(4) Pores, which are vessels cut thru, appearing as holes in cross-section, in longitudinal section as channels, scratches, or identifications. (See p. 23 and Figs. 129 and 130.) They appear only in the broad-leaved, so called, hard woods; their relative size (large, medium, small, minute, and indistinct when they cease to be visible individually by the naked eye) and manner of distribution in the ring being of much importance, and especially in the summer wood, where they appear singly, in groups, or short broken lines, in continuous concentric, often wavy lines, or in radial branching lines. (5) Resin ducts (see p. 26 and Fig. 128) which appear very much like pores in cross-section, namely, as holes or lighter or darker colored dots, but much more scattered. They occur only in coniferous woods, and their presence or absence, size, number, and distribution are an important distinction in these woods. (6) Pith rays (see p. 21 and Figs. 129 and 130), which in cross-section appear as radial lines, and in radial section as interrupted bands of varying breadth, impart a peculiar luster to that section in some woods. They are most readily visible with the naked eye or with a magnifier in the broad-leaved woods. In coniferous woods they are usually so fine and closely packed that to the casual observer they do not appear. Their breadth and their greater or less distinctness are used as distinguishing marks, being styled fine, broad, distinct, very distinct, conspicuous, and indistinct when no longer visible by the naked (strong) eye. (7) Concentric lines, appearing in the summer wood of certain species more or less distinct, resembling distantly the lines of pores but much finer and not consisting of pores. (See Fig. 129.) Of microscopic features, the following only have been referred to: (8) Tracheids, a description of which is to be found on page 28. (9) Pits, simple and bordered, especially the number of simple pits in the cells of the pith rays, which lead into each of the adjoining tracheids. For standards of weight, consult table on pages 50 and 192; for standards of hardness, table on page 195. Unless otherwise stated the color refers always to the fresh cross-section of a piece of dry wood; sometimes distinct kinds of color, sometimes only shades, and often only general color effects appear. [Footnote A: From Forestry Bulletin No. 10, _U. S. Department of Agriculture_.] HOW TO USE THE KEY.