and several hundred cell rows in height, which amount
commonly to one or more inches. These large rays are conspicuous on
all sections. They appear as long, sharp, grayish lines on the
cross-sections; as short, thick lines, tapering at each end, on the
tangential or "bastard" face, and as broad, shiny bands, "the
mirrors," on the radial section. In addition to these coarse rays,
there is also a large number of small pith rays, which can be seen
only when magnified. On the whole, the pith rays form a much larger
part of the wood than might be supposed. In specimens of good white
oak it has been found that they form about sixteen to twenty-five per
cent of the wood.

[Illustration: Fig. 7. Portion of the Firm Bodies of Fibres
with Two Cells of a Small Pith Ray _mr_ (Highly Magnified).]

[Illustration: Fig. 8. Isolated Fibres and Cells, _a_, four
cells of wood, parenchyma; _b_, two cells from a pith ray;
_c_, a single joint or cell of a vessel, the openings _x_
leading into its upper and lower neighbors; _d_, tracheid;
_e_, wood fibre proper.]

Minute Structure

[Illustration: Fig. 9. Cross-section of Basswood (Magnified).
_v_, vessels; _mr_, pith rays.]

If a well-smoothed thin disk or cross-section of oak (say
one-sixteenth inch thick) is held up to the light, it looks very much
like a sieve, the pores or vessels appearing as clean-cut holes. The
spring-wood and gray patches are seen to be quite porous, but the firm
bodies of fibres between them are dense and opaque. Examined with a
magnifier it will be noticed that there is no such regularity of
arrangement in straight rows as is conspicuous in pine. On the
contrary, great irregularity prevails. At the same time, while the
pores are as large as pin holes, the cells of the denser wood, unlike
those of pine wood, are too small to be distinguished. Studied with
the microscope, each vessel is found to be a vertical row of a great
number of short, wide tubes, joined end to end (see Fig. 8, _c_). The
porous spring-wood and radial gray tracts are partly composed of
smaller vessels, but chiefly of tracheids, like those of pine, and of
shorter cells, the "wood parenchyma," resembling the cells of the
medullary rays. These latter, as well as the fine concentric lines
mentioned as occurring in the summer-wood, are composed entirely of
short tube-like parenchyma cells, with square or oblique ends (see
Fig. 8, _