Apart from the general shape of the tree, the bark on the trunk and branches is a constant help in identification. It is hard and smooth on some trees, like that of the hornbeam and beech, fissured into ridges like the sugar maple on others, it sometimes flakes off in rough plates like those of the shagbark hickory, and again in thin, brittle strips like those of the hop hornbeam, the bark peels off laterally as in the canoe birch, and occasionally becomes ridged and corky as we find it on the branches of the liquidamber and cork elm. Very often the color of the bark is distinctive as is that of the green stems of the sassafras and moosewood maple and the white, brown, pink, and yellow trunks of different birches. The taste and odor of the bark are also characteristic of certain species, as, for instance, the unpleasant, bitter taste of the black cherry, the mucilaginous taste of the slippery elm, and the aromatic fragrance of the stems of the mockernut hickory. The little dots on young bark are called _lenticels_, they are openings for admitting air to the inner tissues. Lenticels are conspicuous in the bark of the birches. The presence of thorns on the trunk and branches of certain trees helps us to distinguish them from others, and the clusters of dry fruit which remain hanging on some trees through the winter are another means of identification. [Sidenote: Stems and Twigs] Stems and twigs vary from the finest, lightest sprays to the most coarsely moulded ones,—from the delicate twigs of the hop hornbeam to the stout shoots of the horsechestnut;—like larger branches their tips either ascend, droop, or grow at right angles from the stem, and may be smooth, downy, or rough to the touch. The pith in cross sections of twigs shows different forms and is a means in itself of distinguishing some trees. It is usually circular, but in some species it takes the form of a pentagon or a star. In a vertical section we sometimes find it in horizontal plates, like the chambered pith of the walnuts. The color is usually white, but sometimes we find it pink, yellowish, green, red, and brown. It is interesting to find that the history of a tree for several years past can be told by studying the scars along the bare stems. The annual growth each year is marked by a circle of scars around the stem, which was left by the scales of the buds when they opened in the spring, and these scars mark each season’s growth for successive years along the stem. [Sidenote: Leaf-scars]