ulose is nearly always
contaminated by various encrusting materials; and in the process of
wood-formation, the cell-wall material continually thickens by the
conversion of the cellulose into ligno-cellulose and the protoplasm of the
cell as continuously diminishes in volume. Thus the protoplasm of the cell
produces a number of different kinds of material which are deposited in the
walls of the cell. All of these, taken together, constitute the general
group known as the celluloses.

These may be divided into three classes: namely, (1) the hemi-celluloses,
(2) the normal celluloses, and (3) the compound celluloses.

The =hemi-celluloses= (pseudo-, or reserve celluloses) include a series of
complex polysaccharides which occur in the cell-walls of the seeds of
various plants. They are found in the shells of nuts, rinds of cocoanuts,
shells of stony fruits, etc., and in the seedcoats of beans, peas and other
legumes. They are much more easily hydrolyzed than the other members of
this group, and when hydrolyzed yield various sugars, chiefly galactose,
mannose, and the pentoses. They bear the same relation to these sugars that
starch does to glucose, and are generally supposed to serve as reserve food
material, although it is difficult to conceive how the shells, etc., in
which they appear can be utilized by a growing seedling. They differ in
structure from the fibrous celluloses and are probably not cell-wall
building material. They appear to be a form of reserve carbohydrates, which
differ from the glucose-polysaccharides in being condensed in, or as a part
of, the external structural material rather than in the internal storage
organs. They are soluble in water and exhibit the properties of gums, and
are often classified with the gums and described under the names
"galactans," "mannosans," "pentosans," etc.

The =normal celluloses=, of which the fibers obtained from cotton, flax,
hemp, etc., are typical examples, are widely distributed in plants and form
the commercial sources for all textile fibers of vegetable origin. Ordinary
cotton fiber contains 91 per cent of cellulose, about 7.5 per cent of
water, 0.4 per cent of wax and fat, 0.55 per cent of pectose derivatives,
and 0.25 per cent of mineral matter; or a total of only 1.2 per cent of
non-cellulose solids. Filter paper is practically pure cellulose.

Pure cellulose is a white, hygroscopic substance, which is insoluble in
water and in most other solvents. If heated