The user of the key must, however, realize that external appearance, such, for example, as color, is not only very variable but also very difficult to describe, individual observers differing especially in seeing and describing shades of color. The same is true of statements of size, when relative, and not accurately measured, while weight and hardness can perhaps be more readily approximated. Whether any feature is distinctly or only indistinctly seen will also depend somewhat on individual eyesight, opinion, or practice. In some cases the resemblance of different species is so close that only one other expedient will make distinction possible, namely, a knowledge of the region from which the wood has come. We know, for instance, that no longleaf pine grows in Arkansas and that no white pine can come from Alabama, and we can separate the white cedar, giant arbor vitæ of the West and the arbor vitæ of the Northeast, only by the difference of the locality from which the specimen comes. With all these limitations properly appreciated, the key will be found helpful toward greater familiarity with the woods which are more commonly met with. 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.