[Footnote 2: To discover the resin ducts a very smooth surface is necessary, since resin ducts are frequently seen only with difficulty, appearing on the cross-section as fine whiter or darker spots normally scattered singly, rarely in groups, usually in the summer wood of the annual ring. They are often much more easily seen on radial, and still more so on tangential sections, appearing there as fine lines or dots of open structure of different color or as indentations or pin scratches in a longitudinal direction.] ==== ADDITIONAL NOTES FOR DISTINCTIONS IN THE GROUP. Spruce is hardly distinguishable from fir, except by the existence of the resin ducts, and microscopically by the presence of tracheids in the medullary rays. Spruce may also be confounded with soft pine, except for the heart-wood color of the latter and the larger, more frequent, and more readily visible resin ducts. In the lumber yard, hemlock is usually recognized by color and the silvery character of its surface. Western hemlocks partake of this last character to a less degree. Microscopically the white pine can be distinguished by having usually only one large pit, while spruce shows three to five very small pits in the parenchyma cells of the pith ray communicating with the tracheid. The distinction of the pines is possible only by microscopic examination. The following distinctive features may assist in recognizing, when in the log or lumber pile, those usually found in the market: The light, straw color, combined with great lightness and softness, distinguishes the white pines (white pine and sugar pine) from the hard pines (all others in the market), which may also be recognized by the gradual change of spring wood into summer wood. This change in hard pines is abrupt, making the summer wood appear as a sharply defined and more or less broad band. The Norway pine, which may be confounded with the shortleaf pine, can be distinguished by being much lighter and softer. It may also, but more rarely, be confounded with heavier white pine, but for the sharper definition of the annual ring, weight, and hardness. The longleaf pine is strikingly heavy, hard, and resinous, and usually very regular and narrow ringed, showing little sap-wood, and differing in this respect from the shortleaf pine and loblolly pine, which usually have wider rings and more sap-wood, the latter excelling in that respect. The following convenient and useful classification of pines into four groups, proposed by Dr. H. Mayr, is based on the appearance of the pith ray as seen in a radial section of the spring wood of any ring: Section I. Walls of the tracheids of the pith ray with dentate projections. _a._ One to two large, simple pits to each tracheid on the radial walls of the cells of the pith ray.--Group 1. Represented in this country only by _P. resinosa_.