Very young trees are all sapwood, but, as they get older, part of the wood is no longer needed to carry sap and it becomes heartwood.  Heartwood is darker than the sapwood, sometimes only slightly, but in other instances it may vary from a light-brown color to jet black.  It tends to fill with gums, resins, pigments and other substances, but otherwise its structure is the same as that of the sapwood.

[Illustration:  FIG. 145.—­Cross-section of Oak.]

The wood of all our common trees is produced by a thin layer of cells just beneath the bark, the cambium.  The cambium adds new wood on the outside of that previously formed and new bark on the inside of the old bark.  A tree grows most rapidly in the spring, and the wood formed at that time is much lighter, softer and more porous than that formed later in the season, which is usually quite hard and dense.  These two portions, known as early wood or spring wood, and late wood or summer wood, together make up one year’s growth and are for that reason called annual rings.  Trees such as palms and yucca do not grow in this way, but their wood is not important enough in this country to warrant a description.

[Illustration:  FIG. 146.—­White Oak Wood. (Magnified 20 times.)]

If the end of a piece of oak wood is examined, a number of lines will be seen radiating out toward the bark like the spokes in a wheel.  These are the medullary rays.  They are present in all woods, but only in a few species are they very prominent to the unaided eye.  These rays produce the “flakes” or “mirrors” that make quartersawed (radially cut) wood so beautiful.  They are thin plates or sheets of cells lying in between the other wood cells.  They extend out into the inner bark.

While much may be seen with the unaided eye, better results can be secured by the use of a good magnifying glass.  The end of the wood should be smoothed off with a very sharp knife; a dull one will tear and break the cells so that the structure becomes obscured.  With any good hand lens a great many details will then appear which before were not visible.  In the case of some woods like oak, ash, and chestnut, it will be found that the early wood contains many comparatively large openings, called pores, as shown in Figs. 146 and 147.  Pores are cross-sections of vessels which are little tube-like elements running throughout the tree.  The vessels are water carriers.  A wood with its large pores collected into one row or in a single band is said to be ring-porous.  Fig. 146 shows such an arrangement.  A wood with its pores scattered throughout the year’s growth instead of collected in a ring is diffuse-porous.  Maple, as shown in Fig. 152, is of this character.

[Illustration:  FIG. 147.—­Example of the Black Oak Group. (Quercus coccinea.) (Magnified 20 times.)]