Often, as in mahogany and sweet gum, the fibers of several layers twist first in one direction and then those of the next few layers twist the other way, Fig. 24. Such wood is peculiarly cross-grained, and is of course hard to plane smooth. But when a piece is smoothly finished the changing reflection of light from the surface gives a beautiful appearance, which can be enhanced by staining and polishing. It constitutes the characteristic "grain" of striped mahogany, Fig. 23. It is rarely found in the inner part of the tree. [Illustration: Fig. 23. Mahogany, Showing Alternately Twisted Grain (full size).] [Illustration: Fig. 24. Spiral Grain in Cypress. _After Roth._] Sometimes the grain of wood is "cross," because it is "wavy" either in a radial or a tangential section, as in maple, Fig. 25, and Fig. 26. [Illustration: Fig. 25. Planed Surface of Wavy-Grained Maple (full size).] [Illustration: Fig. 26. Split Surface of Wavy-Grained Maple (full size).] "Curly grain" refers to the figure of circlets and islets and contours, often of great beauty, caused by cutting a flat surface in crooked-grained wood. See Fig. 27, curly long-leaf pine, and Fig. 28, yellow poplar. When such crookedness is fine and the fibers are contorted and, as it were, crowded out of place, as is common in and near the roots of trees, the effect is called "burl," Fig. 29. The term burl is also used to designate knots and knobs on tree trunks, Fig. 31. Burl is used chiefly in veneers. [Illustration: Fig. 27. Curly Grained Long-leaf Pine (full size).] [Illustration: Fig. 28. Curly Yellow Poplar (full size).] [Illustration: Fig. 29. Redwood Burl (full size).] [Illustration: Fig. 30. Bird's-eye Maple (full size.)] [Illustration: Fig. 31. Burl on White Oak.] Irregularity of grain is often caused by the presence of adventitious and dormant buds, which may be plainly seen as little knobs on the surface of some trees under the bark. In most trees, these irregularities are soon buried and smoothed over by the successive annual layers of wood, but in some woods there is a tendency to preserve the irregularities. On slash (tangent) boards of such wood, a great number of little circlets appear, giving a beautiful grain, as in "Bird's-eye maple," Fig. 30. These markings are found to predominate in the inner part of the tree. This is not at all a distinct variety of maple, as is sometimes supposed, but the common variety, in which the phenomenon frequently appears. Logs of great value, having bird's-eyes, have often unsuspectingly been chopped up for fire wood. The term "grain" may also mean the "figure" formed by the presence of pith rays, as in oak, Fig. 32, or beech, or the word "grain" may refer simply to the uneven deposit of coloring matter as is common in sweet gum, Fig. 33, black ash, or Circassian walnut. [Illustration: Fig. 32. Figure Formed by Pith Rays in Oak (full size).] [Illustration: Fig. 33. Sweet Gum, Showing Uneven Deposit of Coloring Matter (full size.)]