulose is represented by the empirical formula C6H10O5, identical with that of starch, with which it has many chemical analogies as well as physiological correlations. The representative “cellulose” is the main constituent of the cotton fibre substance, and is obtainable by treating the raw fibre with boiling dilute alkalis, followed by chlorine gas or bromine water, or simply by alkaline oxidants. The cellulose thus purified is further treated with dilute acids, and then exhaustively with alcohol and ether. Chemical filter-paper (Swedish) is practically pure cellulose, the final purification consisting in exhaustive treatment with hydrofluoric acid to remove silicious inorganic residues. The “cellulose” group, however, comprises a series of substances which, while presenting the characters generally similar to those of cotton cellulose, also exhibit marked divergences. The resemblances are maintained in their synthetical reactions; but reactions involving the decomposition of the complex show many variations. For example, cotton cellulose is difficultly hydrolysed; other celluloses are more or less readily split up by dilute acids, the extreme members readily yielding sugars: the hexoses—dextrose, mannose and galactose; and the pentoses—xylose and arabinose; these less resistant cell-wall constituents are termed hemi-celluloses.

The celluloses proper are essentially non-nitrogenous, though originating in the cell protoplasm. The cell-walls of the lower cryptogams, similarly purified, retain a notable proportion—2.0-4.0%—of constitutional nitrogen. When hydrolysed these fungoid celluloses yield, in addition to monoses, glucosamine and acetic acid. The celluloses of the phanerogams are generally associated, in a degree ranging from physical mixture to chemical union, with other complicated substances, constituting the “compound celluloses.” The nature of the associated groups affords a convenient classification into pecto-celluloses, ligno-celluloses and cuto-celluloses. Pecto-celluloses are so named because the associated substances—carbohydrates, together with their oxidation products, i.e. containing either two carbonyls (CO) in the unit group or carboxyl (CO·OH) groups in a complex—are readily hydrolysed by weak acids to the gelatinous “pectic acids” or their salts. Ligno-celluloses are the substances of lignified tissue, the non-cellulose constituents of which are characterized by the presence of benzenoid and furfuroid groups; and although essentially complex, they may be regarded as homogeneous, and are conveniently grouped under the name lignone. The lignone complex reacts, by its unsaturated groups, with the halogens. It is a complex containing but little hydroxyl; and is of relatively high carbon percentage (55.0–57.0%). Cuto-celluloses predominate in the protective coatings of plant organs, and are characterized by constituent groups, the decomposition products of which are compounds of the fatty series, and also wax alcohols, acids, cholesterols, &c.

The typical pecto-cellulose is the flax fibre, i.e. the bast fibre of the flax plant (Linum usitatissimum), as it occurs in the plant, or as the commercial textile fibre in its raw state. Rhea, or ramie, is another leading textile fibre in which the cellulose occurs associated with alkali-soluble colloidal carbohydrates. Pecto-celluloses are found in the stems of the Gramineae (cereal straws, esparto), and in the fibro-vascular bundles of monocotyledons used as textile and rope-making fibres. They are the chief constituents of the fleshy parenchyma of fruits, tubers, rhizomes. Ligno-celluloses find their chemical representative in the jute fibre. They constitute the woods, and are therefore of the widest distribution and the highest industrial utility. It is important to note that a complex having all the chemical characteristics of a ligno-cellulose occurs in a soluble colloidal form in the juice of the white currant. The formation of ligno-cellulose is the chemical equivalent of the morphological change of the plant cell known as “lignification.” The topical cuto-celluloses are the epidermal tissues of all growing plants or organs, which are easily detached from the underlying tissues which it is their function to protect. To subserve this function, they are extremely resistant to the attack of reagents. The associated groups are mostly of the normal saturated series, and of very high molecular weight.

Cellulose and Botanical Science.—The elaboration of cellulose, i.e. of the cell walls, and its morphological and physiological aspects are discussed in the articles : Physiology, Anatomy: and ; while in the article the part played by cellulose in the formation of these deposits receives treatment: here we may deal with its general relation to agriculture. In the analysis of fodder plants and other vegetable produce, the residue obtained after successive acid and alkaline hydrolysis is the “crude fibre” of the agricultural chemist, and is generally taken as a measure of the actual cellulose contents of the raw material. We give in tabular form the average percentage of crude fibre in typical food-stuffs and agricultural produce:—