Cellulose

Cellulose. The membrane forming the walls of plant cells, and which generally forms the framework of all vegetable tissues, is, in all cases, composed chiefly of the same chemical substance, termed cellulose. Cotton wool, elder pith, hemp, etc., consist almost entirely of cellulose, which may be obtained pure from any of these sources by removing the impurities by different solvents. When so obtained, it is a white solid, insoluble in water or alcohol, but soluble in strong sulphuric acid, which converts it into sugar. If pure it is stable, but in the presence of other vegetable matter it soon decomposes. Its chemical composition maybe represented by (C₆H₁₀O₅)_n, the exact complexity of the molecule being unknown. By treatment with concentrated sulphuric acid it may be entirely converted into grape-sugar, a process by which sugar for brewing and other purposes is largely manufactured from linen-rags or other refuse. Though cellulose does occur as a rare substance in the animal kingdom, it is more especially characteristic of the vegetable cell, and the cellulose walls can be seen to be formed by nascent or "primordial" cells out of the protoplasmic substance In the metasperm of the date-palm, and probably in other cases, cellulose occurs as a reserve substance, the cell-walls being thick in the ripe seed and thinning during germination. More commonly the walls of old cells undergo one of three changes. becoming impregnated with lignin, cutin, or mucin, substances richer in carbon than is cellulose. In the first case, lignification, characteristic of wood-cells, the wall becomes less plastic, less permeable, hard, and susceptible of a distinct yellow coloration by iodine alone and brown with sulphuric acid. In the second case, cuticularisation, characteristic of cork and cuticle, the wall becomes elastic, impermeable, and non-absorbent, and offers great resistance to the action of even strong acids. It becomes yellow on treatment with iodine and does not swell, as do unaltered or lignified walls, on treatment with sulphuric acid. The third case, conversion into mucilage, as in the testa of linseed, renders the walls extremely absorbent, swelling considerably with water. These three changes are known compendiously in German as vcrholzung, verkorkung, and verschleimung respectively.