Cellulose is the substance secreted by the living protoplasm of a vegetable cell to form its investing membrane or cell-wall, passing through the various ligneous, corky, and colloid changes, new arrangement and union in cell-walls, &c. (see CELL, LEAF, TISSUE, VEGETABLE PHYSIOLOGY, WOOD). It is obtained pure by treating any unaltered cellular tissue with alkalies and acids to remove mineral matter and protoplasm, and successive washings with water, alcohol, and ether to remove soluble substances. Cotton-pith or vegetable-ivory, although much contrasted in histological properties, are alike remarkably pure cellulose; in bast the proportion of associated mineral matter becomes much more considerable. Cellulose has the chemical composition , and spec. grav. 1.52. Among its familiar natural modifications gum is an isomer, and starch-dextrin and grape-sugar are all of similar ultimate composition, while its woody and corky modifications (lignin and suberin) possess an increasing proportion of carbon. Iodine alone stains cellulose yellow or brown, but blue when strong sulphuric acid has been previously added. Strong hot sulphuric acid chars it, while brief immersion in the cold converts it into a tough and dense modification, well known in parchment paper, and prolonged treatment dissolves it altogether. Dextrine may thus be prepared and next transmuted, by boiling the watery solution, into grape-sugar (see DEXTRINE, GLUCOSE). By immersion in a mixture of strong nitric and sulphuric acid we obtain Gun-cotton (q.v.), while dilute nitric acid or potash oxidises it into oxalic acid. Ammoniacal oxide of copper dissolves it without change, as is shown by its reprecipitation on dilution. By heating in closed vessels under pressure a dense coal-like mass is formed, while in ordinary dry distillation, gas, tar, and acetic acid are given off, processes which throw light on the formation of coal in nature and on the chemistry of gas-making. In natural decomposition cellulose turns yellow and brown with gradual formation of humus. See SOILS.
Although so constant and characteristic a product of vegetable life, the conditions and mode of its formation are still very obscure. From that cell-cycle or rhythm of change between the passive and cellulose-walled state and an active and wall-less one, which is so characteristic of the lowest forms of life, and of which we find surviving traces (e.g. the rejuvenescence of the pollen-grain) in the reproductive processes of even the highest plants (see CELL), it would appear that there is some relation between this increased passivity and the formation of cellulose. And in this way arises the speculation that cellulose may be viewed essentially as a (mechanically coherent and thus useful) excretion, an incompletely utilised waste product corresponding to the carbonic acid and water given off by the completer respiratory oxidation and larger evolution of energy of the active phase. Once formed by the plant, it may be again absorbed, as is well seen in the union of a row of cells into a continuous vessel, or in the consumption of endosperm of a seed during germination. Many seeds, such as vegetable-ivory or date, have a great proportion of their reserve material in this form; and this must be digested into glucose by the growing embryo, and again worked up into new protoplasm, which deposits cellulose as before. Like the plant itself, the similar digestive ferments of the animal might thus be naturally expected to digest cellulose; and this is actually, to some extent, the case with the delicate young cell-walls of many green vegetables, as can be experimentally verified, even in man; while in herbivorous animals this power is much developed, and the nutritive utilisation of their fodder is thus increased to an important extent.
The cysts of amœbæ and other protozoa appear to be at least largely composed of cellulose, and the external tunic of ascidians (see TUNICATA) is of identical, or at least isomeric, composition. Cellulose has been described as a pathological product, even in brain-tissue; and Chitin (q.v.), a very characteristic and in many respects comparable animal product, has been sometimes viewed as cellulose in association with a proteid substance.