g. 17, which represents an extreme case observed in
beech. This same form also occurs on the radial plane, causing the
tangential section to appear wavy or in transverse folds.

When wavy grain is fine (_i.e._, the folds or ridges small but
numerous) it gives rise to the "curly" structure frequently seen in
maple. Ordinarily, neither wavy, spiral, nor alternate grain is
visible on the cross-section; its existence often escapes the eye even
on smooth, longitudinal faces in the sawed material, so that the only
guide to their discovery lies in splitting the wood in two, in the two
normal plains.

[Illustration: Fig. 15. Spiral Grain. Season checks, after
removal of bark, indicate the direction of the fibres or
grain of the wood.]

[Illustration: Fig. 16. Alternating Spiral Grain in Cypress.
Side and end view of same piece. When the bark was at _o_,
the grain of this piece was straight. From that time, each
year it grew more oblique in one direction, reaching a climax
at _a_, and then turned back in the opposite direction. These
alternations were repeated periodically, the bark sharing in
these changes.]

Generally the surface of the wood under the bark, and therefore also
that of any layer in the interior, is not uniform and smooth, but is
channelled and pitted by numerous depressions, which differ greatly in
size and form. Usually, any one depression or elevation is restricted
to one or few annual layers (_i.e._, seen only in one or few rings)
and is then lost, being compensated (the surface at the particular
spot evened up) by growth. In some woods, however, any depression or
elevation once attained grows from year to year and reaches a maximum
size, which is maintained for many years, sometimes throughout life.
In maple, where this tendency to preserve any particular contour is
very great, the depressions and elevations are usually small
(commonly less than one-eighth inch) but very numerous.

On tangent boards of such wood, the sections, pits, and prominences
appear as circlets, and give rise to the beautiful "bird's eye" or
"landscape" structure. Similiar structures in the burls of black ash,
maple, etc., are frequently due to the presence of dormant buds, which
cause the surface of all the layers through which they pass to be
covered by small conical elevations, whose cross-sections on the sawed
board appear as irregular circlets or islets, each with a dark speck,
the se