The features which have been utilized in the key and with which—their names as well as their appearance—therefore, the reader must familiarize himself before attempting to use the key, are mostly described as they appear in cross-section. They are:
(1) Sap-wood and heart-wood (see [p. 17]), the former being the wood from the outer and the latter from the inner part of the tree. In some cases they differ only in shade, and in others in kind of color, the heart-wood exhibiting either a darker shade or a pronounced color. Since one can not always have the two together, or be certain whether he has sap-wood or heart-wood, reliance upon this feature is, to be sure, unsatisfactory, yet sometimes it is the only general characteristic that can be relied upon. If further assurance is desired, microscopic structure must be examined; in such cases reference has been made to the presence or absence of tracheids in pith rays and the structure of their walls, especially projections and spirals.
(2) Annual rings, their formation having been described on [page 19]. (See also Figs. 128-130.) They are more or less distinctly marked, and by such marking a classification of three great groups of wood is possible.
(3) Spring wood and summer wood, the former being the interior (first formed wood of the year), the latter the exterior (last formed) part of the ring. The proportion of each and the manner in which the one merges into the other are sometimes used, but more frequently the manner in which the pores appear distributed in either.
(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.)