, presents a picture like that shown
in Fig. 2. Only short pieces of the tubes or cells of which the wood
is composed are represented in the picture. The total length of these
fibres is from one-twentieth to one-fifth inch, being the smallest
near the pith, and is fifty to one hundred times as great as their
width (see Fig. 3). They are tapered and closed at their ends,
polygonal or rounded and thin-walled, with large cavity, lumen or
internal space in the spring-wood, and thick-walled and flattened
radially, with the internal space or lumen much reduced in the
summer-wood (see right-hand portion of Fig. 2). This flattening,
together with the thicker walls of the cells, which reduces the lumen,
causes the greater firmness and darker color of the summer-wood.
There is more material in the same volume. As shown in the figure, the
tubes, cells or "tracheids" are decorated on their walls by
circlet-like structures, the "bordered pits," sections of which are
seen more magnified as _a_, _b_, and _c_, Fig. 2. These pits are in
the nature of pores, covered by very thin membranes, and serve as
waterways between the cells or tracheids. The dark lines on the side
of the smaller piece (1, Fig. 2) appear when magnified (in 2, Fig. 2)
as tiers of eight to ten rows of cells, which run radially (parallel
to the rows of tubes or tracheids), and are seen as bands on the
radial face and as rows of pores on the tangential face. These bands
or tiers of cell rows are the medullary rays or pith rays, and are
common to all our lumber woods.

In the pines and other conifers they are quite small, but they can
readily be seen even without a magnifier. If a radial surface of
split-wood (not smoothed) is examined, the entire radial face will be
seen almost covered with these tiny structures, which appear as fine
but conspicuous cross-lines. As shown in Fig. 2, the cells of the
medullary or pith are smaller and very much shorter than the wood
fibre or tracheids, and their long axis is at right angles to that of
the fiber.

[Illustration: Fig. 3. Group of Fibres from Pine Wood. Partly
schematic. The little circles are "border pits" (see Fig. 2,
_a-c_). The transverse rows of square pits indicate the
places of contact of these fibres and the cells of the
neighboring pith rays. Magnified about 25 times.]

In pines and spruces the cells of the upper and lower rows of each
tier or pith ray have "bordered" pits, like those of the w