FIBROUS MATERIALS 7

cross-bindings which can be seen in the diagram (Fig. 2, No. 4). Further, if
the fibres are stained with a solution of iodine and zinc chloride they give a
bright yellow colour.

Cellulose pulps from coniferous woods consist entirely of fibrous elements
(tracheids). The fibres of spring or summer wood are broad, flat and thin-
walled, owing to the quicker and more juicy growth of these seasons. Those
of autumn growth are thicker-walled and rounded, with pointed ends. The

I. Birch, with cell. z. Spruce. 3. Poplar, with cell.

FIG. 2.—THE FIBRES OF WOOD PULP

4, Mechanical Spruce.

fibres have circular pores surrounded by depressions, and these are very charac-
teristic, and may be distinctly seen under high magnification (Eg. 2, No. 2).
In pinewood cellulose the pores are oval-shaped. Some of the thick-walled
fibres are very similar to cotton, and they vary greatly in length.
The fibres of deciduous (foliage) trees (Fig. 2, Nos, I and 3) are quite short
and rounded, with fairly thick walls, and narrow slits or pores disposed diagon-
ally. Poplar fibres sometimes have nodes, or knots, like linen. The structure
does not lend itself to beating or tearing into fibrillae.